Maintenance Of Endemic Dendroctonus ponderosae
Populations Through Interactions With A
Secondary Bark Beetle, Pseudips mexicanus

Gregory David Smith
B.Sc, University of Victoria, 2002

Thesis Submitted In Partial Fulfillment Of
The Requirements For The Degree Of
Master of Science
In
Natural Resources And Environmental Studies
(Biology)

The University of Northern British Columbia
July 2008
© Gregory David Smith, 2008

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ABSTRACT
Endemic Dendroctonus ponderosae are often found inhabiting weakened lodgepole pine
(Pinus contorta var. latifolia) in western Canada previously attacked by the little known bark
beetle Pseudips mexicanus. Interactions between the two species have not been elucidated, so
three studies were conducted. The first examined the life history of P. mexicanus, describing
developmental characteristics. The second explored the interaction between the two species and
found that D. ponderosae attacking ephemeral hosts previously occupied by P. mexicanus do so
in greater densities with earlier offspring emergence when compared to hosts D. ponderosae
attacked alone. P. mexicanus may alter host chemistry, allowing D. ponderosae to be more
successful when compared to trees D. ponderosae attacked alone. The third study investigated
the attraction of D. ponderosae to P. mexicanus-produced volatiles, revealing no response by D.
ponderosae. The scope of this work contributes to the understanding of bark beetle ecology in
the lodgepole pine ecosystem.

TABLE OF CONTENTS
Page
ABSTRACT

ii

TABLE OF CONTENTS

iii

LIST OF TABLES

v

LIST OF FIGURES

vi

ACKNOWLEDGEMENTS

viii

CHAPTER 1

GENERAL INTRODUCTION

Literature cited
CHAPTER 2

5
THE LIFE HISTORY OF A SECONDARY BARK BEETLE,
PSEUDIPS MEXICANUS, IN LODGEPOLE PINE

Abstract
Introduction
Methods
Field study
Laboratory study
Data analysis
Results and Discussion
Field study
Laboratory study
Literature cited

CHAPTER 3

1

9
9
10
13
13
16
20
21
21
32
41

AN EXAMINATION OF THE INTERACTION BETWEEN A
SECONDARY BARK BEETLE, PSEUDIPS MEXICANUS AND
ENDEMIC DENDROCTONUS PONDEROSAE IN WEAKENED
LODGEPOLE PINE TREES

Abstract
Introduction
Methods
Study area
Tree selection
Beetle rearing and gallery examination
Analysis
Results
Tree characteristics

in

48
48
49
54
54
55
56
57
58
58

Page
Attack density and gallery length
Phloem use
Number of offspring emerging
Offspring size
Sex ratio
Brood emergence phenology
Discussion
Literature cited
CHAPTER 4

60
60
67
67
70
70
72
81

THE RESPONSE OF ENDEMIC DENDROCTONUS PONDEROSAE
TO VOLATILES ASSOCIATED WITH THE SECONDARY BARK
BEETLE PSEUDIPS MEXICANUS
90

Abstract
Introduction
Methods
Study Site
Experiment 1 - Pheromone
Trap layout
Experiment 2 - Pine bolt
Host material collection
Beetle collection
Bolt infestation
Trap layout
Beetle trapping and identification
Data analysis
Results
Experiment 1 - Pheromone
Experiment 2 - Pine bolt
Discussion
Literature cited
CHAPTER 5

90
91
94
94
95
95
95
95
96
96
97
97
98
99
99
102
106
111

GENERAL CONCLUSIONS

Literature cited

116
120

IV

LIST OF TABLES
Page
Table 2.1. Characteristics of lodgepole pine trees (Pinus contorta var. latifolia)
assessed by variable radius plot from seven stands at Angstad Creek, B C ,
and individual trees attacked by Pseudips mexicanus.

22

Table 2.2. Mean attack and ovipositional gallery characteristics (± SE) for wild
populations of Pseudips mexicanus reared from naturally attacked
lodgepole pine trees.

24

Table 2.3. Comparison of mean head capsule (HC) widths, range (mm ± SE) and rate
of change (ROC) between instars of Pseudips mexicanus from manually
infested lodgepole pine logs (Smith) with those reported by Trimble
(1924). Trimble did not report standard error or standard deviation.

39

Table 3.1. Characteristics of lodgepole pine trees attacked by Dendroctonus
ponderosae (D. ponderosae) alone (D. ponderosae only) and trees
attacked by D. ponderosae after P. mexicanus infestation (D. ponderosae
and P. mexicanus). Also included are tree characteristics of lodgepole
pine from seven stands (Stand). These data were used to assess the
interaction between endemic D. ponderosae and Pseudips mexicanus.

59

Table 3.2. Dendroctonus ponderosae (D. ponderosae) and P. mexicanus attack
characteristic descriptions from trees attacked by D. ponderosae alone (D.
ponderosae only) and by D. ponderosae after P. mexicanus infestation {D.
ponderosae and P. mexicanus). Mean ± SE.

61

Table 3.3. Summary of a multiple linear regression analysis examining the effects of
volume of phloem utilized (cm ) by Pseudips mexicanus per m and
Dendroctonus ponderosae attack density (attacks/m ) on female endemic
D. ponderosae mean pronotal width (mm).

69

Table 4.1. Analysis of variance test on log transformed capture data for bark beetle
species collected from a field trapping experiment with treatment as a
fixed factor and block as a random. Treatments consisted of a control trap
and a trap baited with Pseudips mexicanus attractant pheromone, racemic
ipsenol and racemic ipsdienol.

100

Table 4.2. Analysis of variance test on log transformed capture data for bark beetle
species collected from a field trapping experiment with sex and treatment
as factors. Treatments were comprised of a control trap, a trap baited with
a lodgepole pine bolt infested with Pseudips mexicanus and a trap baited
with an uninfested pine bolt.

103

LIST OF FIGURES
Page
Figure 2.1. (A) Amorphous gallery of Pseudips mexicanus typical of overwintering
beetles. Note lack of egg niches and apparent randomness in gallery
length and direction. (B) Ovipositional gallery of Pseudips mexicanus.
Note distinctive shape of the gallery and the presence of egg niches on
the outside of the curve. NC = nuptial chamber, EN = egg niche.

15

Figure 2.2. Mean pronotal widths (mm ± SE) of Pseudips mexicanus by sex reared
from naturally infested lodgepole pine trees. Different letters above bars
indicate significant differences (P < 0.05).

29

Figure 2.3. Development rate (d~ ) of Pseudips mexicanus from infestation to fully
mature adult versus temperature. Quadratic curve applied to determine
optimum temperature of development (arrow). Open circles indicate data
collected in 2004; solid circles indicate data collected in 2005. Error bars
represent 1 SE of the mean.

34

Figure 2.4. Minimum temperature of development (arrow) extrapolated from
regression of temperatures within normally encountered conditions of
Pseudips mexicanus. Open circles indicate data collected in 2004; solid
circles indicate data collected in 2005. Error bars represent 1 SE of the
mean.

36

Figure 2.5. Histogram of Pseudips mexicanus larval head capsule widths from logs
incubated at 23.5 °C indicating four distinct instars, n = 195.

38

Figure 3.1. Proportion of phloem used per lodgepole pine log by Pseudips mexicanus
in relation to P. mexicanus attack density (number of attacks per m 2 ).

62

Figure 3.2. Proportion of phloem used per lodgepole pine log by Dendroctonus
ponderosae in relation to D. ponderosae attack density (number of
attacks per m 2 ). Solid dots indicate trees attacked previously by Pseudips
mexicanus, followed by D. ponderosae in the same year and open circles
indicate trees attacked solely by D. ponderosae.

64

Figure 3.3. Lodgepole pine phloem volume (cm3) utilized per attack by endemic
Dendroctonus ponderosae in relation to D. ponderosae attack density.
Solid dots indicate trees attacked previously by Pseudips mexicanus,
followed by D. ponderosae in the same year and open circles indicate
trees attacked solely by D. ponderosae.

65

vi

"5

Page

Figure 3.4. Lodgepole pine phloem volume (cm ) utilized per beetle by endemic
Dendroctonus ponderosae in relation to the total number of emerged D.
ponderosae offspring per tree. Solid dots indicate trees attacked
previously by Pseudips mexicanus, followed by D. ponderosae in the
same year and open circles indicate trees attacked solely by D.
ponderosae.

66

Figure 3.5. Number of female endemic Dendroctonus ponderosae emerging per m 2
in relation to phloem volume used (cm3) by Pseudips mexicanus per m 2
in lodgepole pine. Solid dots indicate trees attacked previously by P.
mexicanus, followed by D. ponderosae in the same year and open circles
indicate trees attacked solely by D. ponderosae.

68

Figure 3.6. Cumulative mean percent emergence of female endemic Dendroctonus
ponderosae from lodgepole pine trees in relation to the mean number of
days since emergence commenced per infestation type. Solid dots
indicate trees attacked previously by Pseudips mexicanus, followed by D.
ponderosae in the same year, open circles indicate trees attacked solely
by D. ponderosae and grey dots represent overlapping data points. Error
bars are 1 SE of the mean.

71

Figure 4.1. Number of bark beetles captured during a field trapping experiment using
a combination of racemic ipsenol and ipsdienol as the pheromone bait;
(A) Pseudips mexicanus; (B) Ipspini. Bars for each sex with the same
letter are not significantly different (P < 0.05, N= 5). Con = control,
Treat = pheromone treatment.

101

Figure 4.2. Number of Pseudips mexicanus captured during a field trapping
experiment with lodgepole pine bolts as attractive baits either alone or
infested by P. mexicanus. Bars for each sex with the same letter are not
significantly different; REGW multiple comparisons test (P < 0.05, N =
9).

104

Figure 4.3. Number of Orthotomicus latidens captured during a field trapping
experiment with lodgepole pine bolts either uninfested or infested by P.
mexicanus as attractive baits. Bars with the same letter are not
significantly different; REGW multiple comparisons test (P < 0.05, N =
9).

105

VII

ACKNOWLEDGEMENTS
I would like to thank my co-supervisors Dr. Staffan Lindgren of the University of Northern
British Columbia and Dr. Allan Carroll of the Canadian Forest Service for their dedication to
facilitating my development as a scientist and for passing along to me a small portion of their
vast knowledge of bark beetle ecology. My thanks to my committee members, Dr. Russ
Dawson and Dr. Lisa Poirier of the University of Northern British Columbia for their time
and energy which helped to guide me through this process. And I would like to thank Dr.
Nadir Erbilgin of the University of Alberta for time and thoughtful input as my external
examiner.
I extend my appreciation for the field and laboratory instruction extensively provided by
Doug Linton of the Canadian Forest Service and for field and laboratory assistance from
Tony Ibaraki, Janine Jell, Heather Linton, Josie Smith, Lara vanAkker, Andrew Copeland
and Michael Henley. Additionally, I would like to thank Dr. Les Safranyik of the Canadian
Forest Service for his insightful comments on my manuscripts and to Dr. Katherine Bleiker
of the Canadian Forest Service for her energy, perception and enlightening discussions.
My appreciation and gratitude I extend to my Mother and Father for instilling in me curiosity
and love of nature throughout my childhood and to my brother Steve for his companionship
on our adventures into the forest.
A special thank you goes to my wife Jessica for her artistic ability (Figs. 4.1 A and B), field
assistance as well as patience and understanding while taking care of our son Thomas
throughout this process. Without her support this work may have never been completed.
Funding for this project was provided by the Federal Mountain Pine Beetle Initiative Project
# 1.03 through the laboratory of A.L. Carroll.

Vlll

CHAPTER 1
GENERAL INTRODUCTION
Competition is generally accepted as the driving force in ecological theory (Kaplan and
Denno 2007), as numerous decades of research have focused on it as the principal
mechanism of interaction between species. Interspecific competition is commonly described
as either interference (where one species reduces the other's ability to obtain resources
through direct interaction) or exploitation (depression of both species when using a shared
resource) (Gotelli 2001). Facilitation, on the other hand, is a positive interaction, where at
least one of the species benefits without harming the other (Bruno et al. 2003). Competition
has been the most commonly reported interaction, but recently plant and marine invertebrate
studies have begun to show that facilitation is much more common than suspected (Callaway
1995, Stachowicz 2001). hi insects, facilitation as a mode of interaction has not been
identified regularly, however; recent work has suggested that interactions between
phytophagous insects do not fit the general pattern of competition and that facilitation should
be examined more rigorously (Kaplan and Denno 2007). Positive interactions have not been
studied extensively in the bark beetles (Coleoptera: Curculionidae: Scolytinae), but some
research has suggested that such interactions do occur (Amman and Schmitz 1988, Carroll et
al. 2006).
To assess the implications of interactions between species, basic biological information is
required for all involved. Although a vast number of organisms have been identified and
named, life history descriptions are often lacking. This is especially true in the insect world,
and can be seen in the bark beetles associated with lodgepole pine trees (Pinus contorta var.
latifolia Engelm.). More than 50 species of bark beetles associated with lodgepole pine have

1

been identified in Canada, but not all have life history information available (Bright 1976).
Life history studies examine traits such as attack dynamics, mating preference (i.e., polygyny
versus monogyny), offspring behaviour, development rate and voltinism. Once established,
these can be used to explain and possibly predict behaviours when interspecific encounters
occur.
Bark beetles are integral to healthy forest ecosystems, contributing to the degradation of
senescent or recently dead woody plant material, and they are often categorized by their host
preference. Apart from the mycophagous ambrosia beetles, all species in the Scolytinae are
primarily phytophagous, and a large proportion of them, the bark beetles, feed subcortically
on conifers in the phloem region (Wood 1982). The majority of bark beetle species are
'secondary' (Wood 1982) and attack weakened or dying trees almost exclusively (Doliner
and Borden 1984), with some species able to kill living trees when populations build up
(Thomas 1961). 'Primary' beetles are the first species to arrive at, and successfully infest,
healthy living hosts (Doliner and Borden 1984), but may have to subsist in weakened trees
when populations are low. Primary beetles have the ability to increase rapidly in numbers
during periods of host stress and to become severe pests (Wood 1982, Safranyik and Carroll
2006).
Initial host selection by bark beetles is conducted by so-called pioneer beetles that often
orient to host volatiles, a behaviour termed primary attraction (Person 1931). Primary
attraction has been studied extensively, e.g. for Dendroctonus ponderosae Hopkins (the
mountain pine beetle). For this species it is generally accepted that pioneer females orient to
host volatiles at long distances (Gara et al. 1984), recognize silhouettes and volatiles at short
distances (Moeck and Simmons 1991, Campbell and Borden 2006a, b), and land on host trees

2

at random coupled with gustatory sampling (Hynum and Berryman 1980, Raffa and
Berryman 1982, Pureswaran and Borden 2003) to select their host tree. Upon selecting a
suitable host, aggregation pheromones are produced attracting both mates and others of the
same sex. These pheromones may also attract individuals of other species of bark beetles,
predators and parasitoids (Poland and Borden 1994, 1997, Savoie et al. 1998, Aukema et al.
2004, Miller et al. 2005).
Bark beetles create ovipositional galleries in the phloem tissue, with some scoring of the
sapwood. Galleries vary in length and orientation to the grain of the wood, and may be
curved or linear. Eggs are laid along the sides of the gallery or in carved niches, singly or in
groups. Larvae feed either gregariously or solitarily away from the ovipositional gallery and
pupation occurs in the phloem or in the outer bark (Wood 1982). Many species feed as
newly eclosed adults prior to emergence to build up resources required for dispersal and host
selection (Wood 1982, McNee et al. 2000).
A pair of species was selected to examine how interspecific interactions affect bark
beetles. D. ponderosae is found in lodgepole pine forests throughout the southern half of the
province of British Columbia and the north-western United States, but is a major pest of pine
trees across its range. A great deal of research has examined the epidemic population phase
of this species (Safranyik et al. 1974, Berryman 1976, Klein et al. 1978, Thomson and
Shrimpton 1984, Safranyik and Linton 1985, Safranyik 1988, Safranyik et al. 1999), but the
endemic phase has had very little work conducted on it (Carroll et al. 2006). Because of this,
there is a knowledge gap in D. ponderosae population dynamics, specifically, the effect that
the presence of other bark beetle species has on endemic D. ponderosae and how this
influences population growth. Endemic D. ponderosae populations tend to be too small to

3

allow for successful attack on healthy hosts, leaving them confined to weakened trees often
previously infested by other bark beetles (Carroll et al. 2006). Carroll et al. (2006) found that
endemic D. ponderosae were directly associated with nine species of secondary bark beetles
on the lower bole, the most common being Pseudips mexicanus (Hopkins). This beetle is
native to British Columbia and is found sympatrically with D. ponderosae (Bright 1976), but
little is known about it. The studies that do exist describe P. mexicanus in Monterey pine in
California (Trimble 1924) and Mexican white pine in Central America (Schwerdtfeger 1956).
A study of the effects of the presence of P. mexicanus in weakened lodgepole pine trees
on endemic D. ponderosae was conducted to determine whether P. mexicanus played a
competitive or facilitative role in its interactions with D. ponderosae.

To successfully

scrutinize this interaction, in Chapter 2, I examine P. mexicanus life history characteristics
(e.g. development rate, voltinism, attack density) in lodgepole pine. Chapter 3 investigates
the effect of P. mexicanus on endemic D. ponderosae in weakened hosts using naturally
attacked trees. Chapter 4 explores the mechanism used by D. ponderosae to locate hosts
previously infested by P. mexicanus with trapping experiments using a variety of potentially
attractive baits. The examination of the relationship between these two species of beetle will
improve our understanding of how an eruptive herbivore maintains populations while in the
endemic state and will close some knowledge gaps in the field of bark beetle ecology.

4

LITERATURE CITED
Amman, G. D. and R. F. Schmitz. 1988. Mountain pine beetle-lodgepole pine interactions
and strategies for reducing tree losses. Ambio, 17:62-68.
Aukema, B. H., G. R. Richards, S. J. Krauth, and K. F. Raffa. 2004. Species assemblage
arriving at and emerging from trees colonized by Ips pini in the Great Lakes Region:
Partitioning by time since colonization, season and host species. Annals of the
Entomological Society of America, 97:117-129.
Berryman, A. A. 1976. Theoretical explanation of mountain pine beetle dynamics in
lodgepole pine forests. Environmental Entomology, 5:1225-1233.
Bright, D. E. 1976. The insects and arachnids of Canada Part 2: the bark beetles of Canada
and Alaska: Coleoptera, Scolytidae. Canada Department of Agriculture, Ottawa,
Ontario.
Bruno, J. F., J. J. Stachowicz, and M. D. Bertness. 2003. Inclusion of facilitation into
ecological theory. Trends Ecology and Evolution, 18:119-125.
Callaway, R. M. 1995. Positive interactions among plants. The Botanical Review, 61:306349.
Campbell, S. A. and J. H. Borden. 2006a. Close-range, in-flight integration of olfactory and
visual information by a host-seeking bark beetle. Entomologia Experimentalis et
Applicata, 120:91-98.
Campbell, S. A. and J. H. Borden. 2006b. Integration of visual and olfactory cues of hosts
and non-hosts by three bark beetles (Coleoptera: Scolytidae). Ecological Entomology,
31:437-449.
Carroll, A. L., B. H. Aukema, K. F. Raffa, G. D. Smith, and B. S. Lindgren. 2006. Mountain
pine beetle outbreak development: the endemic - incipient transition. Mountain Pine
Beetle Initiative, Project 1.03 Working Paper, Natural Resources Canada, Canadian
Forest Service. 21 p.
Doliner, L. H. and J. H. Borden. 1984. Pesterms: a glossary of forest pest management terms.
British Columbia Ministry of Forests, Pest Management Report #3. 34 pp.

5

Gara, R. I., D. R. Geiszler, and W. R. Littke. 1984. Primary attraction of the mountain pine
beetle to lodgepole pine in Oregon. Annals of the Entomological Society of America,
77:333-334.
Gotelli, N. J. 2001. A Primer of Ecology. 3rd edition. Sinauer Associations Inc., Sunderland,
Massachusetts.
Hynum, B. G. and A. A. Berryman. 1980. Dendroctonus ponderosae (Coleoptera:
Scolytidae): Pre-aggregation landing and gallery initiation on lodgepole pine. The
Canadian Entomologist, 112:185-191.
Kaplan, I. and R. F. Denno. 2007. Interspecific interactions in phytophagous insects
revisited: a quantitative assessment of competition theory. Ecology Letters, 10:977994.
Klein, W. H., D. L. Parker, and C. E. Jensen. 1978. Attack, emergence, and stand depletion
trends of the mountain pine beetle in a lodgepole pine stand during an outbreak.
Environmental Entomology, 7:732-737.
McNee, W. R., D. L. Wood, and A. J. Storer. 2000. Pre-emergence feeding in bark beetles
(Coleoptera: Scolytidae). Population Ecology, 29:495-501.
Miller, D. L., J. H. Borden, and B. S. Lindgren. 2005. Dose-dependent pheromone responses
of Ips pint, Orthotomicus latidens (Coleoptera: Scolytidae), and associates in stands
of lodgepole pine. Environmental Entomology, 34:591-597.
Moeck, H. A. and C. S. Simmons. 1991. Primary attraction of mountain pine beetle,
Dendroctonus ponderosae Hopk. (Coleoptera: Scolytidae), to bolts of lodgepole pine.
The Canadian Entomologist, 123:299-304.
Person, H. L. 1931. Theory in explanation of the selection of certain trees by the western pine
beetle. Journal of Forestry, 29:696-699.
Poland, T. M. and J. H. Borden. 1994. Semiochemical-based communication in interspecific
interactions between Ips pini (Say) and Pityogenes knechteli (Swaine) (Coleoptera:
Scolytidae). The Canadian Entomologist, 126:269-276.
Poland, T. M. and J. H. Borden. 1997. Attraction of a bark beetle predator, Thanasimus
undulatus (Coleoptera: Cleridae), to pheromones of the spruce beetle and two

6

secondary bark beetles (Coleoptera: Scolytidae). Journal of the Entomological
Society of British Columbia, 94:35-41.
Pureswaran, D. S. and J. H. Borden. 2003. Test of semiochemical mediated host specificity in
four species of tree killing bark beetles (Coleoptera: Scolytidae). Environmental
Entomology, 32:963-969.
Raffa, K. F. and A. A. Berryman. 1982. Gustatory cues in the orientation of Dendroctonus
ponderosae (Coleoptera: Scolytidae) to host trees. The Canadian Entomologist,
114:97-104.
Safranyik, L. 1988. Mountain pine beetle: Biology overview. Pages 9-13 in Symposium on
the Management of Lodgepole Pine to Minimize Losses to the Mountain Pine Beetle,
Kalispell, MT.
Safranyik, L. and A. L. Carroll. 2006. The biology and epidemiology of the mountain pine
beetle in lodgepole pine forests. Pages 3-66 in L. Safranyik and B. Wilson, editors.
The Mountain Pine Beetle: a Synthesis of its Biology, Management and Impacts on
Lodgepole Pine. Natural Resources Canada, Canadian Forest Service, Pacific
Forestry Centre, Victoria, BC.
Safranyik, L. and D. A. Linton. 1985. The relationship between density of emerged
Dendroctonus ponderosae (Coleoptera: Scolytidae) and density of exit holes in
lodgepole pine. The Canadian Entomologist, 117:267-275.
Safranyik, L., T. L. Shore, D. A. Linton, and L. Rankin. 1999. Effects of induced competitive
interactions with secondary bark beetle species on establishment and survival of
mountain pine beetle broods in lodgepole pine. Information Report BC-X-384,
Pacific Forestry Centre, Canadian Forest Service, Victoria, BC.
Safranyik, L., D. M. Shrimpton, and H. S. Whitney. 1974. Management of lodgepole pine to
reduce losses from the mountain pine beetle. Forestry Technical Report 1,
Environment Canada, Canadian Forest Service, Pacific Forestry Centre, Victoria, BC.
Savoie, A., J. H. Borden, J. Pierce, H.D., R. Gries, and G. Gries. 1998. Aggregation
pheromone of Pityogenes knechteli and semiochemical-based interactions with three
other bark beetles. Journal of Chemical Ecology, 24:321-337.
Schwerdtfeger, F. 1956. Scolytidae (Col.) an Pinus-Arten in Mittleamerika: I. Das genus Ips
De Geer. Zeitschrift fur Angewandte Entomologie, 39:34-57.
7

Stachowicz, J. J. 2001. Mutualism, facilitation, and the structure of ecological communities.
BioScience, 51:235-246.
Thomas, J. B. 1961. The life history of Ips pint (Say) (Coleoptera:
Canadian Entomologist, 93:384-390.

Scolytidae). The

Thomson, A. J. and D. M. Shrimpton. 1984. Weather associated with the start of mountain
pine beetle outbreaks. Canadian Journal of Forest Research, 14:255-258.
Trimble, F. M. 1924. Life history and habits of two Pacific coast bark beetles. Annals of the
Entomological Society of America, 17:382-391.
Wood, S. L., editor. 1982. The bark and ambrosia beetles of North and Central America
(Coleoptera: Scolytidae), a taxonomic monograph. Brigham Young University,
Provo, Utah.

8

CHAPTER 2

THE LIFE HISTORY OF A SECONDARY BARK BEETLE, PSEUDIPS
MEXICANUS, IN LODGEPOLE PINE

ABSTRACT
Pseudips mexicanus (Hopkins) is a secondary bark beetle native to western North and
Central America that attacks most species of Pinus within its range. A pair of life history
studies examined P. mexicanus in other host species, but until now, no work has been
conducted in lodgepole pine {Pinus contorta var. latifolia). P. mexicanus in lodgepole pine
was found to be polygynous. Gallery lengths were shorter, offspring smaller, and the number
of eggs laid per niche and the number of potential progeny fewer than in populations from
California and Guatemala. The size of each larval instar was corrected from previous work.
The development rate from the time of female attack to emerged adult offspring was less than
50 days at 26.5 °C and the heat accumulation required to complete the lifecycle was
determined to be 889.2 degree days above 8.5 °C, indicating that P. mexicanus in the
northern portion of its range is univoltine. Determination of these life history traits will
allow discussion of interactions between P. mexicanus and other bark beetles with which it
often cohabitates in weakened lodgepole pine.

Key words: sex ratio; development rate; voltinism; fecundity; Dyar's rule

9

INTRODUCTION
Numerous species of bark beetles (Coleoptera: Curculionidae: Scolytinae) are
phloeophagus, feeding subcortically on a wide range of tree species. Some specialize on
particular hosts, while others are generalists, often attacking all of the species within a genus
or across a number of genera (Wood 1982). The majority of bark beetles attack weakened
hosts, while some species successfully attack healthy trees and may erupt into large, often
economically devastating epidemics (Wood 1982). Host defences can be altered by a
number of intrinsic and extrinsic factors such as age, injury, pathogen infection and drought,
all of which influence a tree's suitability as a bark beetle host (Shrimpton 1973, Raffa and
Berryman 1983, Safranyik and Carroll 2006).
In Canada, different bark beetle species go through life cycles that range from bivoltine to
semivoltine, depending upon latitude and elevation (Bright 1976). They may overwinter in
the adult stage and emerge early the following summer or overwinter as juveniles and emerge
later in the summer (Wood 1982). Upon emergence, bark beetles locate hosts through a
combination of olfactory cues, visual recognition and gustatory sampling of host material
(Hynum and Berryman 1980, Moeck and Simmons 1991, Campbell and Borden 2006).
Depending on the species, either sex may be responsible for host location and initial attack.
Upon selection of a suitable host, the pioneering sex releases aggregation pheromones, which
usually attract both sexes (Wood 1982). In systems where the female initiates attack, she
usually mates with a single male that may stay the duration of the egg laying period or may
leave soon after copulation to mate with other females (monogyny) (Kirkendall 1983).
Scolytinae species in which the male attacks first are usually polygynous, where multiple
females join a single male (Kirkendall 1983).

10

Ovipositional gallery construction is conducted mostly in the phloem tissue, often with
some scoring of the sapwood. Galleries vary in length and orientation to the grain of the
wood, and may be curved or linear. Eggs are laid along the sides of the gallery or in egg
niches, singly or in groups (Wood 1982). An egg niche is a small notch carved into the side
of the ovipositional chamber in which eggs are laid and often packed in with frass (a mixture
of wood fragments and feces). Larvae feed either gregariously or solitarily away from the
ovipositional gallery, and pupation occurs in the phloem or in the outer bark (Wood 1982).
Many species feed as newly eclosed adults prior to emergence to reach sexual maturity, and
to build up resources required for dispersal and host selection (Wood 1982, McNee et al.
2000).
One of the approximately 50 species of bark beetles that use lodgepole pine {Pinus
contorta var. latifolia Engelm.) as a host (Bright 1976) in western Canada is Pseudips
mexicanus (Hopkins). P. mexicanus is native to the lodgepole pine ecosystem of central and
southern British Columbia, but its range extends northwest into Alaska and south along the
west coast of North and Central America into Guatemala (Bright 1976, Wood 1982). Typical
of most bark beetles (Wood 1982), it attacks unthrifty trees (Trimble 1924) at low densities,
and it has not been noted as an economic pest in Canada. P. mexicanus is often found
cohabiting trees with endemic or low populations of the mountain pine beetle, Dendroctonus
ponderosae Hopkins (Chapter 3), an eruptive species currently in outbreak across British
Columbia and parts of Alberta.

Potential interaction effects between P. mexicanus and

endemic D. ponderosae have not been investigated; however; to assess how two species
interact with each other, life history knowledge must be available for both.

11

D. ponderosae has been studied extensively, especially in the epidemic phase (Amman
and Cole 1983, Raffa and Berryman 1983, Safranyik et al. 1999, Safranyik and Carroll
2006).

In contrast, the life history of P. mexicanus has been studied by only a few

researchers. Trimble (1924) examined some aspects of the basic biology of P. mexicanus in
Monterey pine (Pinus radiata D. Don), e.g., attacking sex ratio, gallery construction, instar
head capsule sizes, the duration of development for each life stage, and the number of eggs
laid per egg niche. Schwerdtfeger (1956) examined Guatemalan specimens in Mexican white
pine {Pinus ayacahuite Ehrenb. ex Schltdl.), and determined the length of time required to
develop from egg to adult. A Forest Pest Leaflet by Struble (1961) marginally expanded on
Trimble's (1924) paper by commenting on predators, parasitoids and potential control
measures. Fox et al. (1991) examined the potential for P. mexicanus to vector pitch canker
fungus (Fusarium subglutenans (Wollenw. and Reink.) Nelson, Toussoun and Marasas) in
Monterey pine.
The objective of this study was to elucidate the life history characteristics of P.
mexicanus in lodgepole pine trees in British Columbia. A field survey was conducted to
examine attack and ovipositional characteristics, and a complementary laboratory study
examined the developmental characteristics of P. mexicanus, voltinism and the number of
larval instars. The determination of these life history traits will provide the necessary
knowledge to allow an examination of how P. mexicanus interacts with other species, such as
endemic D. ponderosae. Additionally, the study will ascertain whether the biology of P.
mexicanus in lodgepole pine trees differs from populations inhabiting other pine species
further south.

12

METHODS
Field study
To study the attack and ovipositional characteristics of P. mexicanus, infested trees were
located in 2005 by conducting a detailed survey of seven lodgepole pine stands totalling 28
ha. The stands were either pine dominant or co-dominant, located at Angstad Creek (49° 51'
N, 120° 46' W, mean elevation = 1300 m), 28 km south of Merritt, British Columbia. Pine
dominant stands are characterized by a pine component of > 50 % of tree species present, and
individual pine trees extending above the crowns of other species, while in pine co-dominant
stands the largest pines are of similar height as surrounding trees species (Avery and
Burkhart 2002). Diameter at breast height (DBH), phloem thickness and tree age were
collected by randomly placing one variable radius plot in each stand. A basal area factor 5
prism was used in all plots. For all pine trees, phloem thickness at DBH was recorded as the
mean of two measurements to 0.1 mm made on the bole at 90° from each other. Phloem was
removed using a 1.5 cm diameter punch.
P. mexicanus-infested trees were identified by the fine grained mixture of reddish boring
dust on the trunk at the base of the tree. Seventeen P. mexicanus-infested trees were located,
and their DBH, phloem thickness and age were determined along with an assessment of
damage (physical injury or infection) (Carroll et al. 2006). The infested trees were harvested
in September 2005, and the bark was peeled between 1.0 m and 1.5 m from the base to check
for the presence of P. mexicanus. As beetles were not noted above 1.0m, the bottom 1.0 m of
the bole of each tree was taken to the laboratory in Victoria BC. The ends of each log were
sealed with hot paraffin wax to reduce desiccation and then stored at 4 °C until they were
examined in detail. In March 2006 the logs were placed in individual rearing cages enclosed

13

in mesh in a greenhouse chamber kept at 20 °C, so the resident beetles could complete their
lifecycle. Emerging beetles were collected daily and placed in 70 % ethyl alcohol for
storage. P. mexicanus were sorted by sex using the large median tubercle present on the
male frons and its absence or reduction on the female frons (Wood 1982) as the identifying
characteristic. The pronotal width of each insect was measured to the nearest 0.1 mm using a
dissecting microscope equipped with an ocular micrometer.

Other beetle species (D.

ponderosae, Orthotomicus latidens (LeConte), Hylurgops porosus LeConte and H.
rugipennis (Mannerheim)) were also collected as they emerged and identified (Bright 1976).
After the emergence of beetles was complete (no emergence in any cage for > 5
consecutive days) the logs were peeled and the galleries of all species were inspected, but
only those of P. mexicanus were examined in detail. During peeling, the sites of attack were
examined for pitch exudation and evidence of necrotic lesions, the presence of which would
indicate a defensive response by the tree upon attack (Reid et al. 1967, Nebeker et al. 1993).
The galleries of other species were identified using descriptions in Bright (1976) and Wood
(1982). A total of 81 attacks on the 17 logs (mean = 7.0 ± 1.3 (SE) attacks/log) were
assessed, and the shape of the gallery system assessed. Gallery systems were categorized as
follows: 'amorphous', numerous gallery arms with no evidence of egg laying (Fig. 2.1 A); or
'ovipositional', at least one gallery arm present with egg niches (Fig. 2.IB). The presence of
a nuptial chamber, the number of ovipositional gallery arms extending from the nuptial
chamber, ovipositional gallery length, and density of egg niches were recorded for
ovipositional galleries only. The number of eggs laid per niche was determined by counting
the number of larval traces moving away from each of three randomly selected egg niches

14

Figure 2.1. (A) Amorphous gallery of Pseudips mexicanus typical of overwintering beetles. Note lack of egg niches and apparent
randomness in gallery length and direction. (B) Ovipositional gallery of Pseudips mexicanus. Note distinctive shape of the gallery
and the presence of egg niches on the outside of the curve. NC = nuptial chamber, EN = egg niche.

from 12 randomly selected galleries. Potential progeny was calculated by multiplying the
mean number of eggs laid per niche by the mean number of niches per gallery. The sex ratio
of attacking adults was not quantified exactly, as not all gallery systems had adults present
when the logs were dissected. However, the sex ratio of attacking adults was estimated by
assuming one female created each gallery arm as suggested by Trimble (1924). One male
was assumed present if a nuptial chamber was evident. The sex ratio of offspring was
determined by counting the number of each sex that had emerged.
Additional qualitative observations of gallery shape were made in early May 2005 on ten
trees mass attacked by D. ponderosae the previous year and subsequently attacked by P.
mexicanus in late 2004 (post-Z). ponderosae). Trees mass attacked by D. ponderosae were
identified by the red crown and pitch tubes on the bole (Safranyik and Carroll 2006). Fresh
boring dust was used to locate P. mexicanus attacks and the lower 1.0 m of the bole was
peeled in situ to reveal gallery systems. Four amorphous galleries with beetles present were
observed on three of these trees. A number of ovipositional galleries were also present but
details were not noted.

Laboratory study
Life history characteristics of P. mexicanus were determined by rearing insects in
lodgepole pine logs at different temperatures. The study was conducted in 2004 and repeated
in 2005. Source (parent) beetles used in the study were reared out of logs cut from infested
trees at Angstad Creek as described above or collected from Lindgren funnel traps (Lindgren
1983) baited with racemic ipsenol and ipsdienol (Phero Tech, Inc., Delta, BC) hung at
Angstad Creek and Aberdeen Plateau (50° 5' N, 119° 11 'W, elevation approximately 1300

16

m). Additional beetles were manually collected from the base of D. ponderosae-infested
trees at Angstad Creek. Beetles were maintained alive until needed in 120 ml capped vials
containing moist paper towel and were used within eight days of emergence or collection.
Two healthy lodgepole pine trees were felled early in the summer of 2004, and three trees
were felled in 2005 at Angstad Creek.

Each tree had a diameter at breast height of

approximately 20 cm. The lower three metres of the bole was removed and cut into 20 cm
long logs. The bark was scored vertically to the sapwood in a one centimetre wide strip on
opposite sides of the log to create two separate resource patches. Hot paraffin wax was
applied to all cuts to reduce desiccation. Each side of the scored log was manually infested
with P. mexicanus using the gelatine capsule technique (Lanier and Wood 1968). One male
was introduced into an entrance hole prepared in the centre of each side and left overnight
(Safranyik and Linton 1983). Beetles which did not initiate attack after 24 hours were
replaced. Once the male beetle had successfully entered the log, two females (Trimble 1924)
were placed in the gelatine capsule. Females that did not enter logs after 24 hours were
replaced. After successful beetle introduction, logs were placed in individual 20 L plastic
buckets, each ventilated by a 5 cm diameter hole covered with 0.25 mm polyethylene mesh,
and fitted with a 20 ml scintillation collecting vial.
Nine rearing buckets were placed into each of three walk-in growth chambers (Conviron
PGV36, Controlled Environments Ltd., Winnipeg, Manitoba, Canada) in 2004 and five
growth chambers in 2005. In 2004 the chambers were set at constant temperatures of 20.0,
23.5 and 28.8 °C and in 2005 at 14.1, 20.5, 23.4, 28.9 and 33.6 °C. Subtle variation in
chamber temperature was due to differences between individual chambers. Temperatures
were determined by placing a data logger ((HOBO® H8 Pro Temp, Onset Computer

17

Corporation, Pocasset, MA, USA) in each chamber for the period of the study and
temperatures were maintained at ± 1.5 °C with ambient humidity. A light: dark regime of
16:8 h was used to encourage emerging beetles to enter the collecting vials. Buckets were
checked every five days until the first beetle appeared in a collection vial, after which time
they were checked daily. Collected beetles were placed in 70 % ethyl alcohol.

Development rate. The developmental characteristics of P. mexicanus were assessed by
recording the amount of time it took P. mexicanus to develop from the time of introduction
of female beetles to the logs until emerging adults appeared in the manually infested pine
logs at different temperatures. From this information, the minimum threshold and optimum
temperature of development were determined and the development rate at each temperature
was calculated by taking the reciprocal of the mean length of time in days (Campbell et al.
1974). The mean length of time for development from infestation to adult was estimated by
counting the number of days between the time of beetle introduction to the logs and the first
day of emergence of greater than four beetles per temperature. The minimum temperature
was determined by extrapolation of a regression line. Optimum temperature was estimated
by finding the maximum of a quadratic curve (see Analysis below).

Voltinism.

Voltinism of P. mexicanus was determined by calculating the number of

degree days required by P. mexicanus to complete its lifecycle from infestation to adult and
comparing with temperature data from Environment Canada weather data (Environment
Canada 2007) and data loggers (see below). The minimum and the optimum temperature of

18

development were used in equation (1) to determine the number of degree days required for
P. mexicanus to fully mature and emerge:
dd = D(OT-MT)

(1)

where dd = number of degree days; D = number of days required for development from
infestation to adult; OT = optimum temperature of development (°C); MT = minimum
temperature of development (°C) (Snyder et al. 1999). The calculated number of degree days
was compared with the total number of available degree days over a period of one year at
two different locations. Environment Canada weather data were used from the Merritt STP
station (50° 6'N, 120° 48'W, elevation 588 m) between June 1, 2004 and May 31, 2005. A
second set of data were collected at Angstad Creek with HOBO temperature data loggers
from July 2003 to June 2004. The Merritt site was chosen as the closest permanent weather
station with current data. If the calculated number of degree days was less than the total
available number of degree days at a particular station then P. mexicanus can be described as
univoltine. If the calculated number of degree days was greater than the available number of
degree days, then P. mexicanus could not complete one generation per year (semivoltine). P.
mexicanus developing in fewer degree days than half of the available degree days at a
weather station would indicate a potential multivoltine population.

Instars. P. mexicanus larvae were collected from the manually infested logs to determine
the head capsule width of each instar. During both 2004 and 2005 one log per growth
chamber temperature was periodically chosen at random and dissected to assess
developmental progress. Larvae were collected from all peeled logs from the 23.5 °C
chamber in 2004 for head capsule measurement. This temperature was chosen to ensure that

19

all larval stages were present at the time of log dissection. At other temperatures, eggs may
not have hatched or larvae had progressed into a later life stage at the time of the dissections,
resulting in one or more missing instars. Head capsule widths and additionally, the number
of larval instars, were determined by examining 195 larvae (Prebble 1933).

Data analysis
Field study. Data not meeting assumptions of normality and homoscedasticity were
transformed by Y = logio (X + 1). The characteristics (DBH, phloem thickness and age) of
infested host trees were compared to healthy trees sampled in the prism plots using an
independent samples Mest. Gallery characteristics were measured in the naturally infested
trees and the means (± SE) reported. An independent samples Mest was used to detect
differences in pronotal widths between the males and females collected from the naturally
infested trees in the field. A log-likelihood ratio test (G-test) was performed to assess
divergence of observed sex ratios in wild populations of both attacking adults and emerged
offspring from the ratios reported in the literature.

Laboratory study. A regression analysis of development rate as a function of temperature
was done. The development rates calculated at six temperatures (not including temperatures
where beetle survival was zero) were used to estimate the optimum temperature of
development for P. mexicanus by determining the maximum point of the curvilinear model
fit to the data. Simple linear regression was used with rate and temperature to estimate the
lower threshold of development through extrapolation of the model to Y = 0.

Only

temperatures through which development rate increased approximately linearly, and

20

excluding temperatures higher than would be normally encountered by the insect in nature,
were included in this model (Gilbert and Raworth 1996). Larval head capsule widths were
grouped graphically by examining natural breaks in the data, and the rate of change between
instars was compared with previous research. All tests followed Sokal and Rohlf (1995) and
were conducted using SPSS 13.0 (SPSS 2004) with a = 0.05.

RESULTS AND DISCUSSION
Field study
Typical of Ipini bark beetles (Scolytinae), P. mexicanus focus their attack on suppressed,
weakened or recently killed trees (Wood 1982). Observations of the 17 trees harvested and
brought to the laboratory in 2005 revealed that the diameter at breast height was significantly
smaller (t = 3.70, df = 59, P < 0.001) and phloem thickness was significantly thinner (t =
4.03, df — 59, P < 0.001) for trees infested with P. mexicanus than for the healthy trees in the
prism plots. Age was not significantly different (t = -1.66, df - 59, P - 0.10) (Table 2.1) in
attacked versus unattacked trees, however. All trees with P. mexicanus attack had sustained
physical injury in the past (broken tops, bole scars) or were infected by lodgepole pine dwarf
mistletoe, Arceuthobium americanum Nutt. ex Engelm. (see Chapter 3). Inspection of the
pine logs revealed no evidence of host defence in the form of resinous pitch tubes or necrotic
lesions under the bark associated with P. mexicanus attack sites, confirming that the selected
hosts had weakened defences at the time of attack. Trees which are capable of resistance
should react to the presence of P. mexicanus by exudation of resin caused by physical trauma
to resin canals and the production of induced lesions in the phloem and xylem tissues, a
response to the inoculation of P. mexicanus fungal associates as seen in other bark beetle
systems (Berryman 1972, Shrimpton 1973, Paine et al. 1997). Dissection of the logs taken to

21

Table 2.1. Characteristics of lodgepole pine trees {Pinus contorta var. latifolia) assessed by
variable radius plot from seven stands at Angstad Creek, BC, and individual trees attacked
by Pseudips mexicanus.
Mean ± SE
Tree characteristic
P. mexicanus-infested trees (n = 17)
Stand (n = 42)
15.7 ±0.9 a
DBH (cm)
21.3±1.0b
Phloem thickness (mm)
1.4±0.1b
1.0±0.1a
Age (years)
111.5±2.3a
118.1 ± 1.9 a
Note: Within each row, means followed by the same letter are not significantly different
from one another (independent samples t-test, a = 0.05).

22

the laboratory, as well as observation in the field of the portion of the bole 1.0 m to 1.5 m on
each tree, revealed that P. mexicanus prefer the lower 1.0 m of suppressed lodgepole pine
trees, 1.0 m of suppressed lodgepole pine trees, as no attack was noted above that point.
Thus, P. mexicanus attack behaviour on lodgepole pine in British Columbia differs from that
of California beetles (Wood 1982), which in P. radiata were found thriving in slash (Trimble
1924) as well as in the boles, branches and cones of trees infected with pitch canker fungus
(Fox et al. 1990). Schwerdtfeger (1956) also found P. mexicanus in the bole, branches and
slash of Mexican white pine, P. ayacahuite. The variation in attack behaviour may be the
result of different trophic interactions, such as competition and symbiotic relationships, or
climatic factors within the different ecosystems. The propensity of P. mexicanus to attack
low on the bole in lodgepole pine may be an overwintering adaptation of northern
populations. Snow often covers the lower trunk, insulating it and possibly reducing risk of
cold-induced mortality.
P. mexicanus was found cohabiting with a number of other bark beetle species such as
endemic D. ponderosae, O. latidens, H. porosus and H. rugipennis. Most of these species,
except for O. latidens and endemic D. ponderosae, attacked at very low densities (< 2 attacks
per tree on average) and were only present in a few trees. The attack density of P. mexicanus
(Table 2.2) was lower than that of endemic D. ponderosae, which was present on
approximately 37 % of the trees at a mean density of 55 attacks/m2. O. latidens was present
on approximately 50 % of the trees at densities slightly greater than P. mexicanus. O.
latidens were normally found higher on the bole than P. mexicanus and appeared to prefer
more deteriorated phloem tissue (Miller and Borden 1985), while the Hylurgops species were

23

Table 2.2. Mean attack and ovipositional gallery characteristics (± SE) for wild populations
of Pseudips mexicanus reared from naturally attacked lodgepole pine trees.
Characteristic
Mean ± SE
Range
N
Attack density (attacks/m )
17.0 ±3.6
1.9- -46.8
17
Number of galleries per attack
1.5 ±0.06
1.0- -3.0
81
Gallery length (cm)
5.4 ±0.2
1.1--17.0
125
Number of egg niches per gallery
7.2 ± 0.4
1.0- -23.0
125
Number of egg niches per cm of gallery
1.4 ±0.07
0.2--5.6
125
Number of eggs per niche
3.5 ±0.2
36
1.0- -4.0
Potential progeny
25.2 ±0.1

24

found lower on the bole than P. mexicanus, at or below the root collar. Trees with endemic
D. ponderosae did not have O. latidens present, although O. latidens attacks may have been
present higher on the bole. D. ponderosae tended to attack slightly above, but never
overlapping, P. mexicanus gallery systems. It was not possible to determine which of the
secondary species arrived first at these trees, but it appeared that all were spring attacks of
the same year, while endemic D. ponderosae attacks followed P. mexicanus infestation.
Observation of gallery systems during the spring field surveys found four amorphous P.
mexicanus galleries in three trees mass attacked by D. ponderosae (Fig. 2.1 A). These types
of galleries have also been noted in weakened hosts, not attacked by D. ponderosae (A.
Carroll, unpubl. data). Each of these four galleries had four or more beetles, and none had
any evidence of egg laying at the time of observation. P. mexicanus has been found to fly in
low numbers throughout the summer, with a peak in late May and early June, and a smaller
peak in early to mid-August (A. Carroll, unpubl. data). Similar late summer flights occur in a
number of secondary species (Ayres et al. 2001). The purpose of this late flight is unclear,
but it may be used to establish a secondary gallery or to find suitable sites for overwintering
in hosts with available resources. Some bark beetle species, e.g., D. ponderosae, overwinter
under the bark, while others, e.g., Ips pini (Say), emerge and drop into the duff at the base of
their natal tree (Thomas 1961, Safranyik et al. 1996). P. mexicanus has not been found to
overwinter in the duff (Safranyik et al. 1999) and so likely overwinters under the bark.
Overwintering in a host with resource still available could be advantageous, as re-attack
rather than spring dispersal could occur, reducing mortality risk. Dispersal in search of new
hosts is very costly (Amman 1984), as beetles must be able to locate a suitable host, avoid
predation and attract mates. The opportunity to ameliorate these risks would be beneficial.

25

All of the 81 P. mexicanus gallery systems observed on the 17 naturally infested trees
brought to the laboratory were ovipositional (Fig. 2.IB). Nuptial chambers were associated
with 97.5 % of the ovipositional gallery systems, indicating that the majority of the time a
male initiated attack (Miller and Borden 1985). The galleries lacking a nuptial chamber
indicate the lack of a male and may have been started by a female which had been previously
mated in an earlier, but unsuitable gallery that she abandoned (Wood 1982). Alternatively,
mating may have occurred prior to leaving the natal tree. P. mexicanus has the opportunity
to engage in sibling mating prior to emergence, as, like many other scolytids (McNee et al.
2000), P. mexicanus undergo a period of pre-emergence feeding as callow adults, often in
groups. This phenomenon of sib-mating has been noted in a number of other bark beetle
species, but not in any Ipini (Kirkendall 1983), suggesting that sib-mating is somehow
avoided by P. mexicanus, as it is in /. pini (Domingue and Teale 2007). Therefore, solitary
galleries without nuptial chambers strongly suggest the presence of females that had mated
during the construction of previous galleries and re-emerged rather than sib-mated females
creating ovipositional galleries for the first time.
The ovipositional gallery in systems where only one gallery was associated with the
nuptial chamber were either straight or curved. Egg niches in straight galleries were on
either side, while niches in the curved galleries were on the outside of the curve (Fig. 2. IB).
In systems where two galleries were present, the galleries were curved away from each other
often in an ' S ' formation with the nuptial chamber at the centre and egg niches on the outside
of the curves as reported by both Trimble (1924) and Schwerdtfeger (1956). Food is often a
limiting factor for larvae (De Jong and Sabelis 1988). The shape of the gallery system when
two ovipositional galleries are present is likely an adaptation to reduce competition among

26

offspring and improve access to host material (Trimble 1924). The 'S' shape permits larvae
to fan out away from the ovipositional gallery without having to come into contact with
larvae from the other gallery.

Although the 'S' shape likely reduces intraspecific

interactions, it does not eliminate them. Cannibalism was observed in two cases during log
dissections where a larger larva altered the direction of its gallery. In both instances, these
larvae turned into neighbouring galleries and partially consumed the inhabitants prior to
moving on.
The mean number of individual ovipositional galleries associated with each nuptial
chamber (Table 2.2) was slightly less than that reported by Trimble (1924) and
Schwerdtfeger (1956), both of whom stated two galleries were the most common. The mean
of 1.5 galleries per attack that I found was significantly less (G = 9.47, df - 1, P < 0.05) and
suggests an attacking sex ratio of approximately 1.5 females per male, assuming one female
was present to create each gallery. Although females in some species have been observed
creating multiple galleries per nuptial chamber (e.g., O. latidens, Reid 1999, Miller and
Borden 1985), my observations could not verify this phenomenon, as parent adults were not
always present when logs were peeled.
P. mexicanus was categorized as a polygynous species by Trimble (1924) and
Schwerdtfeger (1956), but as monogynous by Bright and Stark (1973). Monogynous scolytid
species in which the male initiates gallery construction are rare, while polygyny is very
common within the Ipini, which includes P. mexicanus (Wood 1982, Kirkendall 1983). I
found that > 50% of the gallery systems examined on trees naturally attacked by P.
mexicanus had at least two radial arms, suggesting that this species is polygynous, but does
not appear to be obligate. This finding and results from a trapping experiment that found five

27

times more females than males being attracted to P. mexicanus attractant pheromone (unpubl.
data), correspond strongly to Kirkendall's (1983) definition of polygyny which states that: 1)
more than one female will join a male in a gallery system; and 2) males produce an
aggregation pheromone attractive to both sexes, but usually more attractive to females.
Additionally, males of polygynous species tend to be the larger sex (Kirkendall 1983), and P.
mexicanus males were on average 9.4% wider across the pronotum than females (Fig. 2.2).
Male P. mexicanus offspring in lodgepole pine were significantly larger than females {t^\ = 10.477, P < 0.001), with mean pronotal widths (± SE) of 1.7 ± 0.1 mm and 1.6 ± 0.1 mm
respectively (Fig. 2.2). Males ranged in pronotal width from 1.4 to 2.0 mm and females from
1.4 to 1.9 mm.
At a mean length of 5.4 cm, galleries in lodgepole pine were shorter (Table 2.2) than in
other species reported by Trimble (1924) and Schwerdtfeger (1956), where they averaged 6.5
cm and 9.0 cm, respectively. The mean number of egg niches per gallery in this study (Table
2.2) was less than half that reported by Schwerdtfeger (1956), who found a range of 16 to 20
niches per gallery. Egg niche density (number of egg niches per cm of gallery) was also
smaller than either Trimble (1924) or Schwerdtfeger (1956) reported with densities of 2.0
niches/cm and 3.5 niches/cm respectively. The mean number of eggs per egg niche was
again less than that found in the previous studies, where 4.0 was the norm (Trimble 1924,
Schwerdtfeger 1956).

The difference between the number of eggs laid per egg

28

1.8

a
CD

o
c
o

T

17

b
T

1.6 --

1.5

'

1

'

Male

1

Female

Figure 2.2. Mean pronotal widths (mm ± SE) of Pseudips mexicanus by sex reared from
naturally infested lodgepole pine trees. Different letters above bars indicate significant
differences (P < 0.05).

29

niche in my study and that reported by others may be a result of counting larval traces instead
of eggs. I may have introduced error and lowered the mean number of eggs per niche by not
accounting for occasional egg mortality prior to hatch.
The potential progeny produced per female was based on the mean number of eggs per
niche and the mean number of niches per gallery, giving approximately 25 eggs per female in
this study. Trimble (1924) reported 90 eggs laid per female, while Schwerdtfeger (1956)
found 64 to 80. The large differences may be the result of (i) warmer temperatures and (ii)
larger beetles in the southern populations. Amman (1972) found that D. ponderosae creating
ovipositional galleries at higher temperatures had greater rates of egg production. Beetles in
the south likely experience warmer temperatures, suggested by the higher egg niche density
found in Mexican white pine (Schwerdtfeger 1956) than in my study.

Beetle size

significantly affects the egg-laying capacity, with larger beetles able to lay more eggs
(McGhehey 1971). Larger size also confers a greater ability to disperse and locate new hosts,
and energetic requirements for the attacking sex are usually greater than for the sex
responding to aggregation pheromones (Pureswaran et al. 2006). Beetles from populations in
Mexican white pine were reported to be larger (Schwerdtfeger 1956) than the ones I found in
lodgepole pine, possibly contributing to the greater reproductive potential. Wood (1982)
reported that P. mexicanus tend to be 2.5 times longer than wide. Using this ratio and length
measurements, Wood's (1982) pronotal width range (no sex distinction) is 1.4 mm to 2.0
mm, almost identical to that of the males in my study. Similarly, Schwerdtfeger (1956)
reported a pronotal range, again with no sex distinction, of 1.6 mm to 2.2 mm, with a
minimum that was 10.6 % greater than the minimum I found for females, and a maximum

30

10.7 % greater than for males in lodgepole pine. The female size difference could account
for the greater egg production by southern beetles.
Many of the P. mexicanus galleries observed in lodgepole pine had stained phloem, but
only moderate staining of the underlying wood was noted. This indicates the presence of a
potentially phytopathogenic fungus associated with P. mexicanus, as suggested by Struble
(1961), who observed staining of the sapwood as a result of fungal inoculation by P.
mexicanus attacking Monterey pine trees. Numerous bark beetles have fungi associated with
them (reviewed in Six 2003). P. mexicanus pupal chambers were noted to contain white
fungal spores and it appears that P. mexicanus teneral adults consume these spores, as they
were not detected in chambers with teneral beetles present. Spore consumption is known to
take place in other species (Whitney 1971). The spores I observed are likely of the fungal
species associate, Leptographium terebrantis Barras & Perry (K. Bleiker, S. MassoumiAlamoutim, G. Smith, A. Carroll and C. Breuil 2007, unpubl. data), which has recently been
isolated from the exoskeleton of P. mexicanus freshly excised from naturally attacked
lodgepole pine. This fungus is associated with a number of other bark beetles species (Six et
al. 2003) and is highly phytopathogenic in pine trees (Eckhardt et al. 2004). In addition to
being detrimental to the host, evidence has been presented that some fungi confer a
nutritional benefit to the associated beetle species when individual teneral adults ingest the
spores (Ayres et al. 2000, Bleiker and Six 2007). The presence of this fungus may be more
beneficial to secondary bark beetles as a nutritional supplement than as an aid to reduce host
defences.
The sex of a total of 296 freshly emerged beetles was determined. The common sex ratio
of one female per male found in Ips species (Miller and Borden 1985) was used for

31

comparison purposes, as Ips are closely related to the small genus Pseudips (Cognato 2000).
The resulting sex ratio for emerging offspring of 1.6 females/male was significantly greater
than the expected ratio (G = 13.36, df = 1, P < 0.001). Cole (1973) suggests that beetles
breeding at low densities should produce offspring of a sex ratio close to 1:1. As crowding
increases, the smaller sex is reduced in number. Crowding is likely not occurring with P.
mexicanus as attack densities are quite low, but if crowding were a limiting factor, the sex
ratio should be biased toward the males, the larger sex (Safranyik 1976). While cold is
normally a factor in differential survival, the beetles in this study did not experience
temperatures below 0 °C, as the logs were collected in the fall prior to frost. It is possible
that the parent beetles are preferentially selecting female offspring when laying eggs,
however this behaviour is only common is species which inbreed (Borsa and Kjellberg 1996,
West et al. 2005), and I have suggested that sibling mating does not occur in P. mexicanus.
The difference between observed and expected sex ratios may be the result of using species
of a different genus as a standard for comparison.

Laboratory study
Development rate. In the 28.9 °C and 33.6 °C chambers all parent beetles died in 2005
prior to oviposition and consequently no data were collected. It is not completely clear why
the beetles in the 28.9 °C chamber died, when beetles in the 28.8 °C chamber from 2004
successfully produced numerous offspring. Temperature and humidity traces from the 28.9
°C chamber did not indicate any erratic behaviour of the chamber, but upon examination, the
parent beetles were found to be covered in an unidentified white fungus. One possible cause
of the rapid mortality of these beetles was the residual presence of an entomopathogenic

32

fungus from a previous study not eliminated during sterilization of the chamber. The 28.9 °C
data were not used in the models, but the 33.6 °C data were assumed to be accurate.
The development rates of insects tend to follow similar patterns of nearly linear increase
followed by a peak and then rapid decrease (Wagner et al. 1984, Logan and Powell 2001).
The apparent decrease in the rate of development for P. mexicanus at 28.8 °C (Fig. 2.3)
indicates that this species does not tolerate temperatures of this magnitude well in at least one
of its life stages. Beetles in the 33.6 °C environmental chamber may have died due to heat
stress or desiccation of the study logs, and it is assumed that this represents the rapid drop-off
to zero development that would be expected. A quadratic curve was fitted to the temperature
data that had beetles surviving from both 2004 and 2005.

The peak of the curve is the

optimum temperature at which development rate is highest (Trudgill et al. 2005). In my
study, this point was approximately 26.5 °C at a rate of 0.0198 d"1 (Fig. 2.3). By taking the
inverse of this rate, the optimum number of days required for P. mexicanus to develop from
infestation to adult was determined to be 50.5 days at a constant temperature of 26.5 °C. This
model (Fig. 2.3) appears to be a fairly good predictor for the species. Schwerdtfeger (1956)
found that P. mexicanus in Mexican white pine took a period of 12 to 14 weeks at 17 °C to
complete their life cycle. By solving the equation in Figure 2.3 for X = 17 °C and taking the
reciprocal, the model predicts it would take 11.4 weeks for complete development, which is
fairly close to Schwerdtfeger's (1956) results. Trimble (1924) reported development being
completed in nine weeks but did not state a temperature at which this occurred. Faster
development in northern climates has been shown for other species as an adaptation to
shorter developmental periods dictated by cooler summers (Bentz et al. 2001)

The

development rate for P. mexicanus within the range of temperatures normally encountered in

33

0.024
0.022
0.020
0.018

26.5 °C

0.016 H
£

0.014
0.012

r2adJ. = 0.939

0.010

F25 = 39.415, P= 0.007
0.008

Y = 0.0045X- (8.4867 x 10 5 )X 2 - 0.0394

0.006

— i —

12

14

16

18

20

— i —

22

24

— i —

— i —

—i

26

28

30

Temperature (°C)
Figure 2.3. Development rate (d 1 ) of Pseudips mexicanus from infestation to fully mature
adult versus temperature. Quadratic curve applied to determine optimum temperature of
development (arrow). Open circles indicate data collected in 2004; solid circles indicate data
collected in 2005. Error bars represent 1 SE of the mean.

34

the environment is shown in Figure 2.4. The minimum temperature of development, the
lowest point at which metabolic activity contributes to growth and development (Trudgill et
al. 2005), was estimated to be 8.5 °C. Assessment of beetles reared at 23.5 °C by periodic
peeling of logs showed that teneral adults were present after 45 days, but no emergence
occurred until midway through day 48. Based on this, it appears that P. mexicanus can
complete maturation feeding, commonly conducted by many bark beetles (Wood 1982), in
less than 4 days and be prepared to emerge.

Voltinism. Degree day calculation can be made for each separate immature life stage or
for the whole life cycle from infestation to emerging adult (Bentz et al. 1991, Bonhomme
2000, Logan and Powell 2001). In my study, development from the time of infestation to
adult was modelled. From equation (1), the number of degree days required for complete
development at 26.5 °C was calculated to be 889.2 degree days above 8.5 °C. At the Merritt
STP weather station, 1662 degree days were available between June 1, 2004 and May 31,
2005. At higher elevation within the Angstad Creek watershed a minimum of 982 degree
days were available between July 2003 and June 2004 for P. mexicanus development. The
number of degree days required for one generation of P. mexicanus corresponds closely with
the number of degree days available at Angstad creek, indicating populations there are
univoltine. In the arid valley near Merritt, P. mexicanus may have 1.5 generations per year,
but not two; however, at that site, Pinus ponderosa P. Laws. Ex C. Laws, is the dominant
cover type.
Univoltine and multivoltine lifecycles may benefit P. mexicanus in both northern and
southern populations as they allow for a more rapid population growth compared to

35

0.024
0.022
0.020
0.018
0.016
0.014
0.012 -\
0.010
0.008
0.006
0.004

r2adj. = 0- 931

8.5 °C

1

0.002
0.000

F,>4 = 54.685, P = 0.005
Y = 0.0013X-0.0110

-1—'

1—

8

10

12

I

i

i

i

i

i

i

14

16

18

20

22

24

26

Temperature (°C)
Figure 2.4. Minimum temperature of development (arrow) extrapolated from regression of
temperatures within normally encountered conditions oiPseudips mexicanus. Open circles
indicate data collected in 2004; solid circles indicate data collected in 2005. Error bars
represent 1 SE of the mean.

36

semivoltine populations, and the shorter development time may reduce the potential effect of
mortality agents, e.g., cold and predation, which may manifest themselves more during a
longer developmental period (Hansen and Bentz 2003).

The rapidity with which P.

mexicanus can progress from infestation to adult at high temperatures suggests that it is well
adapted to produce more than one generation in regions which have longer summers, as
shown by Struble (1961), where three generations are common in California. Although
degree day estimations are often calculated for pest insect species to establish timing of
control measures to coincide with susceptible life stages, P. mexicanus has not been the
subject of any such practices. However, it is important to know the number of generations
that P. mexicanus is capable of producing per year so evaluation of interactions with other
species can be made (see Chapter 3).

Instars.
The head capsule widths of P. mexicanus larvae were reported by Trimble (1924), but it
appears an error occurred either in his measurements or during the printing of his findings,
where the same head capsule width is listed for both 3rd and 4th instars (Table 2.3). I
confirmed that P. mexicanus does have four instars (Fig. 2.5) and report the correct head
capsule width for each (Table 2.3). Another indication of the possible error in Trimble's
(1924) data was found by comparing the rate of change from one instar to the next between
our data and those of Trimble (1924). Out data revealed a linear progression as was expected
(Dyar 1890), while Trimble's data were non-linear (Table 2.3), not following the regular
geometric progression described by Dyar (1890). Dyar (1890) found that for Lepidoptera,
the ratio of the head capsule width of any instar to that of the next instar should be a constant

37

4th

40 -A

30 -\

5*
O

3rd

c
0

3
cr

1st

20

2nd

10 H

I
0.30

0.40

0.50

0.60

0.70

0.80

0.90

1.00

1.10

Head capsule width (mm)
Figure 2.5. Histogram of Pseudips mexicanus larval head capsule widths from logs
incubated at 23.5 °C indicating four distinct instars, n = 195.

38

Table 2.3. Comparison of mean head capsule (HC) widths, range (mm ± SE) and rate of
change (ROC) between instars oiPseudips mexicanus from manually infested lodgepole pine
logs (Smith) with those reported by Trimble (1924). Trimble did not report standard error or
standard deviation.
Study
Trimble 1924
Smith
Instar
HC width
HC width
ROC
ROC
HC width range
0.40
1
0.38 ±0.003
0.37 - 0.40
0.90
2
0.44
0.50 ± 0.006
0.75
0.43-0.57
1.20
3
0.75
0.68 ± 0.005
0.75
0.60-0.78
1.20
4
1.00
0.93 ± 0.004
0.73
0.83-1.04

39

and therefore the increments in size between instars should closely resemble a straight line
(Ghent 1956). Prebble (1933) and Reid (1962) showed that this rule was applicable for the
bark beetles Dendroctonus simplex LeConte and D. ponderosae, respectively. The rate of
change in these two studies was approximately 0.75, similar to our findings for P. mexicanus
(Table 2.3).
In lodgepole pine forests, P. mexicanus differ from southern populations in a number of
ways. Northern beetles may produce between 1.0 and 1.5 generations per year, unlike
populations in California which can have as many as three generations per year (Struble
1961). Attack behaviour differs as well, as northern populations overwinter in the lower
portion of the bole, which may be an adaptation to use snow cover to gain protection from
lethal cold temperatures. Southern beetles tend to make use of the whole tree plus downed
material.

Similarities across the populations in terms of polygyny, ovipositional

characteristics (i.e. # of eggs per egg niche) and development rate are apparent. In Canada,
P. mexicanus is a common forest insect that is not an economic threat to the forestry industry.
Populations of this species do not increase to levels where live, healthy trees are attacked,
unlike I. pini which can cause mortality when conditions are optimal (Thomas 1961). P.
mexicanus has, however; been a concern in Monterey pine because of its ability to
disseminate the pitch canker fungus in California (Fox et al. 1991). Moribund trees are
typical hosts in Canada, and through removal of low-quality material, aided by other bark
beetle species, P. mexicanus is likely beneficial by maintaining stand health. Determination
of attack dynamics, development rate and voltinism will facilitate future discussions
examining interactions between P. mexicanus and other species such as endemic D.
ponderosae.

40

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Sokal, R. R. and F. J. Rohlf. 1995. Biometry. 3rd edition. W.H. Freeman and Company, San
Francisco.
SPSS. 2004. SPSS 13.0 for Windows Graduate Student Version. SPSS Inc., Chicago, IL.
Struble, G. R. 1961. Monterey pine Ips. United States Department of Agriculture, Forest
Service, Forest Pest Leaflet 56.
Thomas, J. B. 1961. The life history of Ips pini (Say) (Coleoptera:
Canadian Entomologist, 93:384-390.

Scolytidae). The

Trimble, F. M. 1924. Life history and habits of two Pacific coast bark beetles. Annals of the
Entomological Society of America, 17:382-391.
Trudgill, D. L., A. Honek, D. Li, and N. M. Van Straalen. 2005. Thermal time - concepts and
utility. Annals of Applied Biology, 146:1-14.
Wagner, T. L., H.-I. Wu, P. J. H. Sharpe, R. M. Schoolfield, and R. N. Coulson. 1984.
Modeling insect development rates: a literature review and application of a
biophysical model. Annals of the Entomological Society of America, 77:208-225.
West, S.A., Shuker, D.M. and Sheldon, B.C. 2005. Sex-ratio adjustment when relatives
interact: A test of constraints on adaptation. Evolution, 59: 1211-1228.

46

Whitney, H. S. 1971. Association of Dendroctonus ponderosae (Coleoptera: Scolytidae) with
blue stain fungi and yeasts during brood development in lodgepole pine. The
Canadian Entomologist, 103:1495-1503.
Wood, S. L., editor. 1982. The bark and ambrosia beetles of North and Central America
(Coleoptera: Scolytidae), a taxonomic monograph. Brigham Young University,
Provo, Utah.

47

CHAPTER 3

AN EXAMINATION OF THE INTERACTION BETWEEN A SECONDARY
BARK BEETLE, PSEUDIPS MEXICANUS AND ENDEMIC DENDROCTONUS
PONDEROSAE IN WEAKENED LODGEPOLE PINE TREES.

ABSTRACT
Endemic populations of the bark beetle Dendroctonus ponderosae attack weakened
lodgepole pine trees that are often previously infested by other bark beetle species, such as
Pseudips mexicanus. Interactions between these two species were assessed by examining
attack dynamics and attributes associated with brood production of D. ponderosae in trees
containing P. mexicanus and trees attacked solely by D. ponderosae. My findings suggest
that D. ponderosae attacking ephemeral hosts previously occupied by P. mexicanus do so in
greater densities and offspring emerge earlier when compared to hosts D. ponderosae
attacked alone. Additionally, D. ponderosae larvae in P. mexicanus-infested trees were
found to require significantly less resource to complete development. The presence of P.
mexicanus may affect internal host chemistry, improving the subcortical environment for
endemic D. ponderosae, and hosts in this condition should be preferentially attacked.

Key words: facilitation; interspecific competition; host selection; phenology

48

INTRODUCTION
Interactions between species sharing a common resource are often characterized as
competitive (Bruno et al. 2003). All such interactions are not negative, however. A great
deal of work has been conducted in the past fifteen years investigating positive interactions
or facilitation, especially in plant (Callaway 1995, Callaway and Walker 1997) and inter-tidal
communities (Stachowicz 2001, Bruno et al. 2003). Facilitation is defined as an interaction
where at least one species benefits from the interaction while none are harmed (Bruno et al.
2003). In the bark beetles (Coleoptera: Curculionidae: Scolytinae), a number of studies have
examined and characterized competition occurring between cohabiting beetles, (Rankin and
Borden 1991, Schlyter and Anderbrant 1993, Poland and Borden 1994, Gara et al. 1995,
Poland and Borden 1998, Safranyik et al. 1999, Miller and Borden 2000, Hedgren 2004). As
yet, only a small number of papers have suggested facilitation occurring between bark beetle
species (Amman and Schmitz 1988, Carroll et al. 2006a).
Most bark beetles attack woody plants, but less than half of the families described are
phloeophagous, i.e., they consume and breed in phloem (Wood 1982). Phloeophagus bark
beetles bore through the bark and create ovipositional galleries in the phloem region of host
trees. Many of them carry fungi, which have been suggested to aid in the killing of the host
and provide nutritional benefits (Paine et al. 1997, Bleiker and Six 2007). While feeding on
the phloem, bark beetle larvae may ingest or come into contact with fungal spores prior to or
after pupation; this facilitates transport of the fungi to new host trees.
Bark beetles are integral to healthy forest ecosystems, contributing to the degradation of
senescent or recently dead woody plant material and are often categorized by their host
preference. 'Primary' beetles are the first species to arrive at and successfully infest healthy

49

living hosts (Doliner and Borden 1984) but may have to subsist in weakened trees when
populations are low. Primary beetles have the ability to increase rapidly in numbers during
periods of host stress and to become severe pests (Wood 1982, Safranyik and Carroll 2006).
The majority of bark beetles are termed 'secondary' (Wood 1982) and attack weakened or
dying trees almost exclusively (Doliner and Borden 1984), with some species able to increase
in numbers for short periods, allowing attacks on living trees (Thomas 1961).
Populations of primary beetles fluctuate through a number of stages (Carroll et al. 2006a,
Safranyik and Carroll 2006). In the endemic stage beetle populations are very low. Eruptive
beetle species are often present within a forest as innocuous endemic populations. During
this period of low numbers, beetles are unable to aggregate and overcome the defences of
vigorous hosts (Wallin and Raffa 2004, Safranyik and Carroll 2006). Aggregation is an
important mechanism for gaining access to a healthy tree. For example, Raffa and Berryman
(1983) found that the mountain pine beetle needed at least 62 attacks per m2 to reduce host
defensive chemicals enough to ensure beetle survival upon entry under the bark.
Consequently, when beetle numbers are below this critical value, i.e., during the endemic
population phase, mountain pine beetles are forced to subsist on weakened trees, where they
have to interact with a number of secondary bark beetle species (Carroll et al. 2006a).
As forest conditions change (aging hosts, drought), the number of vigour-impaired trees
increase and larger hosts become accessible (Safranyik and Carroll 2006). When this occurs,
populations increase to the endemic-incipient stage (Carroll et al. 2006a). Small groups of
large diameter trees are infested while interactions with other bark beetle species decrease as
increasing numbers of the eruptive species engage in 'primary' attack behaviour (Wallin and
Raffa 2004). As populations continue to increase the epidemic stage is reached, during

50

which large healthy trees can be colonized (Carroll et al. 2006a). Throughout an epidemic,
eruptive species attacking living trees are the only ones able to utilize the well-defended
hosts, and interactions with other species are relatively minimal. The epidemic stage lasts
until most large host trees with thick phloem have been killed.

Beyond this point,

populations begin to decline into the post-epidemic or collapse stage, characterized by
increased interspecific interactions with other bark beetle species and reliance upon
weakened hosts again (Safranyik and Carroll 2006).
The mountain pine beetle, Dendroctonus ponderosae Hopkins, is found in lodgepole pine
forests throughout the southern half of the province of British Columbia and the northwestern United States, but is a major pest of pine trees across its range. A great deal of
research has examined the epidemic population phase of this species (Safranyik et al. 1974,
Berryman 1976, Klein et al. 1978, Thomson and Shrimpton 1984, Safranyik and Linton
1985, Safranyik 1988, Safranyik et al. 1999) and many management strategies have been
suggested (McMullen et al. 1986, Raffa and Berryman 1986, Amman et al. 1989, Lindgren et
al. 1989, Lindgren and Borden 1993, Anonymous 2001, Carroll et al. 2006b). The endemic
phase on the other hand, has been examined very little (Carroll et al. 2006a). Because of this,
there is a knowledge gap in mountain pine beetle population dynamics, specifically, the
effect that the presence of other bark beetle species has on endemic D. ponderosae and how
this influences population growth. Previous work by Amman and Schmitz (1988) and more
recently by Carroll et al. (2006a) suggests that the presence of secondary bark beetles in trees
attacked by endemic D. ponderosae may assist in population maintenance and possibly aid in
population increases.

51

Endemic D. ponderosae populations tend to be too small to allow for successful attack on
healthy hosts, leaving them confined to weakened trees often previously infested by other
bark beetles (Carroll et al. 2006a). In lodgepole pine ecosystems, over 50 bark beetle species
make use of various parts of the tree, the majority of which attack vigour-impaired hosts
(Bright 1976). Carroll et al. (2006a) found that endemic D. ponderosae were directly
associated with nine species of secondary bark beetles on the lower bole. The most common
of those beetles was Pseudips mexicanus (Hopkins).

This beetle is native to British

Columbia and is found sympatrically with D. ponderosae (Bright 1976). P. mexicanus has
one generation per year in Canada and overwinters either under the bark in the same host in
which it developed or in a new host (Chapter 2). They attack early in the summer, unlike D,
ponderosae, which attack in late July to early August (Safranyik and Jahren 1970). Because
P. mexicanus attacks earlier than D. ponderosae, their broods have time to mature to
adulthood before D. ponderosae emerge for their annual dispersal. This provides a partial
temporal separation between the two species, where larval-larval interaction does not take
place, reducing the likelihood of interference competition.
The trees suitable for P. mexicanus and endemic D, ponderosae are an ephemeral
resource and likely require the beetles to search for long periods (Hanski 1987). Carroll et al.
(2006a) found that in the endemic phase less than two trees per hectare were infested by D.
ponderosae and up to five or six per hectare in the endemic-incipient phase. P. mexicanusinfested trees were more common, with up to 15 trees attacked per hectare. In a hypothetical
stand stocked with 1000 stems, less than two percent of the total number of trees would be
infested by either species, making both relatively rare on the landscape during the endemic
phase of D. ponderosae. In the study by Carroll et al. (2006a), an examination of the trees

52

attacked by endemic D. ponderosae showed that approximately 65 % of them had been
previously infested by secondary bark beetles.
Some eruptive bark beetles, such as Dendroctonus rufipennis Kirby, survive in endemic
phases by exploiting ephemeral habitats (Lewis and Lindgren 2002, Wallin and Raffa 2004),
where populations are sustained in weakened material.

Mountain pine beetles likely

maintain populations in a comparable fashion, even though suppressed trees often have thin
phloem, which provides a sub-optimal resource for the insects. Brood production in these
types of hosts is low, which prevents build-up of a large enough population to facilitate
aggregation and successful attacks on larger, healthier trees (Amman 1972a).
Interaction between two species of bark beetles can have a number of consequences for
each. Although species often partition resources when they cohabitate, competition often
takes place where niche overlap occurs (Ayres et al. 2001). Competition can be exploitative
or interference, reducing the reproductive capacity of both species (Wood 1982, Light et al.
1983). Poland and Borden (1994) showed that Ips pini Say and Pityogenes knechteli Swaine
both had reduced numbers of progeny in trees naturally attacked as a result of exploitation of
the phloem resource.
As P. mexicanus was found to be the most common secondary beetle in the study by
Carroll et al. (2006a), it was used to assess the potential effects of the presence of secondary
bark beetles on endemic D. ponderosae.

Facilitation by secondary beetles has already been

suggested (Carroll et al. 2006a), however; it is unclear what role P. mexicanus may be
playing. To determine what effect the presence of P. mexicanus in weakened lodgepole pine
hosts has on endemic D. ponderosae, their interaction was examined in naturally infested
trees. It was predicted that if P. mexicanus is competing with D. ponderosae, D. ponderosae

53

would have lower attack densities in P. mexicanus-infested trees than in trees D. ponderosae
attacked alone.

Additionally, D. ponderosae females should produce fewer and smaller

offspring as a result of P. mexicanus resource utilization forcing D. ponderosae into poorer
quality sections of the host with reduced overall quantity of resource available when
compared to broods from trees D. ponderosae attacked alone. However, if P. mexicanus is
facilitating D. ponderosae, then D. ponderosae should attack P. mexicanus-infested trees in
greater numbers and D. ponderosae offspring should be more numerous and larger than in
trees attacked by D. ponderosae alone. Regardless of the direction of the interaction, the
results of this study will improve understanding of D. ponderosae population dynamics in the
endemic phase.

METHODS
Study area.
The field site was located near Angstad Creek, 28 km south of Merritt BC (49° 51' N,
120° 46' W, mean elevation = 1300 (± 200) m). Trees were selected from 28 ha (seven 4 ha
stands) of forest where lodgepole pine was either the dominant or co-dominant species. Pine
dominant stands are characterized by the pine component being greater than 50 % of the
species present and individual pine trees extending above the crowns of other species, while
in pine co-dominant stands these are of similar height to surrounding tree species (Avery and
Burkhart 2002). Tree characteristics were collected by randomly placing one variable radius
plot in each stand. A BAF 5 prism was used in all plots. The level of beetle infestation was
endemic to endemic/incipient where beetle numbers were too low to overcome large

54

diameter, healthy trees (Safranyik and Carroll 2006). The study ran from May 2005 - March
2006.

Tree selection.
Study trees were a minimum of 50 metres from any other D. ponderos ae-attacked tree
and were attacked in 2005. Because of the rarity of such trees (Carroll et al. 2006a), only
twelve trees were found, seven with both D. ponderosae and P. mexicanus present and five
with D. ponderosae only. These will be referred to as the 'infestation types' throughout the
chapter. Identification and determination of the presence of each attacking species was
accomplished by assessing each pine tree in the 28 ha study area. The initial assessment in
June 2005 identified trees with new P. mexicanus attacks by observing boring dust collected
at the base of the tree. Stands were reassessed in August and D. ponderosae attacks were
identified in a similar manner, along with any new P. mexicanus attacks occurring since the
first assessment. Pitch tubes, indicators of D. ponderosae attack stimulating host resistance,
were not noted on any of the trees, suggesting a diminished host defensive capacity (Amman
1984), which is typical in endemic D. ponderosae attacks.
Diameter at breast height (1.3 m) (DBH), phloem thickness, age, ten-year mean annual
increment of growth and physical damage or pathogen infestation were recorded from each
infested tree. Phloem thickness at DBH was recorded as the mean of two measurements, 90°
from each other. Phloem was removed using a 1.5 cm diameter punch. Assessment of
injuries from physical damage or pathogens was made following the methods of Carroll et al.
(2006a). Trees were felled in late September and the bottom 1 m of each was transported to
the Pacific Forestry Centre, Victoria, BC, where they were placed in individual rearing cages.

55

All cuts made to the logs were sealed with paraffin wax to prevent desiccation of the phloem
and sapwood. Rearing of all beetles took place in a greenhouse room maintained at an
ambient air temperature of 20 °C.

Beetle rearing and gallery examination.
Emerging beetles were collected beginning in January 2006, placed in labelled vials, and
frozen until later examination. After emergence was complete (determined as no emergence
for a period of 7 days for all logs), the logs were placed in an autoclave at 120 °C and 2
atmospheres of pressure for an hour to soften the bark, which was then peeled off for
examination of galleries (Carroll et al. 2006a). Remaining adult beetles under the bark were
collected. Logs were assessed and attributes such as attack density (number of attacks per
m2), interspecific gallery overlap (percent of total gallery systems overlapping systems of
other species) and percent phloem-use were recorded for both species. Phloem-use was
defined as any portion of the phloem consumed or discoloured as a result of proximate
occupation by a beetle or its associated fungal growth. Surface area of the outer face of the
phloem was calculated using a conic frustum formula with radius equal to the radius of the
sapwood plus the phloem thickness. Phloem volume was calculated by subtracting the
surface area of the sapwood from the surface area of the outer phloem, multiplying by the
phloem thickness and the percent use.
D. ponderosae were counted, and their sex and size determined. Sex of mountain pine
beetles was established by looking for the male stridulatory apparatus on the 7th abdominal
tergite (Lyon 1958). Since P. mexicanus had mostly emerged by the time the trees were cut,
the sex and size of these beetles was not determined. The width of D. ponderosae female

56

pronotums was measured as an indicator of size to the nearest 0.1 mm using an ocular
micrometer (Haack et al. 1984). Only female D. ponderosae were examined, as they are the
attacking and egg-laying sex. Where more than 100 mountain pine beetles had emerged, at
least 40 females were sub-sampled using methods of Campbell and Borden (2006a). Unless
otherwise indicated, all values are reported as mean ± SE (one standard error).

Analysis.
Transformation of data by Y = sqrt(X + 0.01) was done as needed to address assumptions of
normality and homoscedasticity. Tree characteristics were compared between trees in both
infestation types and healthy trees from the prism plots using analysis of variance (ANOVA)
and a Ryan-Einot-Gabriel-Welsch F-test (REGW) for multiple comparisons. DBH and 10
year mean annual increment (MAI) data were transformed as described above. Regression
analyses were used to test the effects of D. ponderosae attack density on (i) D. ponderosae
ovipositional gallery length, (ii) phloem volume used per D. ponderosae attack, (iii)
proportion of phloem used by D. ponderosae per log and (iv) D. ponderosae offspring mean
pronotal widths. Regression was also used to test the effects of (i) the total number of D.
ponderosae offspring per log on the phloem volume used per D. ponderosae offspring, (ii) P.
mexicanus phloem volume use on the number of D. ponderosae female offspring produced
per m and on D. ponderosae offspring mean pronotal widths, (iii) P. mexicanus attack
density on P. mexicanus ovipositional gallery length and (iv) the mean number of Julian days
after the emergence of the first D. ponderosae brood adult per infestation type on the
cumulative percent D. ponderosae emergence. Curvilinear and non-linear regressions were
used only when they explained significantly (P < 0.05) more of the variation than simple

57

linear regressions. Differences in sex ratios of D. ponderosae offspring were compared to an
expected value using a log-likelihood ratio test (G-test). Independent samples t-tests were
used to compare variables between infestation types. All tests were referenced from Sokal
and Rohlf (1995) and statistical analyses were conducted with SPSS 13.0 (SPSS 2004) and
SigmaPlot 9.01 (Systat 2004).

RESULTS
Tree characteristics.
Descriptions of study-tree characteristics and stand tree measurements are shown in
Table 3.1. All of these trees were alive and had green crowns at the time of attack, but were
stressed in some manner. Damage assessment by visual examination (Carroll et al. 2006a)
revealed that all trees had at least one putative vigour-impairing injury (forked tops, broken
tops, scars) and all had evidence of lodgepole pine dwarf mistletoe,
americanum Nutt. ex Engelm., infestation in the branches.

Arceuthobium

Additionally, all trees were

suppressed or had crowns which did not extend into the main canopy.
The mean DBH of trees attacked by D. ponderosae either with P. mexicanus or alone was
significantly smaller than the mean stand DBH (Table 3.1, F2,s\ = 4.17, P = 0.02). Amman
and Pace (1976) characterized different phloem thickness classes and defined 'thin phloem'
as having a mean of 1.8 ± 0.3 (SD) mm, which was greater than the mean phloem thickness
of all trees examined in this study, including the healthy ones. A comparison of mean
phloem thicknesses of trees infested by both D. ponderosae and P. mexicanus, trees infested
by D. ponderosae alone and uninfested trees examined during prism plot data collection

58

Table 3.1. Characteristics of lodgepole pine trees attacked by Dendroctonus ponderosae (D.
ponderosae) alone (D. ponderosae only) and trees attacked by D. ponderosae after Pseudips
mexicanus infestation (D. ponderosae and P. mexicanus). Also included are tree
characteristics of lodgepole pine from seven stands (Stand). These data were used to assess
the interaction between endemic D. ponderosae and P. mexicanus.
Tree characteristic
Mean ± SE
D.
ponderosae
and P.
D. ponderosae
Stand (n = 42)
mexicanus (n = 7)
only (n = 5)
DBH (cm)
14.8 ±1.2 a
16.2 ±0.8 a
21.3± 1.0b
Phloem thickness (mm)
1.5 ±0.2 a
1.4±0.1 a
1.4±0.1a
Age (years)
105.6 ±4.6 a
118.0±3.3a
111.5±2.3a
10 year MAI (mm)
0.20 ± 0.05 a
0.20 ± 0.04 a
0.45 ± 0.04 b
Note: Within each row, means followed by the same letter are not significantly different
from one another (REGW multiple range test, a = 0.05).

59

was not significant (Table 3.1, jf<2,5i = 0.16, P = 0.85).
different between D. ponderosae-only

Tree age was not significantly

attacked trees, D. ponderosae with P. mexicanus-

attacked trees and stand data (Table 3.1, i ^ i = 1.24, P = 0.30). Ten-year MAI were small
for all attacked trees, indicating senescence and prolonged stress and did not differ between
P. mexicanus and D. ponderos ae-attacked trees and D. ponderosae-only

attacked trees.

However, when compared to stand MAI, trees in both infestation types had growth reduced
by over half (Table 3.1, F2,s\ - 4.06, P = 0.02). All attacked trees had evidence of staining in
the phloem and sapwood resulting from ophiostomatoid fungal infection.

Attack density and gallery length.
The mean D. ponderosae attack density was significantly greater (approximately four
times) in trees attacked previously by P. mexicanus than in trees attacked by D. ponderosae
alone (Table 3.2, t = -3.88, df = 10, P < 0.05). P. mexicanus attack density tended to be
lower than 17 attacks/m2, with a mean of 5.9 ±2.1 attacks/m2. Comparison of D. ponderosae
ovipositional gallery lengths between infestation types revealed no significant difference (t =
0.32, df = 10, P = 0.76). Gallery lengths were expected to decrease as D. ponderosae attack
density increased (Coulson et al. 1976), however no significant relationship was found with
simple linear regression (N= 12, r adj. = 0.16, P - 0.18).

Phloem use.
P. mexicanus attack density exhibited a strong linear relationship with proportion of P.
mexicanus phloem use (Fig. 3.1, N = 7, r2^, = 0.62, P = 0.02) as expected. P. mexicanus

60

ON

Table 3.2. Dendroctonus ponderosae (D. ponderosae) and P. mexicanus attack characteristic descriptions from trees attacked by D.
ponderosae alone (D. ponderosae only) and by D. ponderosae after P. mexicanus infestation (D. ponderosae and P. mexicanus). Mean
±SE.
# D. ponderosae
P. mexicanus
D. ponderosae
D. ponderosae
Attacking
D. ponderosae
P. mexicanus
female offspring
phloem
phloem volume
gallery length
species
attacks/m
attacks/m
volume (%)
produced per
(%) used per log
(cm)/attack
used per log
attack
D. ponderosae
14.0 ±4.0 a
n/a
20.1 ±2.2 a
n/a
37.6 ±8.0 a
4.8 ±1.6 a
only
D. ponderosae
55.1 ± 8.4 b
5.9 ±2.1
19.4±l.la
5.0 ±1.4
68.6 ± 2.4 b
and P.
7.0 ± 1.4 a
mexicanus
Note: Within each column, means followed by the same letter are not significantly different from one another (independent samples ttest, a = 0.05).

0.12

r2
' adj.

= 0.620
=

F1 6 = 10.804, P= 0.022
>,

0.10

J2
•o
3

y = 0.018+ 0.005X
C 0.08

Eno

fl) -ft
Q.

c

0.06 -

*• !5

a0o
v.
0.

0.02

0.00
0.0

2.0

4.0

6.0

8.0

10.0

12.0

14.0

16.0

18.0

20.0

Pseudips mexicanus attack
density (attacks/m2)
Figure 3.1. Proportion of phloem used per lodgepole pine log by Pseudips mexicanus in
relation to P. mexicanus attack density (number of attacks per m 2 ).

62

phloem use did not exceed 10 % of total phloem volume available on any log and the overall
mean consumption was very low (Table 3.2). The proportion of phloem consumed by D.
ponderosae, however; was nonlinear and increased rapidly to approximately 70 %, where it
levelled off at a density of approximately 25 attacks/m2 (Fig. 3.2, N = 12, i?2adj. = 0.89, P <
0.0001). The maximum phloem use by both species combined was approximately 80 %,
leaving at least 20 % of the total phloem resource per log unused by either species (Table
3.2). The greatest phloem use was by D. ponderosae in trees previously occupied by P.
mexicanus (solid dots, Fig. 3.2; Table 3.2, t = -4.30, df = 10, P < 0.05).
Phloem use per D. ponderosae attack exhibited an inverse relationship with attack
density (Fig. 3.3, N = 12, r2^. = 0.750, P < 0.001) and hosts attacked solely by D.
ponderosae used the greatest amount of phloem per attack (Fig. 3.3, open dots, t = 3.251, df
= 10, P < 0.05) with a mean of 0.93 ±0.19 cm3/attack compared to 0.33 ± 0.08 cm3/attack in
trees previously attacked by P. mexicanus. Unused areas of phloem were recorded between
D. ponderosae and P. mexicanus gallery systems, and no interspecific egg or larval gallery
overlap was observed.
The amount of phloem utilized per individual D. ponderosae

offspring decreased

dramatically with increasing number of offspring produced (Fig. 3.4, N= 12, R2adj. = 0.70, P
< 0.0001). D. ponderosae offspring in hosts attacked by D. ponderosae alone used 0.19 ±
0.08 cm3 per attack and 0.03 ± 0.01 cm3 in hosts attacked by both species. There was a
significant difference in phloem use by D. ponderosae offspring between the two types of
infested hosts (t = 2.40, df = 10, P = 0.04).

63

0.8
0.7
0.6
0.5 0.4
R2adj= 0.887

0.3 -

F2 „ = 44.295, P< 0.0001

0.2

0.679
y=

-| + e -((x-11.066)/5.028)

0.1
0.0
0.0

20.0

40.0

60.0

80.0

100.0

Dendroctonus ponderosae attack
density (attacks/m2)

Figure 3.2. Proportion of phloem used per lodgepole pine log by Dendroctonus ponderosae
in relation to D. ponderosae attack density (number of attacks per m 2 ). Solid dots indicate
trees attacked previously by Pseudips mexicanus, followed by D. ponderosae in the same
year and open circles indicate trees attacked solely by D. ponderosae.

64

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100.0

Dendroctonus ponderosae attack
density (attacks/m2)

Figure 3.3. Lodgepole pine phloem volume (cm3) utilized per attack by endemic
Dendroctonus ponderosae in relation to D. ponderosae attack density. Solid dots indicate
trees attacked previously by Pseudips mexicanus, followed by D. ponderosae in the same
year and open circles indicate trees attacked solely by D. ponderosae.

65

0.6
r2adj. = 0.696

0.5 H

O

F r i 1 =26.189, P< 0.0001
y = 0.007 +

0.4

4.268

0.3

0.2

0.1 -\

0.0 H
0

100

200

300

400

500

600

Number of Dendroctonus ponderosae offspring
Figure 3.4. Lodgepole pine phloem volume (cm3) utilized per beetle by endemic
Dendroctonus ponderosae in relation to the total number of emerged D. ponderosae
offspring per tree. Solid dots indicate trees attacked previously by Pseudips mexicanus,
followed by D. ponderosae in the same year and open circles indicate trees attacked solely by
D. ponderosae.

66

Number of offspring emerging.
The volume of phloem utilized per m by P. mexicanus significantly influenced the total
number of emerging D. ponderosae females per m 2 , where more offspring emerged as
phloem use increased (Fig. 3.5, N = 12, r2^. = 0.30, P - 0.04).

Trees infested by P.

mexicanus (solid dots, Fig. 3.5) produced the most D. ponderosae offspring per m2 (/ = -2.89,
df = 10, P = 0.02) with a mean of 356.0 ± 74.7 females compared to a mean of 76.9 ± 42.8 in
trees D. ponderosae attacked alone. The number of D. ponderosae brood females produced
per attack was not significantly related to D. ponderosae attack density (N= 12, r2a(jj. = -0.09,
P = 0.08).

The number of female D. ponderosae produced per attack did not differ

significantly between the two infestation types (t = -0.98, df = 10, P = 0.35), with D.
ponderosae-only trees producing 4.8 ±1.6 females and P. mexicanus-infested trees 7.0 ± 1 . 4
D. ponderosae females (Table 3.2). The total phloem volume available to D. ponderosae did
not significantly affect the number of D. ponderosae emerging offspring (N = 12, r a dj. =
0.002, P = 0.89).

Offspring size.
Pronotal widths of female D. ponderosae were measured as an indicator of beetle size. A
multiple regression analysis indicated that P. mexicanus phloem volume consumption per m 2
did not contribute significantly to the model, but pronotal widths were negatively affected by
D. ponderosae attack density (Table 3.3). However, the effect was small and the sizes of D.
ponderosae from the two infestation types were not significantly different (t = 1.41, df = 10,
P = 0.19), with D. ponderosae female offspring from hosts attacked solely by D. ponderosae

67

6UU "

•

.2
(0
E

500 -

55 Ne

400 -

•

•

<D
vff
0)

o
V. l _

•

0) a)
^S CL

§ o 300 -

S..E

JO Q .

2 w

So

•

o
200 -

Den dro

o

•

100 - o

o-

r2adj. = 0.297

•

F1f11 = 5.645, P = 0.040
y= 136.052+ 184.285x

@
I

0.0

....

.

I

i

i

i

0.5

1.0

1.5

2.0

2.5

Pseudips mexicanus phloem use (cm3)/m2
Figure 3.5. Number of female endemic Dendroctonus ponderosae emerging per m in
relation to phloem volume used (cm ) by Pseudips mexicanus per m in lodgepole pine.
Solid dots indicate trees attacked previously by P. mexicanus, followed by D. ponderosae in
the same year and open circles indicate trees attacked solely by D. ponderosae.

68

Table 3.3. Summary of a multiple linear regression analysis examining the effects of volume
of phloem utilized (cm ) by Pseudips mexicanus
tus per m and Dendroctonus
Dendroi
ponderosae attack
2
density (attacks/m ) on female endemic D. ponderosae
nderosae mean pronotal
pronota width (mm).
Coefficients
SE
D. ponderosae attack density
-0.003
0.001
-3.126
0.012
P. mexicanus phloem use per log
0.054
0.034
1.573
0.150
Intercept
2.204
0.034
64.888
< 0.001
Multiple R'adj. = 0.41, F2>9 = 4.89, P = 0.04

69

having a mean pronotal width of 2.17 ± 0.03 mm and female offspring from P. mexicanus
infested trees having a mean width of 2.10 ± 0.03 mm.

Sex ratio.
Mountain pine beetle offspring were expected to have a 1:2 male to female sex ratio
(Reid 1958, McGhehey 1969). A comparison of the sex ratios of emerging adults for the two
types of infestations to the expected ratio showed significant differences. Both offspring
from trees attacked solely by D. ponderosae and offspring from trees attacked by both
species had a sex ratio of 1:1.4 (G = 4.02, df = 1, P < 0.05 and G = 13.98, df = 1, P < 0.001
respectively).

Brood emergence phenology.
The number of days after first emergence and the cumulative mean percent D.
ponderosae female emergence for both infestation types was described by a sigmoidal curve
(Fig. 3.6, N = 12, i?2adj. = 0.997, P < 0.0001). D. ponderosae offspring from trees infested
with P. mexicanus emerged earlier than offspring from trees attacked by D. ponderosae
alone, as determined by observing the length of time required to reach 50 % emergence. D.
ponderosae offspring reached this point after approximately 25.5 days in hosts previously
colonized by P. mexicanus and after 32.5 days in hosts attacked by D. ponderosae alone.
Initial emergence was more rapid in trees previously attacked by P. mexicanus, but faster in
the later half of the emergence period in trees occupied by D. ponderosae only. All logs had
stopped producing brood after 48 days and were peeled on day 55 to collect any remaining
adults.

70

ao> 100.0
c

R2adi. = 0-997
F 2 9 = 1315.990, P< 0.0001

0)

E? 80.0
Qi

E

60.0

c
re
0)

E 40.0
>
•|

20.0

E
3

°

o.o H
— i —

10

— i —

30

20

40

50

60

Julian days after first emergence

Figure 3.6. Cumulative mean percent emergence of female endemic Dendroctonus
ponderosae from lodgepole pine trees in relation to the mean number of days since
emergence commenced per infestation type. Solid dots indicate trees attacked previously by
Pseudips mexicanus, followed by D. ponderosae in the same year, open circles indicate trees
attacked solely by D. ponderosae and grey dots represent overlapping data points. Error bars
are 1 SE of the mean.

71

DISCUSSION
The findings of this study indicate that decadent trees previously colonized by P.
mexicanus were better resources for endemic D. ponderosae than trees D. ponderosae
attacked alone. Hosts attacked in both infestation types were similar both physically and
likely chemically (Raffa and Berryman 1982). Even so, hosts occupied by P. mexicanus
were more attractive to endemic D. ponderosae,

resulting in larger numbers of D.

ponderosae attacks and subsequently greater offspring production per tree. The number of
female D. ponderosae offspring produced was positively affected by the volume of phloem
consumed by P. mexicanus (Fig. 3.5), indicating that the presence of P. mexicanus
contributed in some manner to the increased D. ponderosae attack density. Additionally, D.
ponderosae offspring sizes were not different between infestation types (Table 3.3), but D.
ponderosae in P. mexicanus-infested

trees emerged earlier than in trees D. ponderosae

attacked alone (Fig. 3.6). Thus, comparison of D. ponderosae fitness characteristics between
infestation types is valid.
P. mexicanus is a secondary bark beetle that attacks weakened hosts and does not develop
into outbreak populations, seldom aggregating within hosts in large numbers (Wood 1982,
Chapter 2). Although there are advantages to aggregation, such as mate attraction (Borden
1974) and predator swamping (Aukema and Raffa 2004), secondary beetles tend not to
benefit from this behaviour, as increases in density produce sharp declines in reproductive
success (Robins and Reid 1997). Aggregation to individual trees by this species is therefore
normally limited (Wood 1982). Additionally, P. mexicanus does not completely utilize the
resource they occupy (Fig. 3.1), leaving opportunity for other species to attack.

72

Endemic D. ponderosae typically attack injured or stressed, over-mature lodgepole pine
trees, similar to the study trees (Carroll et al. 2006a). The lack of visible signs of defence
(pitch tubes, necrotic lesions) on the study trees, in conjunction with the observed thin
phloem, reduced growth and advanced age, suggest that these trees were highly susceptible
to attack by endemic D. ponderosae, with greatly diminished constitutive and induced
defences (Safranyik and Carroll 2006).

In resistant hosts, attacking beetles initially

encounter the preformed constitutive defence and the induced defence shortly thereafter
(Raffa and Berryman 1987), both of which contain compounds toxic to attacking beetles
(Cates and Alexander 1982). Constitutive defences are made up of large amounts of stored
resin (secondary metabolites consisting of monoterpenes and resin acids) (Lombardero et al.
2000), but in a weak or non-resistant host, the constitutive defence is minimal or absent
(Shrimpton 1973). The induced defence, a combination of necrotic lesion formation and
secondary resin and associated chemical production, is not strongly linked to the magnitude
of constitutive resin flow (Lombardero et al. 2000), as monoterpenes are either transported to
the attack site or manufactured there de novo (Raffa and Berryman 1982). Trees encountered
by endemic D. ponderosae do not have strong defences, but may still reduce beetle fitness.
A reduction in defences by damage, pathogen infection or previous attack by other bark
beetles could, therefore, benefit D. ponderosae, even in weakened hosts.
The induced defence is typically initiated in response to the presence of pathogenic fungi,
inoculation of which by P. mexicanus may reduce host defences. Most bark beetle species
carry fungal and yeast spores between hosts and many, such as D. ponderosae, have
specialized invaginations of the exoskeleton called the mycangia, in which these spores are
carried (Whitney and Farris 1970, Six and Paine 1998). These mycangial fungi are often

73

mutualistic and are fundamental components of brood success in healthy hosts (Six and Paine
1998). Other fungal species are transported phoretically on the exoskeleton (Six 2003). P.
mexicanus carries a diverse phoretic yeast and fungal complex, including Leptographium
terebrantis Barras & Perry (Bleiker, Massoumi-Alamoutim, Smith, Carroll and Breuil 2007,
unpub. data). This pathogenic fungus (Raffa and Smalley 1988) and others associated with
P. mexicanus, may contribute to further weakening of attacked hosts. In a recent study,
terpene production was compared in trees unattacked and previously attacked by P.
mexicanus through inoculation with Grosmannia clavigera (Robinson-Jeffrey and Davidson)
Zipfel, de Beer and Wingf. (previously Ophiostoma clavigerum), a D. ponderosae fungal
associate. Prior to inoculation, trees had similar terpene concentrations, but trees previously
attacked by P. mexicanus had a much weaker induced defensive response (less terpene
production) than did the unattacked trees (C. Boone, unpubl. data).
Carroll et al. (2006a) suggested, based on evidence presented by Christiansen et al.
(1987), that D. ponderosae may be relying upon secondary bark beetle species to reduce the
defensive capacity of smaller, but still resistant trees in order to successfully reproduce.
Christiansen et al. (1987) stated that small, suppressed trees do not use accumulated carbon
stores for growth, but instead store the energy, making it available for inducible defence.
The study trees had very low recent growth and lacked any sign of either a strong constitutive
(pitch tubes) or induced (lesions) defensive response at P. mexicanus attack sites. If the trees
were able to produce a strong induced response, the presence of L. terebrantis carried by P.
mexicanus should have triggered one (Raffa and Smalley 1988). This lack of visible induced
response, coupled with the work of Boone (unpubl. data), supports my hypothesis, and that of
Carroll et al. (2006a), that P. mexicanus-infested trees are weaker after attack than similar

74

unattacked trees within the stand, and, therefore, are potentially better hosts for endemic D.
ponderosae.
In hosts attacked solely by D. ponderosae, more phloem was used per attack (Fig. 3.3)
and per individual offspring (Fig. 3.4) than in hosts previously attacked by P. mexicanus. It
has been suggested that insects feeding on poor resources increase consumption to
compensate for lower nutritional value (Mattson 1980, Six and Paine 1998, Safranyik and
Carroll 2006). The greater resource use by individual D. ponderosae in hosts they attack
alone suggests that offspring inhabit a less suitable host than D. ponderosae offspring from
P. mexicanus-infested trees, and, therefore, may require larger amounts of resource per
individual to obtain required nutrients. Phloem consumption was not complete on any study
tree, and remained at fairly constant levels in P. mexicanus-infested hosts compared to
consumption in hosts attacked by D. ponderosae alone, where the proportion of phloem used
increased rapidly with attack density, but peaked at approximately 0.55 (Fig. 3.2). Overall
consumption per log was higher in P. mexicanus-trees, due to the larger numbers of attacks
and subsequent offspring production (Fig. 3.2), even though the mean total phloem volume
available was not different between infestation types (P = 0.70).
Subcortical spacing between bark beetle species may be facilitated by rendering phloem
unpalatable to other bark beetles by the presence of the mutualistic fungi of another species
(Yearian et al. 1972). A number of authors have suggested that mutualistic fungi may
increase the nutritional value of an otherwise poor host (Whitney et al. 1987, Six and Paine
1998, Ayres et al. 2000, Bleiker and Six 2007) and it has been proposed that the presence of
fungi associated with secondary bark beetles may improve host conditions for D. ponderosae
(Carroll et al. 2006a). Very little is known about the fungi or yeasts associated with P.

75

mexicanus or their potential effects on adult and brood D. ponderosae. There was no visible
evidence of fungal staining spreading throughout the logs prior to D. ponderosae attack;
however; if P. wex/canws-associated fungi had colonized the logs, D. ponderosae may have
benefited nutritionally from their presence, decreasing the amount of phloem required per
beetle to obtain requisite nutritional requirements (Six and Paine 1998).
Consumption of greater volumes of phloem by beetles in trees attacked by D. ponderosae
alone indicates that a longer period of time may have been required to reach maturity. The
timing of egg laying was not determined, so exact length of development time could not be
measured, but my results clearly show that D. ponderosae offspring took longer to emerge
from trees they attacked alone. Since both groups were reared under the same conditions, the
longer emergence time probably reflects slower development resulting from compensatory
feeding due to poor nutritional value of the resource. Other insects have been shown to
experience similar reductions in rate of growth and delays in emergence (Carisey and Bauce
1997).
The ability to develop quickly with minimal loss in body mass could confer a number of
benefits to a beetle. Earlier emergence would give beetles the opportunity to locate and
choose from among the best available hosts (Pureswaran et al. 2006). Once a host is
selected, an optimum attack site can be located and oviposition commenced. The ability to
begin egg laying prior to the arrival of conspecifics gives offspring time to develop into more
robust life stages which can better survive intraspecific competition. Additionally, the ability
to arrive earlier in relatively large numbers may also reduce losses due to predation while
host searching, essentially beating predators to the host trees (Dixon and Payne 1979,

76

Aukema and Raffa 2004). Beetles emerging later would have to contend with predators that
have accumulated as a result of attraction to previous flights of beetles.
Offspring size is affected by growth rates as well as density of conspecifics. In this
study, the size of female D. ponderosae offspring were found to be negatively affected by
parental attack density, but the effect did not reduce offspring size significantly.

D.

ponderosae females from hosts attacked by this species alone were found to be 3.2 % wider
across the pronotum than offspring from trees previously infested by P. mexicanus. Under
laboratory conditions, McGhehey (1971) found that larger females tended to produce more
eggs. Using the mean pronotal widths of female beetles from each infestation type and
extrapolating from McGhehey's (1971) data, 140 eggs could potentially be laid by females
emerging from D. ponderosae-orAy trees, compared to only 124 by the smaller females from
P. mexicanus-infested hosts. This is a difference in potential fecundity for the larger females
of approximately 11.4 % (Fig. 3 in McGhehey 1971). However, Amman (1972b) found that
there was a great deal of variation in egg-laying capacity among individual beetles and Reid
(1962) pointed out that only a fraction of the potential total number of eggs is ever laid in
natural populations, suggesting that this potential improvement in fecundity for D.
ponderosae in trees they attack alone may not be biologically significant. The number of D.
ponderosae eggs laid could not be determined for the study trees, so actual fecundity was not
calculated.
Larger body size has a number of associated costs and benefits. Bigger beetles tend to
have greater fat stores (Atkins 1966), providing energy for longer periods spent searching for
ephemeral hosts. Beetles emerging from a poor resource, regardless of their size, will have a
diminished capability to secure hosts as a result of reduced lipid content (Elkin and Reid

77

2005). Size also has been shown to affect pheromone production. More robust male D.
ponderosae

produce greater amounts of anti-aggregation pheromone, however; the

production of aggregation pheromone by females is not affected by size (Pureswaran and
Borden 2003). Therefore, the ability to attract mates is not diminished for smaller females.
Additionally, smaller beetles may benefit in an endemic environment as smaller offspring
would be more suited to colonize hosts with thin phloem (Pureswaran and Borden 2003).
Larger beetles may accidentally break through the protective bark layer when the subcortical
environment is not of suitable thickness, resulting in abandonment of the tree and exposure to
the risks of finding a new host (Safranyik and Carroll 2006). Larger beetles attacking hosts
with thin phloem may have to expend more energy engraving the sapwood (Amman 1972b),
while smaller beetles would be able to mine more fully within the softer phloem tissue.
The sex ratio of emerging D. ponderosae was significantly different than that normally
found in epidemic populations, where Reid (1958) and McGhehey (1969) found a ratio of 1:2
males to females, while I found ratios of 1:1.4 in hosts attacked by D. ponderosae only and in
those attacked by P. mexicanus and D. ponderosae.

During periods of stress, D. ponderosae

has been shown to alter sex ratio in favour of females (Amman and Cole 1983); however; the
number of females produced in this study was lower than that found in outbreak conditions,
when numerous high quality hosts are available. I did not subject the beetles to cold, which
could account for the large numbers of males present upon emergence, as they tend to be the
most susceptible to freezing mortality (Safranyik 1976). However, it is possible that the ratio
found during the endemic phase is the norm and that during an outbreak intraspecific
competition reduces the number of male offspring, resulting in a female-biased ratio.

78

Alternatively, the risk of not obtaining a mate may be so great during the endemic phase that
the number of males is kept high to ensure there are enough to find the attacking females.
Amman and Schmitz (1988) suggested that endemic D. ponderosae attacking hosts
previously infested by secondary bark beetles should reduce risk of mortality through
reduction of the time needed to locate a host by attraction to a tree infested with secondary
beetles.

They proposed that endemic D. ponderosae would be attracted to either the

pheromones of the secondary beetles or the host volatiles emitted as a result of secondary
attack. P. mexicanus attractant pheromone is not attractive to (Chapter 4) and, therefore, host
location must be conducted through either detection of a volatile signature that D.
ponderosae orients to (Moeck and Simmons 1991) or random landings (Hynum and
Berryman 1980) or a combination of both (Campbell and Borden 2006).
I have shown that D. ponderosae attacking ephemeral hosts previously occupied by P.
mexicanus do so in greater densities and offspring emerge earlier when compared to hosts D.
ponderosae attacked alone. Additionally, D. ponderosae larvae in P. mexicanus-infested
trees required significantly less resource to complete development. P. mexicanus likely has
an effect on host chemistry, improving the subcortical environment in which endemic D.
ponderosae can be more successful than in hosts they attack alone and possibly causing hosts
to produce volatiles attractive to endemic D. ponderosae. Hosts with P. mexicanus present
should be actively sought by endemic D. ponderosae, whereas D. ponderosae attacking trees
with no secondary beetles present would likely do so as a last ditch effort to reproduce.
The form of the interaction between the two species appears to be facilitation, with D.
ponderosae benefiting from the presence of P. mexicanus in weakened lodgepole pine trees.
However, it is possible that during an emergence event P. mexicanus may re-infest the same

79

tree in order to overwinter. This behaviour may help avoid dispersal losses, but has not been
investigated. If this does occur, the presence of D. ponderosae may force P. mexicanus to
find new host material, resulting in weak asymmetric exclusion competition, where D.
ponderosae benefits greatly from the presence of P. mexicanus while P. mexicanus
experiences mortality due to the presence of D. ponderosae. Regardless, the presence of P.
mexicanus in these trees prior to D. ponderosae facilitates the maintenance of D. ponderosae
populations during the endemic phase in lodgepole pine forests.

Further work should

examine how P. mexicanus alters the host chemistry upon attack and the chemical cues that
endemic D. ponderosae respond to in order to find hosts initially. Additionally, the role of
mutualistic fungi associated with P. mexicanus needs to be studied in terms of altering hosts
chemistry and improving nutritional value of phloem for endemic D. ponderosae.

80

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Scolytidae). The

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CHAPTER 4
THE RESPONSE OF ENDEMIC DENDROCTONUS PONDEROSAE TO
VOLATILES ASSOCIATED WITH THE SECONDARY BARK BEETLE PSEUDIPS
MEXICANUS

ABSTRACT
A pair of experiments was conducted to assess the role of lodgepole pine host volatiles
and the attractant pheromone of Pseudips mexicanus in host selection by endemic
Dendroctonus ponderosae and other associated bark beetle species. Endemic D. ponderosae
are often found in weakened hosts previously colonized by P. mexicanus, but did not respond
to either pheromones or host volatiles produced from P. mexicanus-iafested or uninfested cut
logs, respectively. Both P. mexicanus and Ips pini were strongly attracted to P. mexicanus
pheromone, a combination of racemic ipsdienol and ipsenol. Host volatiles presented alone
or in combination with chemicals produced by feeding P. mexicanus were attractive to P.
mexicanus and Orthotomicus latidens, while no /. pini were captured. These results suggest
that D. ponderosae in the endemic stage locate hosts using cues not directly attributable to
the presence of P. mexicanus, but rather identify volatiles associated with living host decline
stemming from P. mexicanus attack.

Key words: pheromone; ipsenol; ipsdienol; population maintenance

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INTRODUCTION
The mountain pine beetle, Dendroctonus ponderosae Hopkins, is a major pest of
lodgepole pine (Pinus contorta var. latifolia Engelmann) forests in western North America.
During epidemics, large healthy trees are attacked and killed by aggregating beetles
(Safranyik and Carroll 2006). Initial selection of these hosts is conducted by pioneering
beetles through primary attraction (orientation to host volatiles) (Person 1931). Primary
attraction has been studied extensively for epidemic populations of D. ponderosae, where it
is generally accepted that orientation to host volatiles, recognition of silhouettes and random
landing coupled with gustatory sampling are used by pioneering females to locate suitable
hosts (reviewed in Safranyik and Carroll 2006). Like many tree-killing bark beetle species,
D. ponderosae normally exists in healthy forests at low or endemic populations (Carroll et al.
2006), potentially for long periods (Raffa 1988). In the endemic stage, populations are so
small that D. ponderosae cannot access healthy trees as there are not enough individuals
available to overcome a well-defended host (Amman and Schmitz 1988). Strong resin flow
and rapid induced responses in the form of monoterpene synthesis and necrotizing tissue
prevent beetles and associated fungi from becoming established. Therefore, endemic beetles
must subsist on smaller diameter, vigour-impaired trees (Wood 1982, Safranyik and Carroll
2006).
Vigour-impaired trees, in addition to being hosts for endemic D. ponderosae, are the
principal resource for a large number of secondary bark beetles (Bright 1976, Safranyik and
Carroll 2006). Over 50 species of bark beetle attack lodgepole pine in Canada, the vast
majority of which only infest weak trees (Bright 1976). One of these beetles, Pseudips
mexicanus (Hopkins) (previously Ips mexicanus, Cognato 2000), is often found in trees

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attacked in the same or subsequent season(s) by endemic D. ponderosae (Carroll et al. 2006).
The presence of P. mexicanus has been found to facilitate D. ponderosae

population

maintenance where trees occupied by P. mexicanus experience greater D. ponderosae attack
density, tend to produce more female offspring per attack that emerge earlier when compared
to trees attacked by D. ponderosae alone (Chapter 3). If D. ponderosae is experiencing
greater fitness in trees previously colonized by P. mexicanus, then there should be strong
selection pressure for endemic D. ponderosae to orient to these beneficial hosts.
It is unclear how endemic D. ponderosae locate suitable hosts.

Trees attacked by

endemic D. ponderosae tend to be smaller than the stand average, relatively rare (Chapter 3)
and produce lower quantities of phytochemicals compared to healthy trees (Raffa and
Berryman 1982).

Wallin and Raffa (2004) showed that in a closely related system,

Dendroctonus rufipennis (Kirby) on spruce, there was a density-dependent host-selection
behaviour governed by monoterpene concentrations.

The study by Carroll et al (2006)

suggested that a similar density-dependent behaviour occurs in D. ponderosae populations,
but did not suggest a mechanism for host selection in the endemic stage.
Given that the majority of endemic D. ponderosae colonize trees that are either currently
infested or were previously infested by other bark beetle species (Carroll et al. 2006),
endemic D. ponderosae may employ secondary cues to locate hosts. Secondary attraction
occurs when a beetle focuses on a cue other than that directly from the host, such as the
aggregation pheromone of another species of beetle or a combination of aggregation
pheromone and host volatiles.

Depending upon the subsequent interaction between the

sender and the receiver, the pheromone is classified as an allomone, kairomone or synomone.
An allomone benefits the emitter upon response by the receiver, kairomonal attraction

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benefits only the receiver of the chemical, e.g. many species of predators and parasitoids
attracted to bark beetle emissions (Poland and Borden 1997, Aukema et al. 2004, Miller et al.
2005), while a synomone benefits both the receiver and sender (Nordlund 1981). Some bark
beetles are cross-attracted to the semiochemicals of other species, for example Pityogenes
knechteli Swaine to Ips pini (Say) (Poland and Borden 1994) and P. mexicanus to
Orthotomicus latidens (LeConte) (Savoie et al. 1998) and may use these chemicals as either a
pheromone or a synomone, contributing to resource partitioning (reviewed in Miller et al.
1991).
P. mexicanus utilize racemic ipsenol and ipsdienol as attractant pheromones (Carroll and
Borden, unpubl.).

Hunt and Borden (1988) examined attraction of epidemic level D.

ponderosae to racemic ipsdienol and determined that beetles were attracted to high
concentrations in the laboratory. In field trials no significant response to ipsdienol was
detected, but upon the addition of host volatiles and D. ponderosae aggregation pheromone,
trap catches improved significantly, however; catches were lower when ipsdienol was
present. If a density-dependent host-selection strategy exists for D. ponderosae, endemic
populations may respond to volatile cues differently than epidemic beetles. Since endemic
D. ponderosae appears to preferentially colonize weakened hosts with P. mexicanus present
(Chapter 3), it is possible that host selection is mediated by kairomonal or synomonal
attraction to racemic ipsdienol with the addition of ipsenol or a combination of the two
pheromones and host volatiles.
This study was designed to test the attractiveness of P. mexicanus attractant pheromone
alone (Experiment 1) or in combination with volatiles generated by the presence of feeding
P. mexicanus in host material (Experiment 2) to endemic level D. ponderosae. I predicted in

93

the first experiment that traps baited with P. mexicanus attractant pheromone would catch
more D. ponderosae than control traps.

In the second experiment, I expected that P.

mexicanus-infested host material (i.e., bolts) would attract more D. ponderosae than either
control traps or non-infested host material. If the number of beetles captured at the boltbaited traps was greater than at the pheromone-baited traps, more than just the pheromone is
acting to make P. mexicanus-infested trees attractive to D. ponderosae. However, if the
number of beetles captured in the pheromone-baited traps was equal or greater than in the P.
mexicanus-infested bolt-baited traps, the pheromone can be said to be the main attractant.

METHODS
Study Site.
Two trapping experiments were conducted during the summer of 2005 in lodgepole pine
stands near Angstad Creek, 28 km south of Merritt BC (49° 51' N, 120° 46' W, mean
elevation = 1300 m). The stands used in Experiment 1 were pure pine, while the stands used
in Experiment 2 were pine-dominant with a minor component of Douglas-fir (Pseudotsuga
menziesii (Mirb.) Franco).

All stands within each experiment had similar species

composition, age classes, height classes and stocking levels.

During this study, D.

ponderosae populations were shifting from endemic to incipient epidemic (Safranyik and
Carroll 2006), where small groups of trees were being mass attacked. For both experiments,
trapping took place between 20 July and 14 September 2005, during the main flight of D.
ponderosae (Safranyik and Carroll 2006). Twelve-funnel Lindgren traps (Lindgren 1983)
were suspended between two trees a minimum of 0.5 m away from the boles, with the
collecting cup approximately 0.5 m above the ground. The collecting cup contained a piece

94

of Vapona No-pest

strip (Monsanto Canada Ltd, Mississauga, Ontario, Canada) to kill all

captured insects (Campbell and Borden 2006).

Experiment 1 - Pheromone
Trap layout.
A randomized complete block design was laid out with 10 replicates, which consisted of
a treatment trap and a control trap.

The control was a blank trap, while the treatment

consisted of a lure containing P. mexicanus attractant pheromone attached to the midpoint of
the trap. The pheromone lure consisted of a pair of polyvinyl bubble caps, one with racemic
ipsenol (2-methyl-6-methylene-7-octen-4-ol) and the other racemic ipsdienol (2-methyl-6methylene-2,7-octadien-4-ol) (Phero Tech, Inc., Delta, B.C., 2005).

Both bubble caps

contained 40 mg of their respective chemical with release rates of 0.4 mg/day for the ipsenol
and 0.2 mg/day for the ipsdienol at 25 °C. The control and treatment traps were spaced > 40
m apart within the blocks and blocks were separated from each other by > 90m.

Experiment 2 - Pine bolt
Host material collection.
This experiment used 20 cm long lodgepole pine bolts as bait treatments either infested
with P. mexicanus or left uninfested. Uninfested and infested pine bolts were used as baits
because of the propensity of Ips and related species to produce pheromones when feeding
(Vite and Gara 1962). Additionally, the bolts were employed as a source of host volatiles to
test additive or synergistic properties when encountered with P. mexicanus attractant
pheromone. Three 25 cm diameter lodgepole pine trees were felled at Angstad Creek on

95

May 28, 2005 and 3.0 m of the lower bole of each tree was removed and taken to the Pacific
Forestry Centre in Victoria, B.C. Twenty-four 20 cm long bolts were cut from each of the
logs and the bark was scored vertically on opposite sides to create two discrete bark sections.
A three cm diameter eyehook was placed in the top of each. All cuts were sealed with
paraffin wax to reduce desiccation and the bolts were placed in a cold room 4 °C until June
18,2005.

Beetle collection.
For purposes of infesting the treatment bolts, P. mexicanus were collected alive at both
Angstad Creek and on the Aberdeen Plateau, 30 km north-east of Kelowna BC (50° 5' N,
119° 11 'W, mean elevation = 1300 m) between June 13 and June 17, 2005, using
pheromone-baited 12-funnel Lindgren traps.

Collections were made daily to minimize

escape, and damage and loss of beetles to predators. Additionally, beetles were collected
manually on June 17, 2005 from active galleries at the base of lodgepole pine trees mass
attacked in 2004 by D. ponderosae at Angstad Creek. All beetles were sorted by sex, placed
in a jar with freshly peeled lodgepole pine phloem and stored at 4 °C until needed. P.
mexicanus males were identified by the presence of a large median tubercle on the frons
(Wood 1982), while females were separated by a reduced or absent tubercle..

Bolt infestation.
On June 18, 2005, four bolts from each of the three harvested trees were randomly
selected to be infested with P. mexicanus.

The gelatine capsule method was used to

introduce beetles to the logs (Lanier and Wood 1968). Holes were made in the middle of

96

each bark section and one male was placed in each hole, secured with a gelatine capsule and
left overnight to initiate a nuptial chamber. Beetles that did not begin a nuptial chamber were
replaced after 24 hours. When all beetles had successfully entered, two females per hole
were introduced. Any female not entering under the bark after 24 hours was replaced
(Safranyik and Linton 1983). All bolts were kept at room temperature for three days and
subsequently stored at 4 °C until deployed on July 20, 2005.

Trap layout.
Traps were laid out in a randomized complete block design with 12 replicates consisting
of three traps each, two treatment traps and a blank control trap. All traps were 12-funnel
Lindgren traps. The two treatments were laid out with either a randomly chosen infested bolt
or a non-infested bolt as bait. The bolts were suspended by the eyehook on a steel pipe
lashed to two trees and were placed close to, but not touching the traps. The bolts were hung
at a height half way down the traps. All of the bolts were covered with sealed mesh sleeves
(Poland and Borden 1994) to prevent the escape of resident P. mexicanus in the infested bolts
and to inhibit attack by local bark beetles and other insects while the bolts were in the field.
The control and treatment traps were spaced > 40 m apart within each replicate and the
replicates were separated from each other by > 90 m.

Beetle trapping and identification.
Traps for both experiments were checked on August 10, August 27, and September 14.
Collected insects were stored in 70 % ethyl alcohol in 20 ml scintillation vials until sorted.
Initial sorting separated bark beetles from other insects. Bark beetles were then identified to

97

species (Linton 2005) and counted.

P. mexicanus, D. ponderosae, Ips pini (Say) and

Pityogenes knechteli Swaine were sorted by sex according to secondary sexual
characteristics.

P. mexicanus were identified as above.

D. ponderosae males were

distinguished by the presence of the stridulation apparatus on the 7th abdominal tergite (Lyon
1958). /. pini males were identified by the capitate 3rd dorsal spine on the elytral declivity
(Bright 1976), while P. knechteli males were distinguished by a pair of hooked spines on the
upper margin of the elytral declivity (Wood 1982).

Orthotomicus latidens (LeConte)

(previously Ips latidens; (Cognato and Vogler 2001)) were not sorted by sex.

Data analysis.
All trap catch data from both experiments were transformed by Y - logio(X + 0.5) to
address assumptions of normality and homoscedasticity (Tabachnick and Fidell 2001). Tests
for outliers were conducted by plotting standardized residuals against dependent variables
(beetle counts by species). Outliers were detected for all species in both experiments and all
outlier traps were noted to have been located within 10 m of three or more trees recently
mass attacked by D. ponderosae. A number of species of secondary bark beetles attack trees
killed by D. ponderosae (Safranyik et al. 1999, Carroll et al. 2006) and, to eliminate any nontreatment effects, all outlier traps were removed from the analysis. In both experiments, an
ANOVA was applied with transformed trap capture data as the dependent variable, treatment
as a fixed factor and block as a random factor. A Ryan-Einot-Gabriel-Welsch (REGW) Ftest was used to assess differences between treatments in the bolt experiment. All means are
reported as untransformed data ± one standard error. Tests were reviewed in Sokal and Rohlf
(1995) and analyses conducted with SPSS 13.0 (SPSS 2004). For all tests, a = 0.05.

98

RESULTS
Experiment 1 - Pheromone
Total catches for D. ponderosae were small with four caught in pheromone traps and
only one in a control trap. Catches for O. latidens were similar, with five beetles caught in
pheromone traps and four in control traps. Both species subsequently were dropped from the
analysis. The number of captured P. mexicanus males and females were significantly
affected by treatment (Table 4.1). The number of males caught in the control traps was zero
and a mean of 5.4 ±1.2 were captured in the pheromone-baited traps, while 0.4 ± 0.4 and
32.0 ± 3.9 females were caught in the control and baited traps, respectively (Fig. 4.1 A).
Females were captured in numbers approximately six times that of males in the pheromonebaited traps. Block effects were not significant for any species except for male /. pini (Table
4.1), where a pheromone trap in one block captured over 60 % of the total beetles (N= 26).
This resulted in male /. pini captures in the pheromone traps not differing significantly from
the control traps due to high variability in pheromone captures (Fig. 4.IB). However,
significantly more females were captured in the pheromone-baited traps (11.8 ± 4.5) than the
control traps (0.2 ± 0.2) (Table 4.1; Fig. 4.IB). Additionally, more than twice as many
females as males were caught per pheromone-baited trap (Fig. 4.IB). Neither sex of P.
knechteli was captured more in the pheromone-baited traps than in the control traps (Table
4.1), but more females were captured than males on average in the treatment traps (4.4 ± 2.6
and 1.4 ± 1.2 respectively).

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Table 4.1. Analysis of variance test on log-transformed capture data for bark beetle species
collected from a field trapping experiment with treatment as a fixed factor and block as a
random. Treatments consisted of a control trap and a trap baited with Pseudips mexicanus
attractant pheromone, racemic ipsenol and racemic ipsdienol.
Male
Female
F
P
F
Species
Effect
d.f.
MS
MS
P
1 2.658 108.596 <0.001
P. mexicanus Treatment
6.894 178.444 <0.001
Block
4 0.024
1.000
0.500
0.073
1.879 0.278
Intercept
1 0.460 18.802 0.012
4.477 61.659 0.001
I. pini
Treatment
1 0.153
1.000
0.500
3.239 14.431 0.019
Block
4
1.684
11.003 0.029
0.044
0.196 0.928
Intercept
1 0.120
0.781 0.427
1.322 30.051 0.005
P. knechteli Treatment
1 0.105
0.581 0.694
1.122
5.295 0.083
Block
4 0.124
0.686 0.454
0.212
1.000
0.500
Intercept
1 0.089
0.845 0.410
0.011
0.054 0.827

100

4U.U -

A - Pseudips
mexicanus

LU
C/>

b

B - Ips pini

+1

i

O
O

les captured

I 30.0-

b

CD
CD

2 10.0 E
3

z

a

a
o.o-

a

b
T

CD

i

i

Con

Treat

Male

a
i

Con

Treat

Female

— i —

Con

Treat

Male

a
T —' 1
Con Treat
Female

Treatment by sex
Figure 4.1. Number of bark beetles captured during a field trapping experiment using a
combination of racemic ipsenol and ipsdienol as the pheromone bait; (A) Pseudips
mexicanus; (B) Ips pini. Bars for each sex with the same letter are not significantly different
(P < 0.05, N- 5). Con = control, Treat = pheromone treatment.

101

Experiment 2 - Pine bolt
D. ponderosae and I. pini were not attracted to either treatment or control traps, resulting
in the capture of only three D. ponderosae and zero I. pini. Neither species was included in
the analysis. Block did not significantly affect trap catches for any species (Table 4.2). A
total of 13 female and five male P. knechteli were caught, but traps baited with either bolt
treatment were not significantly more attractive than the control traps (Table 4.2). P.
mexicanus males were more attracted to the traps baited with the uninfested bolts (4.4 ± 1.3)
than to the control traps (0.2 ± 0.1), while attraction to the infested bolt-baited traps (2.8 ±
1.0) did not differ from either of the other two treatments (Table 4.2; Fig. 4.2). Female P.
mexicanus were not captured at either treatment trap (8.0 ± 2.4 and 7.4 ± 2.2 uninfested and
infested, respectively) in numbers significantly different from the control traps (1.9 ± 1.0)
(Table 4.2). Although captures at the traps baited with the bolts tended to be relatively high,
the variance associated with the number of beetles caught in the control traps was large
compared to control trap catches for P. mexicanus males and I. latidens (Figs. 4.2 & 4.3),
resulting in the lack of difference between treatments. The number of females caught in traps
baited with the uninfested bolts was approximately twice as high as the number of males, and
the number of females caught in traps baited with infested bolts was over 2.5 times greater
than the trap catches for males. Comparing the number of P. mexicanus caught between the
two experiments, the pheromone treatment in Experiment 1 captured approximately four
times as many females per trap as those caught in each of the bolt treatments in Experiment 2
(Figs. 4.1 A & 4.2). Males were caught in similar numbers between treatments in each
experiment.

102

Table 4.2. Analysis of variance test on log-transformed capture data for bark beetle species
collected from a field trapping experiment with sex and treatment as factors. Treatments
were comprised of a control trap, a trap baited with a lodgepole pine bolt infested with
Pseudips mexicanus and a trap baited with an uninfested pine bolt.
Male
Female
Species

Effect

P. mexicanus

P. knechteli

0. latidens*

MS

F

Treatment
Block
Intercept
Treatment
Block
Intercept

d.f.
2
8
1
2
8
1

7.080
1.141
0.242
1.505
1.092
4.504
0.034
1.000
0.040
1.188
1.221 30.493

Treatment
Block
Intercept

2
8
1

1.740
0.350
5.136

P

MS

F

P

0.006
0.231
0.067
0.390
0.365
0.001

0.810
0.471
5.395
0.114
0.074
0.513

2.424
1.408
11.462
1.224
0.798
6.927

0.120
0.266
0.010
0.320
0.613
0.030

4.897 0.022
0.986 0.482
14.668 0.005

* O. latidens were not segregated by sex; therefore, mean square, F and P values are reported
for total number of beetles captured.

103

IZ.U

a
CO
+i 1 0 . 0 c

T

a

CD
CD

^

8.0 -

b

1

b

capture

T3

CO
.CD

"5)
®

ab

4.0-

<*o

a

CD

|

2.0-

2

a
- i -

0.0 ^I

u ^ ^

1

—

Control Uninfested Infested
Male

1

Control Uninfested Infested
Female

Treatment by sex
Figure 4.2. Number of Pseudips mexicanus captured during a field trapping experiment with
lodgepole pine bolts as attractive baits either alone or infested by P. mexicanus. Bars for
each sex with the same letter are not significantly different; REGW multiple comparisons test
(P<0.05,#=9).

104

^u.u-

a

15.0a
10.0-

5.0b

o.oUninfested

i

i

Infested

Control

Treatment
Figure 4.3. Number of Orthotomicus latidens captured during a field trapping experiment
with lodgepole pine bolts either uninfested or infested by P. mexicanus as attractive baits.
Bars with the same letter are not significantly different; REGW multiple comparisons test (P
< 0.05, TV =9).

105

DISCUSSION
Contrary to predictions, my findings showed that endemic D. ponderosae was not
attracted to either P. mexicanus attractant pheromone alone or in combination with volatiles
associated with feeding P. mexicanus. Additionally, D, ponderosae did not perceive the
uninfested bolts in Experiment 2 as attractive. Improved fitness correlates associated with
endemic D. ponderosae in trees attacked by P. mexicanus (Chapter 3) suggest that these
hosts should be actively sought by dispersing D. ponderosae. However, it appears that the
presence of ipsenol does not mitigate the repellent effect of ipsdienol to flying D. ponderosae
(Hunt and Borden 1988), explaining the lack of attraction to P. mexicanus pheromone-baited
traps. Lack of attraction to the infested bolt treatment in Experiment 2 by D. ponderosae
may have been a reaction to the production of ipsdienol by feeding male P. mexicanus, but
since D. ponderosae was not captured at either of the bolt treatments, it is likely that the bolts
were not producing the appropriate host volatile cues and may not have been sufficiently
fresh. Interestingly, both P. mexicanus and O. latidens were strongly attracted to these traps,
and both species are found in weak living hosts subsequently attacked by endemic D.
ponderosae (Chapter 3) suggesting D. ponderosae seeks the same sort of host material. The
lack of response by D. ponderosae to either P. mexicanus attractant pheromone or the two
bolt treatments (infested or not) indicates endemic D. ponderosae use cues other than those
employed in this study. Alternate cues may be host chemicals produced by living trees
during defensive responses to P. mexicanus attack or inoculation with fungi associated with
P. mexicanus or a combination thereof.
As was expected, attraction to the pheromone source was very strong for P. mexicanus in
Experiment 1, especially for females.

The sex ratio of attacking P. mexicanus is

106

approximately two females/male (Chapter 2) while the observed response to the pheromone
trap was six females/male. A number of factors conceivably contributed to the observed
female bias. First, the two sexes are likely differentially attracted to attractant pheromones
(Miller et al. 1991, Savoie et al. 1998, Miller et al. 2005). Lower numbers of males attracted
to a pheromone source would be adaptive, helping to limit over-utilization of a scarce
resource through moderation of attack densities, while attracting multiple mates to maximize
fitness (Raffa et al. 1993). Second, the pheromone baits may have been releasing chemicals
at a higher rate than would be experienced in the field, drawing more females to the traps
than seen on attacked trees, as shown for I. pini (Miller et al. 2005). Third, the synthetic
blend of the attractant pheromone did not include any host volatiles, which may have
contributed to lower catches of males (Borden et al. 1983, Hunt and Borden 1988,
Pureswaran and Borden 2005). The combination of these three factors may have artificially
skewed the trap catches to favour female P. mexicanus, but findings for other species such as
D. pseudotsugae Hopkins suggest that bias toward responders well beyond the sex ratio seen
during attack and mating is not unusual (Pureswaran and Borden 2005).
Experiment 2 revealed a positive response by P. mexicanus to the host material,
irrespective of the presence of feeding beetles. Based on the high catches at the pheromonebaited traps in Experiment 1, it was expected that the bolts with P. mexicanus feeding in
them would attract more conspecific beetles, as Ips and related species produce attractant
pheromone while the male is feeding and clearing frass (Vite and Pitman 1968), however;
others suggest that as males acquire a complete harem, pheromone production decreases
(reviewed in Miller and Borden 1990). The lack of difference between bolt treatments
suggests a complete harem was present in the infested bolts and that the amount of

107

pheromone being produced was not as compelling as the bolt volatile cues for either sex.
The strong, but not significant (Fig. 4.2), kairomonal attraction of the females indicates that
both sexes are capable of detecting suitable host material. This supports the findings in
Chapter 2 of previously mated females having the ability to locate hosts in the absence of
males. Other species, such as O. latidens, have not been observed to exhibit sex-specific
responses to monoterpenes (Miller and Borden 1990). Thus, it is unclear why more female P.
mexicanus than males would be attracted, since the males are the pioneering sex, and of the
81 galleries inspected in Chapter 2, 97.5 % had males associated with them.
Additional secondary bark beetle species commonly associated with D. ponderosae and
P. mexicanus were captured. In the first experiment, Ips pini (Say) was attracted to the
pheromone-baited traps (Fig. 4.IB), however; catches may have been limited by the presence
of the ipsenol component (Furniss and Livingston 1979, Borden et al. 1992) which has been
shown to reduce, but not completely preclude, /. pini attraction to ipsdienol. The second
experiment captured no /. pini, clearly indicating a lack of attraction to the host material and
to the presence of P. mexicanus within the infested bolts. I. pini colonizes recently dead,
often horizontal trees and slash, as well as the tops of standing trees killed by D. ponderosae
(Wood 1982). Because of this, it was suspected that I. pini would find both bolt treatments
attractive, especially the uninfested ones; however, the lack of attraction suggests that the
appropriate host volatile cues were not present or that populations of I. pini in those areas
were particularly small.
O. latidens was caught in very small numbers in Experiment 1, indicating potential
inhibition by P. mexicanus attractant pheromone.

This was expected since Miller and

Borden (1992) found trap catches of O. latidens significantly reduced when racemic

108

ipsdienol was added to racemic ipsenol. In Experiment 2, O. latidens was found to be
attracted to both bolt treatments. The slightly lower numbers caught in traps baited with P.
mexicanus-infested bolts compared to uninfested bolts may be due to inhibition by ipsdienol
produced by feeding P. mexicanus (Vite and Gara 1962). However, bark beetles are able to
distinguish quantitative differences in monoterpene plumes (Wallin and Raffa 2000,
Pureswaran et al. 2004), so the presence of P. mexicanus degrading the host material by
opening the phloem to oxidization and desiccation, inoculating the subcortical environment
with fungi and yeasts (Whitney 1982), and physically destroying portions of the phloem and
sapwood through gallery construction may have acted as an additional deterrent.
The final species to be caught in these experiments, P. knechteli, was caught in higher
numbers in Experiment 1 than in Experiment 2. Savoie et al. (1998) found ipsdienol to be
the aggregation pheromone of this species, but noted that the addition of (S)-(-)-ipsenol
significantly decreased the number of beetles caught, however; the inclusion of host volatiles
with racemic ipsdienol alone elicited significant attraction responses (Miller and Borden
2003). The number of beetles caught in Experiment 1 are very similar to the trap catches of
Savoie et al. (1998), suggesting racemic ipsenol is also inhibitory to P. knechteli. The low
numbers of beetles caught at the bolt treatments in Experiment 2 may be the result of missing
host volatiles required to stimulate attack, and since P. knechteli are normally found
attacking high in the canopy (Safranyik et al. 2000), they may not be present in large
numbers within 1.5 m of the ground.
The results of this study demonstrate that the members of the community of bark beetles
that exist in lodgepole pine forests in south-central British Columbia are differentially
attracted to either P. mexicanus attractant pheromone or infested host material, likely as a

109

mechanism to reduce competitive interactions between them (Safranyik et al. 2000). The
focal species of this investigation, D. ponderosae, in the endemic stage attacks trees
previously infested by P. mexicanus, but does not use secondary attraction to P. mexicanus
attractant pheromones to locate such hosts. Additionally, dead hosts, regardless of the
presence or absence of P. mexicanus, are not attractive to D. ponderosae. However, a living
but severely weakened host previously occupied by P. mexicanus and subsequently attacked
by D. ponderosae benefits D. ponderosae by facilitating population maintenance (Chapter 3).
To take advantage of this, D. ponderosae must rely upon other stimuli such as host volatile
signatures and silhouette recognition to locate suitable material while in the endemic stage.

110

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and strategies for reducing tree losses. Ambio, 17:62-68.
Aukema, B. H., G. R. Richards, S. J. Krauth, and K. F. Raffa. 2004. Species assemblage
arriving at and emerging from trees colonized by Ips pini in the Great Lakes Region:
Partitioning by time since colonization, season and host species. Annals of the
Entomological Society of America, 97:117-129.
Borden, J. H., J. E. Conn, L. M. Friskie, B. E. Scott, L. J. Chong, J. Pierce, H.D., and L. M.
Oehlschlager. 1983. Semiochemicals for the mountain pine beetle, Dendroctonus
ponderosae (Coleoptera: Scolytidae), in British Columbia: baited-tree studies.
Canadian Journal of Forest Research, 13:325-333.
Borden, J. H., D. R. Devlin, and D. R. Miller. 1992. Synomones of two sympatric species
deter attack by the pine engraver, Ips pini (Coleoptera, Scolytidae). Canadian Journal
of Forest Research, 22:381-387.
Bright, D. E. 1976. The insects and arachnids of Canada Part 2: the bark beetles of Canada
and Alaska: Coleoptera, Scolytidae. Canada Department of Agriculture, Ottawa,
Ontario.
Campbell, S. A. and J. H. Borden. 2006. Integration of visual and olfactory cues of hosts and
non-hosts by three bark beetles (Coleoptera: Scolytidae). Ecological Entomology,
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Carroll, A. L., B. H. Aukema, K. F. Raffa, G. D. Smith, and B. S. Lindgren. 2006. Mountain
pine beetle outbreak development: the endemic - incipient transition. Mountain Pine
Beetle Initiative, Project 1.03 Working Paper, Natural Resources Canada, Canadian
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Cognato, A. I. 2000. Phylogenetic analysis reveals new genus of Ipini bark beetle
(Scolytidae). Annals of the Entomological Society of America, 93:362-366.
Cognato, A. I. and A. P. Vogler. 2001. Exploring data interaction and nucleotide alignment
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Furniss, M. M. and R. L. Livingston. 1979. Inhibition by ipsenol of pine engraver attraction
in northern Idaho. Environmental Entomology, 8:369-372.
Hunt, D. W. A. and J. H. Borden. 1988. Response of mountain pine beetle, Dendroctonus
ponderosae Hopkins, and pine engraver, Ips pini (Say) to ipsdienol in southwestern
British Columbia. Journal of Chemical Ecology, 14:277-293.
Lanier, G. N. and D. L. Wood. 1968. Controlled mating, karyology, morphology, and sexratio in the Dendroctonus ponderosae complex. Annals of the Entomological Society
of America, 61:517-526.
Lindgren, B. S. 1983. A multiple funnel trap for scolytid beetles (Coleoptera). The Canadian
Entomologist, 115:229-302.
Linton, D. 2005. A colour key to the bark beetles in lodgepole pine forests in British
Columbia (Draft Copy). 10.
Lyon, R. L. 1958. A useful secondary sex character in Dendroctonus bark beetles. The
Canadian Entomologist, 90:582-584.
Miller, D. L. and J. H. Borden. 2003. Responses of Ips pini (Say), Pityogenes knechteli
Swaine and associated beetles (Coleoptera) to host monoterpenes in stands of
lodgepole pine Journal of Entomological Science, 38:602-611.
Miller, D. L., J. H. Borden, G. G. S. King, and K. N. Slessor. 1991. Ipsenol: An aggregation
pheromone for Ips latidens (LeConte) (Coleoptera: Scolytidae). Journal of Chemical
Ecology, 17:1517-1527.
Miller, D. L., J. H. Borden, and B. S. Lindgren. 2005. Dose-dependent pheromone responses
of Ips pini, Orthotomicus latidens (Coleoptera: Scolytidae), and associates in stands
of lodgepole pine Environmental Entomology, 34:591-597.
Miller, D. R. and J. H. Borden. 1990. The use of monoterpenes as kairomones by Ips latidens
(LeConte) (Coleoptera: Scolytidae). The Canadian Entomologist, 122:301-307.
Miller, D. R. and J. H. Borden. 1992. (S)-(+)-Ipsdienol: Interspecific inhibition of Ips
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Nordlund, D. A. 1981. Semiochemicals: a review of the terminology. Pages 13-28 in D. A.
Nordlund, R. L. Jones, and W. J. Lewis, editors. Semiochemicals: their role in pest
control. Wiley & Sons, New York.
Person, H. L. 1931. Theory in explanation of the selection of certain trees by the western pine
beetle. Journal of Forestry, 29:696-699.
Poland, T. M. and J. H. Borden. 1994. Semiochemical-based communication in interspecific
interactions between Ips pini (Say) and Pityogenes knechteli (Swaine) (Coleoptera:
Scolytidae). The Canadian Entomologist, 126:269-276.
Poland, T. M. and J. H. Borden. 1997. Attraction of a bark beetle predator, Thanasimus
undulatus (Coleoptera: Cleridae), to pheromones of the spruce beetle and two
secondary bark beetles (Coleoptera: Scolytidae). Journal of the Entomological
Society of British Columbia, 94:35-41.
Pureswaran, D. S. and J. H. Borden. 2005. Primary attraction and kairomonal host
discrimination in three species of Dendroctonus (Coleoptera: Scolytidae).
Agricultural and Forest Entomology, 7:219-230.
Pureswaran, D. S., R. Gries, and J. H. Borden. 2004. Antennal responses of four species of
tree-killing bark beetles (Coleoptera: Scolytidae) to volatiles collected from beetles,
and their host and nonhost conifers. Chemoecology, 14:59-66.
Raffa, K. F. 1988. The mountain pine beetle in western North America. Pages 506-530 in A.
A. Berryman, editor. Dynamics of forest insect populations: Patterns, causes and
implications. Plenum, New York, NY.
Raffa, K. F. and A. A. Berryman. 1982. Physiological differences between lodgepole pines
resistant and susceptible to the mountain pine beetle and associated microorganisms.
Environmental Entomology, 11:486-492.
Raffa, K. F., T. W. Phillips, and S. M. Salom. 1993. Strategies and mechanisms of host
colonization by bark beetles. Pages 103-128 in T. D. Schowalter and G. M. Filip,
editors. Beetle-pathogen interactions in conifer forests. Academic Press, New York.
Safranyik, L. and A. L. Carroll. 2006. The biology and epidemiology of the mountain pine
beetle in lodgepole pine forests. Pages 3-66 in L. Safranyik and B. Wilson, editors.
The Mountain Pine Beetle: a Synthesis of its Biology, Management and Impacts on

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Lodgepole Pine. Natural Resources Canada, Canadian Forest Service, Pacific
Forestry Centre, Victoria, BC.
Safranyik, L. and D. A. Linton. 1983. Brood production by three spp. of Dendroctonus
(Coleoptera: Scolytidae) in bolts of host and non-host trees. Journal of the
Entomological Society of British Columbia, 80:10-13.
Safranyik, L., D. A. Linton, and T. L. Shore. 2000. Temporal and vertical displacement of
bark beetles (Coleoptera: Scolytidae) captured in barrier traps at baited and unbaited
lodgepole pines the year following attack by the mountain pine beetle. The Canadian
Entomologist, 132:799-810.
Safranyik, L., T. L. Shore, D. A. Linton, and L. Rankin. 1999. Effects of induced competitive
interactions with secondary bark beetle species on establishment and survival of
mountain pine beetle broods in lodgepole pine. Information Report BC-X-384,
Pacific Forestry Centre, Canadian Forest Service, Victoria, BC.
Savoie, A., J. H. Borden, J. Pierce, H.D., R. Gries, and G. Gries. 1998. Aggregation
pheromone of Pityogenes knechteli and semiochemical-based interactions with three
other bark beetles. Journal of Chemical Ecology, 24:321-337.
Sokal, R. R. and F. J. Rohlf. 1995. Biometry. 3rd edition. W.H. Freeman and Company, San
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Bacon, Needham Heights, MA.
Vite, J. P. and R. I. Gara. 1962. Volatile attractants from ponderosa pine attacked by bark
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Vite, J. P. and G. B. Pitman. 1968. Bark beetle aggregation: effects of feeding on the release
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Wallin, K. F. and K. F. Raffa. 2000. Influences of host chemicals and internal physiology on
the multiple steps of postlanding host acceptance behavior of Ips pini (Coleoptera:
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115

CHAPTER 5

GENERAL CONCLUSIONS

This group of studies was undertaken to quantify the potential effects of the presence of a
secondary bark beetle, P. mexicanus, on endemic D. ponderosae

cohabiting weakened

lodgepole pine trees. To properly evaluate this interaction, the life history of P. mexicanus
required elucidation (Chapter 2). Prior work had examined a number of life history traits of
P. mexicanus, but these studies involved different host-tree species in different climatic
regions (Trimble 1924, Schwerdtfeger 1956, Struble 1961, Fox et al. 1990). Three of these
studies were conducted on populations breeding in Monterey pine in California and one in
Mexican white pine in Guatemala.

The populations of P. mexicanus that I studied in

lodgepole pine forests of south-central British Columbia were found to be univoltine, but
developed more rapidly than Californian or Central American beetles (Schwerdtfeger 1956).
Beetles in lodgepole pine laid substantially fewer eggs than in either of the other tree species
(Trimble 1924, Schwerdtfeger 1956), a likely result of different climatic and host conditions,
since beetles in BC may have stopped egg laying early in the summer to allow offspring to
prepare for overwintering.

The attack behaviour of northern populations also differed

significantly. In BC, the beetles attacked and overwintered exclusively in the lower portion
of the bole whereas southern beetles tend to colonize the whole tree (Trimble 1924,
Schwerdtfeger 1956). This may be an adaptation to diminish the effect of cold-induced
mortality by confining attacks to the portion of the bole that is covered by snow during the

116

coldest periods of the winter. Southern US and Central American beetles also tended to
make use of downed trees and slash, behaviours not noted during these investigations.
Northern populations were similar to southern ones in terms of polygyny, ovipositional
characteristics (i.e., # of eggs per egg niche) and development rate. In Canada, P. mexicanus
is a common forest insect that is not an economic threat to the forestry industry. It has,
however; been a concern in California because of its ability to disseminate the pitch canker
fungus in Monterey pine (Fox et al. 1991). Moribund trees are typical hosts for P. mexicanus
in Canada and, through removal of suppressed and/or damaged trees, P. mexicanus likely
functions to maintain lodgepole pine stand health.
The second study examined interactions between D. ponderosae and P. mexicanus
(Chapter 3). Evidence from Carroll et al. (2006a) suggested that P. mexicanus may somehow
improve host conditions for endemic D. ponderosae.

Amman and Schmitz (1988) also

proposed that endemic D. ponderosae may benefit from cohabitation with secondary bark
beetles. My study quantified this phenomenon, by showing that D. ponderosae attacking
hosts previously occupied by P. mexicanus did so in greater densities than in trees where P.
mexicanus was absent, with no loss in offspring size from crowding.

D. ponderosae

offspring emerging from P. mexicanus-infested trees developed faster than in uninfested
trees, suggesting that endemic D. ponderosae

attacking trees previously infested by P.

mexicanus have access to better resources than they do when attacking trees alone.
The final study was conducted based on predictions that D. ponderosae searching for
hosts would actively seek trees infested with P. mexicanus.

However, results of trapping

experiments (Chapter 4) showed that D. ponderosae was not attracted to P. mexicanus
attractant pheromone or to bolts of lodgepole pine containing feeding P. mexicanus.

117

Since

secondary attraction to P. mexicanus attractant pheromones did not occur, it appears that
other volatile phytochemicals from living trees play a dominant role in host selection during
endemic D. ponderosae dispersal.
The principal results of my research provide evidence for population maintenance by an
eruptive herbivore when in a non-eruptive population state. Furthermore, the interaction
between D. ponderosae

and P. mexicanus suggests that management of P. mexicanus

populations may contribute to a reduction of endemic D. ponderosae on the landscape. Both
species are a natural component of a healthy lodgepole pine ecosystem and when D.
ponderosae is in endemic populations, both species play a role in reducing the number of
standing unthrifty trees, keeping stands sanitized. However, the development of over-mature
forests (Taylor and Carroll 2004), coupled with climate change trends (Carroll et al. 2004),
have resulted in the most devastating D. ponderosae outbreak in recorded history (e.g. Kurz
et al. 2008). Management in the mid-1990's for P. mexicanus may not have prevented this
D. ponderosae outbreak, but the potential of future epidemics may be reduced by observing
secondary bark beetle populations (Carroll et al. 2006a) and reacting accordingly through
either direct (reduction of D. ponderosae populations) or indirect (reduction of susceptible
trees) control methods (Carroll et al. 2006b).
Another important aspect of my work is the determination of positive interactions
occurring between insect species. This example of facilitation in the insect world supports
the work of Kaplan and Denno (2007), who advocate for the adoption of a new paradigm to
account for positive interactions between insect species within a competition framework.
Understanding that P. mexicanus improves host conditions for endemic D. ponderosae leads
to the question of what role the other secondary species commonly associated with P.

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mexicanus (Orthotomicus latidens, Hylurgops spp.) play in D. ponderosae

population

dynamics. Further work to examine this complex trophic interaction would be needed to
fully understand the bark beetle community ecology in weakened lodgepole pine trees.

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