United States
Department of
Agriculture
Forest Service
Pacific Southwest
Research Station
General Technical Report
PSW-GTR-138
Proceedings of a Workshop
on Bark Beetle Genetics:
Current Status of Research
May 17-18, 1992, Berkeley, California
Hayes, Jane L.; Robertson, Jacqueline L, tech. coords. 1992. Proceedings of a workshop on bark beetle genetics: current
status of research. May 17-18, 1992, Berkeley, California. Gen. Tech. Rep. PSW-GTR-138. Albany, CA: Pacific
Southwest Research Station, Forest Service, U.S. Department of Agriculture; 31 p.
The Proceedings reports the results of a workshop focusing on the topic of bark beetle genetics. The workshop evolved
because of the growing interest in this relatively unexplored area of bark beetle research. Workshop participants submitted brief
descriptions of their views of the current status of bark beetle genetic research and needs for the future. Contributions vary from
broad overviews to detailed descriptions of specific projects. A common theme is the potential for rapid advances in this area of
research given recent technological advances in the field of genetics. Also included is an extensive reference list.
Retrieval Terms: Dendroctonus species, Ips species, scolytid genetics, taxonomy
Dedication
We dedicate this report to the memory of Gerald Lanier, a valued colleague to all who share an interest in bark beetle
research, and a dear friend of many of the workshop participants. It is particularly fitting that we pay tribute to Gerry for his
pioneering efforts in bark beetle genetics and in recognition of his lasting contributions in the area of scolytid systematics.
Technical Coordinators:
Jane L. Hayes is a research entomologist and Project Leader of the Ecology, Behavior, and Management of Southern Pine Beetle
Unit of the Southern Forest Experiment Station, USDA Forest Service, 2500 Shreveport Highway, Pineville, LA 71360.
Jacqueline L. Robertson is a research entomologist and Project Leader of the Insect Biochemistry, Genetics, and Biometry Unit
of the Pacific Southwest Research Station, 800 Buchanan Street, Albany, CA 94710.
Acknowledgments:
We thank Sue Moore for handling the innumerable workshop arrangements and mailings, and for assembling the draft
report. We are also grateful to David Wood and the School of Forestry, University of California, Berkeley, for accommodating the
Workshop on Day 1 and to Garland Mason and the Pacific Southwest Research Station for accommodations on Day 2. We extend
special thanks to Maureen Stanton for providing the "outsider's" point of view and invaluable insight during the course of the
Workshop.
Publisher:
Pacific Southwest Research Station
Albany, California
(Mailing address: P.O. Box 245, Berkeley, California 94701-0245
Telephone: 510-559-6300)
November 1992
Proceedings of a Workshop on Bark Beetle
Genetics: Current Status of Research
May 17-18, 1992, Berkeley, California
Jane L. Hayes and Jacqueline L. Robertson, Technical Coordinators
CONTENTS
Preface....................................................................................................................................................iii An (Ecologically Biased) View of the Current Status of Bark Beetle Genetics and Future Research Needs ............................................................................................................... 1 Jane L. Hayes and Jacqueline L. Robertson
Qualitative Genetics of Mountain Pine Beetle in Central Oregon .......................................................... 3 Lula E. Greene
Biochemical Indicators of Population Status of Pine Bark Beetles ........................................................ 4 M. Carol Alosi and Jacqueline L Robertson
Spatial Analysis in Bark Beetle Research ............................................................................................... 5 Haiganoush K. Preisler
Bark Beetle Genetics .............................................................................................................................. 6 Wyatt W. Anderson and C. Wayne Berisford
Isozyme Studies of Bark Beetle Population Genetics and Systematics .................................................. 7 Molly W. Stock, Gene D. Amman, and Barbara J. Bentz
Biosystematics of Ips mexicanus and Ips plastographus (Coleoptera: Scolytidae) and Their Fungal Symbionts .................................................................................................................. 10 David L Wood, Andrew J. Storer, Thomas R. Gordon, Steven J. Seybold, and Marion Page
The Status of Scolytid Genetics―A Reductionist's View .................................................................... 12 Steven J. Seybold Genetic Characters for Bark Beetle Phylogenies .................................................................................. 14 James H. Cane
Cuticular Hydrocarbon Research .......................................................................................................... 16 Marion Page
CONTENTS
Bark Beetle Genetics―An Overview ................................................................................................... 17 Stephen Teale and Barbara Hager Current Status and Critical Research Needs in Scolytid Genetics ........................................................ 18 Kenneth F. Raffa
Genetic Basis of Semiochemically Mediated Bark Beetle-Predator Coevolution:
Implications for Developing Mass-Trapping Technologies ............................................................ 20 Robert A. Haack, Daniel A. Herms, Bruce D. Ayres, Matthew P. Ayres, and M.L Doozie Snider
Population Genetics of Spruce Bark Beetle Ips typographus (Col., Scolytidae) and Related Ips Species.................................................................................................................... 22 Christian Stauffer, Renate Leitinger, and Erwin Fuehrer Novel Bark Beetle Research Possible with New Genetic Techniques .................................................. 23 Ken Hobson and Owain Edwards
Systematics Research ............................................................................................................................ 25 Donald E. Bright
References ............................................................................................................................................. 26 Workshop Participants .......................................................................................................................... 31 ii
USDA Forest Service Gen. Tech. Rep. PSW-138. 1992.
Preface
This report is the result of a workshop focusing on the
topic of bark beetle genetics held May 17-18, 1992, in Berkeley, Calif. Bark beetles play a significant role in pine forest
ecosystems throughout the world; this group of insects often
causes severe economic damage and loss of wildlife habitat. In
general, bark beetles threaten forest health in the short term. In
the long term, however, they have served as natural agents to
thin overcrowded stands in pine forests. Although bark beetles
have been the focus of intensive research efforts, relatively
little effort has been devoted to understanding bark beetle
genetics. Our Workshop evolved because of (and reflects) the
growing interest in this relatively unexplored area of bark
beetle research. The objectives of this workshop were fourfold:
1. To promote interaction among researchers and exchange
of ideas;
2. To foster collaboration;
3. To summarize the state of knowledge; and
4. To identify research needs for the future.
To accomplish the last two goals and to set the stage for
discussions, participants were asked before the workshop to
submit brief descriptions of their views of the current status of
bark beetle genetics research and the needs for the future. How
this assignment was fulfilled was as diverse as the participants'
approaches to the topic. Contributions varied from broad overviews to detailed descriptions of specific projects. As a whole,
these statements provide an insightful description of the state of
knowledge and research in the area of bark beetle genetics.
Students of bark beetle biology will find this review and
the combined reference list a valuable source of current information on this topic. Significant advances have been made in
the available technology in the decade since the topic of bark
beetle genetics was first reviewed (Mitton and Sturgeon 1982).
Our knowledge of bark beetle biology has advanced slowly;
however, as many contributors indicate, the potential for more
rapid advances is imminent.
During the two-day workshop, participants gave brief presentations on the status of their ongoing research efforts. A
general group discussion of research needs and goals followed.
A topic of considerable concern to the group was the lack of
recognition of the need and support for a continuous effort in
the area of systematics. Declining support for systematics work
has resulted in lack of training and career opportunities for the
next generation of skilled systematicists. Without a commitment by academic and funding institutions to support this
fundamental research, we face a future without advances in this
essential specialty area.
USDA Forest Service Gen. Tech. Rep. PSW-138. 1992.
To structure our general discussions, priority research needs
were identified by participants. The group recommended that
research be initiated or continued on these 15 topics:
• understanding ancestry of scolytid species
• biosystematics of important species, including symbionts
• monitoring endemic vs. epidemic conditions, and understanding processes that trigger changes
• spatial statistical techniques for mapping geographic patterns of gene frequency (and changes in relation to population dynamics of bark beetles)
• interaction between phylogeography and population growth
status
• genetics of variation in pheromone systems
• understanding individual variation in pheromone and
other qualitative characteristics of individual beetles
• genetics of variation in interaction with natural enemies
genetics of cold-hardiness
• understanding selection pressure (i.e., stage and mechanism)
• artificial rearing techniques for major species (especially
Dendroctonus spp.)
• molecular population analyses that focus on phylogeography, gene flow, paternity
• gene expression (description of genome, protein and
enzyme level, transcription) for key traits
• survey of polymorphisms identified from different analytical techniques
• applying information from other well-researched systems (e.g., Drosophila, Tribolium) to bark beetle
genetics
Because items on this list were intentionally expressed in
relatively broad terms and covered multiple levels of analysis
(i.e., molecular to population to phylogenic levels), we did not
attempt to refine or prioritize this general list any further.
Instead, we sought to identify more specific questions and
technological advances necessary to address these questions.
This discussion is summarized in table 1.
Through this report we seek to continue the productive
efforts described at this workshop: to share information and
ideas, and to encourage the pursuits of others with interests in
the challenges of research into genetics of bark beetles. All
participants welcome further inquiries―addresses and telephone numbers are provided. We hope that this publication will
aid those seeking to learn more about the topic, and we encourage efforts in studying new topics that are not mentioned here.
Jane L. Hayes
Jacqueline L. Robertson
Technical Coordinators
iii
Table 1―Specific questions and technological advances necessary to address priority research needs.
Levels of Analysis
Specific Questions
Techniques
1. Molecular genetics
a. Study patterns of expression for genes involved in
semiochemical pathways, e.g., detoxification, pheromones.
Determine more biochemical pathways.
b. Study genetics of development, e.g., stage-specific gene
(adult vs. juvenile, overwintered vs. pre-winter, etc.)
Develop DNA probes associated with key
developmental events.
c. Screen for specific patterns of expression associated with
dispersal vs. feeding adults; cold hardiness; different seasonal
cohorts; host-tree species.
Develop systems to describe transformation of
trees or beetles.
d. Development of molecular markers for ecological genetics.
RAPDS, VNTR loci, mtDNA
e. Studies of sex ratio anomalies (e.g., in Ips spp. etc.) (see Level 2)
2. Quantitative genetics of
ecologically important
traits.
Once appropriate rearing techniques are established for
controlling environmental and genetic background, focus on
traits, establish role of genes and environment as influencing
variations among individuals.
Need rearing techniques for studying similarity
within "constant" environment and for
manipulation of environment.
a. Semiochemicals, correlations between them (production
and response profiles)
Artificial media vs. outdoor trees, logs.
b. Cuticular hydrocarbons, e.g., changes during development
and relationship with host compounds
Explore Kermit Ritland's technique for h2
estimation based on electrophoretic data.
c. Cold-hardiness: examine patterns of expression in larvae
d. Microbial interactions: involvement with overcoming host
defenses, increased attack success, pheromone synthesis
e. Morphological/physiological traits (e.g., elytra loadings, lipid
contents)
Fluctuating asymmetry analysis.
f. Genetics of host selection
g. Trees: genetic and environmental determinants of susceptibility
3. Selection and population dynamics
a. Stages and sources of selection in mortality during the life cycle
due to host stress and predation and parasitism at various stages.
b. Dynamics of beetle interactions (e.g., competition) as a function
of population phase in the context of tree interactions.
c. Do patterns of selection differ between population phase? (e.g., rvs. k-type)
Rearing techniques.
d. Potential for selection experiments for key characterization
(e.g., thick vs. thin phloem) (See level 4c and d).
dispersal: density and "phase"-dependency
Techniques for following cohorts of beetles
below the bark (acoustics, radiography, infrared detection)
population structure: “phase”-dependency
continued
iv
USDA Forest Service Gen. Tech. Rep. PSW-138. 1992.
Table 1, continued
Levels of Analysis
Specific Questions
Techniques
4. Population structure and
phylogeography
a. Calculate hierarchical F-statistics: between geographic regions,
between populations, within populations (between host plants,
pheromone "groups", emergence groups, families) (See Level 5a)
Also relevant to Biosystematics
b. Exploit existing data sets: Dendroctonus spp., I. pini (requires
systematic sampling and coordination between groups).
Spatial statistics development.
c. Spatial analysis, interaction with GIS data now available.
d. Dependence of population structure on:
1. local density
2. stage of population growth (See Level 3d)
e. Direct analysis of dispersal, gene flow in relation to elemental
labelling rare or unique alleles, mutants (beetles or symbionts)
(See Level 3d)
5. Phylogenetic reconstruction and
biosystematics.
a. Species taxonomy of bark beetles is relatively complete, but
need phylogenetic reconstruction.
b. Focus on Ips, Dendroctonus, especially 1) species relationships
within these groups, and 2) ancestral relationships between them.
Elemental labelling
(Mark-Release-Recapture).
Develop discrete characters for:
1. morphometrics
2. mtDNA, nuclear markers
3. sequence data on rib.
DNA (larger scale)
1. develop discrete characters for cladistic analysis,
2. continue applying isozymes.
c. Based on phylogeny, consider evolution of following traits:
Apply new methods of phylogeny
reconstruction and analysis.
1. pheromone production and composition (semiochemistry),
2. behavior, including isolation mechanisms,
3. aggregation response, and
4. cuticular hydrocarbons
USDA Forest Service Gen. Tech. Rep. PSW-138. 1992.
v
An (Ecologically Biased) View of the Current Status of Bark Beetle Genetics
and Future Research Needs1
Jane L. Hayes and Jacqueline L. Robertson2
Bark beetles are commonly considered the most destructive insect pests in pine forests in North America. Despite the
relatively intense interest in this group because of their pest
status and uniquely complex ecological and evolutionary relationships, many aspects of their biology, including genetics, are
poorly understood. Most genetic research on bark beetles has
concerned inferences about the evolution of species, especially
the phylogeny of important genera (e.g., Cane and others 1990b,
Lanier 1967, 1981, Lewis and Cane 1990b, Mitton and Sturgeon 1982). Relatively little emphasis has been placed on
population genetics, particularly on genetic variation within
and among populations over time and space (reviewed in Milton
and Sturgeon 1982, and ref. below).
The natural history of this group is probably the single
most significant deterrent to studies of population genetics,
because sampling and laboratory rearing is either difficult or
impossible. Improved sampling of adults has paralleled advances in semiochemical research, which in turn have prompted
and permitted recent investigations into variation in the production and response to semiochemicals at the population level
(e.g., Birgersson and others 1984, Herms and others 1991,
Miller and others 1989, Teale and Lanier 1991). However,
most of the life cycle is spent under bark, limiting observations
and necessitating destructive sampling of brood in order to
obtain requisite estimates of fitness (e.g., fecundity, age to first
reproduction). Most species are relatively intractable in the
laboratory, thus precluding classical genetic studies, estimations of heritability, and development of molecular genetics
techniques based on phenotypic studies. It is not surprising that
recent advances in our understanding of both molecular and
phenotypic variation are concentrated in the more easily reared
species such as Ips spp. (e.g., Cane and others 1990b, Gast
1987, Lanier and others 1972, 1980, Miller and others 1989,
Piston and Lanier 1974, Stauffer and others 1992a; but see
Langor and Spence 1991, Teale 1990, Teale and Lanier 1991).
The newest molecular techniques (e.g., Restriction Fragment
Length Polymorphism and Randomly Amplified Polymorphic
DNA) may mitigate sampling deficits, are likely to lead to gene
mapping, and vastly accelerate the molecular exploration of
genetic variation. Even with these advanced techniques, however, dramatic advances depend on novel application and well1
An abbreviated version of this paper was presented at the Bark Beetle
Genetics Workshop, May 17-18, 1992, Berkeley, California.
2
Supervisory Research. Entomologist, Southern Forest Experiment Station, USDA Forest Service, 2500 Shreveport Highway, Pineville, LA 71360,
and Supervisory Research Entomologist, Pacific Southwest Research Station,
USDA Forest Service, Albany, Calif.
USDA Forest Service Gen. Tech. Rep. PSW-138. 1992.
designed experimentation. Ultimately, these results must be
tied to ecologically relevant characteristics (phenotypic expression).
Emphasizing an ecological genetics approach, our (other
team members include Alosi, Greene, and Preisler, see contributions this proceedings) investigations have involved estimates of both genotypic and phenotypic variation within and
among bark beetle populations and communities, as well as
across ecologically similar species (mountain, western, and
southern pine beetles). Our research includes aspects of the
technology such as starch gel electrophoresis that were used in
the earlier studies of differences between populations (e.g.,
Roberds and others 1987, Stock and others 1979, 1984). Besides genetic inferences made by electrophoresis, we are examining differences in the levels of the key enzymes that help to
adapt beetles to the presence of toxic secondary chemicals in
their host trees (see Alosi this proceedings). In contrast with
previous gross examinations of terpene levels in host trees
(Coyne and Lott 1976, Smith 1975)―their primary defense―
we are examining differences in terpene composition by means
of more sensitive analyses to detect and monitor changes consistent with changes in population structure.
To date, information acquired from five years (1985-90,
see Greene this proceedings) of monitoring mountain pine
beetle populations in Central Oregon suggests that patterns of
esterase enzymes detected by electrophoresis have shifted concomitantly with a shift in population status from endemic to
epidemic and back to endemic. In addition, quantities of esterases measured biochemically have changed, even for mountain pine beetles in the same host species. Glutathion S-transferase levels also seem to be involved in the transition. We have
initiated (1991-present) comparable studies with western and
southern pine beetles and begun investigations of phenotypic
variation between and within populations including analyses of
behavioral (pheromone response), physiological (lipid content)
and morphological (fluctuating asymmetry) variation. These
studies represent modifications to previous efforts to quantify
and relate phenotypic variations to genotypic variation among
beetle populations (e.g., Langor and Spence 1991, Sturgeon
and Mitton 1986). For example, morphological variability
within individuals may provide a valuable early guide to changes
in beetle quality. Fluctuating asymmetry (nondirectional deviations from bilateral symmetry) has been shown to be an
effective and inexpensive tool for detecting stress in natural
populations (Leary and Allendorf 1989) and in cultures (Clarke
and McKenzie 1992), because it provides a reasonably direct
measure of developmental stability (Palmer and Strobeck 1986).
Environmental and genetic perturbations could affect fluctuat-
1
ing asymmetry analogously, i.e., as the level of stress of any
kind increases, fluctuating asymmetry and its variability should
increase provided that the stress is sufficiently intense to produce an effect (Parsons 1990a, b).
In addition, in most genetic studies done to date, standard
statistical methods involving chi-square comparisons of observed gene frequencies versus those predicted by HardyWeinberg equilibrium have been used for data analyses. Our
investigations emphasize the use of exploratory statistics to
examine actual numbers rather than frequencies (see Preisler
this proceedings). Uses of these new procedures will permit us
to model patterns in biotic communities of which bark beetles
are a part, to estimate realistic models of species overlap, and to
formulate causal hypotheses about changes in bark beetle populations in transition from endemic to epidemic states.
Priority Research Needs
1. Long-term (multi-generation, multi-season) qualitative
and quantitative genetics studies are needed to detect genetic
changes that result in changes on the population structure.
2. Likewise, changes in the component(s) that comprise
the gene-environment interaction (e.g., host-insect interactions)
2
must be assessed to understand the sources of pest population
variation.
3. Continuous rearing technology (e.g., artificial bark
beetle diet) must be developed for Dendroctonus spp.
Needs in Quantitative Studies
1. Estimate heritabilities of key traits over space and time
2. Identify genetic markers and develop DNA/RNA probes,
estimate frequencies, and study changes in population structure
over space and time
3. Having identified one or more markers that differ constantly with space, time (or both), quantitative investigations
are needed to establish sources of variation (selection pressures), including
a. Biochemical traits: levels of key enzymes such as
esterases, mixed function oxidases, or any other
enzyme that can be directly linked to survival
(fitness)
b. Phenotypic or quantitative traits: the expression of
life history traits (e.g., fecundity, longevity, age to
first reproduction, phenology), or behavioral, physiological or morphological traits.
USDA Forest Service Gen. Tech. Rep. PSW-138. 1992.
Qualitative Genetics of Mountain Pine Beetle in Central Oregon1
Lula E. Greene2
The mountain pine beetle (MPB), Dendroctonus
ponderosae, is a predator of all native and many introduced
pines in western North America (Furniss and Schenk 1969,
McCambridge 1975, Smith and others 1981, Wood 1982). The
most important hosts of MPB are ponderosa, lodgepole, sugar,
and western white pine. Although these pine species occur
sympatrically throughout portions of their ranges, MPB are
rarely found in more than one host species in any given locality.
Along the Front Range in the Colorado Rockies, this bark
beetle is found primarily in ponderosa pine, but throughout the
northern Rockies, its preferred host is lodgepole pine. In
Oregon, the major hosts are both ponderosa and lodgepole
pines, but in California, the major host is sugar pine. Nevertheless, small areas can be found in which this beetle has colonized
two or more hosts at the same time.
No factor in the insect/host interaction influences the life
of an endoparasitic phytophagous insect more than its host
(Diel and Bush 1984). Because plant species differ chemically,
physically, and biologically, they can present markedly different selective environments to insects feeding on them (Bush
1969, Wood 1980). Differential selection pressures provided
by different host species can cause genetic differentiation of an
insect species along host lines. Significant genotype variation
in forest trees (the hosts) have been identified for bark beetles
(Berryman 1972, Smith 1963, Smith 1972). Host species affect
emergence, egg gallery characteristics, fecundity, development,
and mortality of MPB (Amman 1982, Amman and Cole 1983,
Amman and Pasek 1986, Schmid 1972).
Sturgeon and Robertson (1985) measured microsomal
polysubstrate monooxygenase (PSMO) activity in western pine
beetle from ponderosa pine and in MPB from both lodgepole
and ponderosa pines. Although they detected no significant
difference in the PSMO activity between the two beetle species, PSMO activity in MPB from ponderosa pine was significantly lower than in MPB from lodgepole pine. These results
suggest that MPB in the two different host species might have
qualities that vary with respect to their ecological genetics.
Isozyme electrophoresis is another means that can be used
to track changes in beetle quality; in such studies, the genetic
structure of insect populations is used to measure genetic variation. Data from electrophoretic surveys have provided direct
measures of genetic variation within and genetic similarity
among populations (Stock and others 1978). Stock and associates (1984) have surveyed much of the northern range of the
MPB for electrophoretically detectable genetic variation. Beetles
collected from populations in Oregon, Washington, Idaho, and
Montana could be differentiated at two enzyme loci, although
1
An abbreviated version of this paper was presented at the Bark Beetle
Genetics Workshop, May 17-18, 1992, Berkeley, California.
2
Research Entomologist, Pacific Southwest Research Station, USDA
Forest Service, 800 Buchanan Street, Albany, CA 94710
USDA Forest Service Gen. Tech. Rep. PSW-138. 1992.
similarity coefficients suggested that they were closely related
(Stock and Guenther 1979). Coastal and inland populations of
MPB collected from varieties of lodgepole pine showed significant differentiation (Stock and others 1978). In Utah, geographically separated populations of MPB from the same host
were more similar than adjacent populations of MPB in diffeent hosts (Stock and Amman 1980). These studies showed that
there is great genetic variation within and among the populations of MPB in the same host in addition to variation due to
geography. Host tree species is implicated as a factor in
maintaining differences among the MPB populations (Stock
and Amman 1985). However, the different host trees were not
sampled at the same site, so the significance of host variation as
a factor cannot be separated from natural variation and geographic variation.
Studies by Sturgeon and Mitton (1986) and Langor (1989)
indicate that significant genetic divergence does exist in cooccurring populations of MPB in different hosts at the same
locality. These differences, they conclude, can be attributed to
either selection, substructuring populations of MPB according
to host, or nonrandom mating in the hosts' population. Either
of these events could lead to host-race formation, and ultimately, to speciation.
To gather more information to clarify insect/host interactions, we initiated a 10-year project on the Deschutes National
Forest in central Oregon in 1985. One of our objectives is to
determine the effects of secondary plant chemicals on MPB
infesting five host trees over time and space. This should aid in
developing a basic understanding of host-induced genetic changes
in bark beetles. Our long-term study will enable us to monitor
changes in allelic frequencies and genotype structuring of populations for trends that indicate changes in the population status
(endemic to epidemic).
At present, we are analyzing data from the first five years
of our study. Esterase enzymes are involved in the insect's
ability to withstand toxins in its environment and its ability to
survive. The esterase enzymes reveal four loci and show the
most genetic variation. At the beta-esterase 1 locus, there is
decided preference for heterozygotes, with one heterozygote
genotype having a frequency that ranges 0.720 to 1.00 in 3 out
of 39 samplings. This locus seems promising as a diagnostic
indicator of host preference and perhaps, population status.
Although our present focus is on MPB, information gained
can provide insights for research into other polyphagous insects
and other insects that persist at low levels for long periods of
time and suddenly explode to devastate entire forests.
No one biotechnology investigatory tool presently available can provide all of the information required to satisfy the
needs of entomologists, population geneticists, and systematicists,
but several used in concert can validate or bring a greater degree
of credibility to the information obtained through their use.
3
Biochemical Indicators of Population Status of Pine Bark Beetles1
M. Carol Alosi and Jacqueline L. Robertson2
Background
The processes by which insects cope with toxic chemicals
in their environment are commonly classified as class 1 and 2
reactions (Dauterman and Hodgson 1978). Class 1 reactions
include oxidations, reductions, and hydrolyses. By these processes, a hydrophilic group is added onto the foreign molecule
and the molecule is then eliminated. Class 2 reactions consist of
conjugations, i.e., glutathione conjugations, glucoside formation, and amino acid conjugation.
Sturgeon and Robertson (1985) examined the comparative
activity of polysubstrate monooxygenases (PSMO = mixed
function oxidases) of mountain pine beetle (MPB) in lodgepole
pine (the preferred host) and ponderosa pine (alternate host).
PSMO activity of western pine beetle (WPB) in ponderosa pine
was compared to that of MPB in lodgepole and ponderosa pine.
Results of this investigation, done with MPB in an endemic
population in the Deschutes National Forest in 1982, suggested
that host preference is reflected in levels of this detoxification
enzyme. By 1985, the Deschutes population reached epidemic
levels; MPB infested five pine species (lodgepole, ponderosa,
western white, sugar, and whitebark).
For the past seven years, we have collected emerging and
attacking MPB from a number of locations in the vicinity of La
Pine, Oregon (Deschutes and Winema National Forests) for
biochemical and qualitative genetic studies. These studies have
been done to test the hypothesis that population status of this
bark beetle species is associated with biochemical diversity
(i.e., differences in class 1 and 2 reactions are associated with
host tree species) that varies over time. We have quantified
levels of PSMO's and esterases in class 1, and glutathione Stransferase (GST) in class 2. We have detected changes in all
three enzyme systems. During 1991, we expanded our attention
to western and southern pine beetles, and to various Ips species.
GST was chosen for study at the molecular level because
methods have been developed for various Dipteran species and
are being developed for a tortricid leafroller, Epiphyas postvittana,
in New Zealand. GST are enzymes that enable cells to detoxify
harmful compounds. Various forms of GSTs, representing allelic variation and sometimes complex gene families, are found
in all eucaryotic organisms. There is some evidence that insects
utilize the GST-enzyme systems to detoxify plant allelochemicals
that they encounter during herbivory (Wadleigh and Yu 1987).
In bark beetles, levels of GST may vary with host and with
population status (endemic versus epidemic) (Robertson and
others in progress). These observations have led us to propose
that a study of the structure and expression of GST genes in
beetles will provide us with molecular indicators of population
status. In addition, studies of GST genes in beetles will provide
insight into the mechanism of metabolic resistance in polyphagous insects.
Method of Study
The immediate goal of our investigation is to isolate cDNA
clones of expressed GST genes from mountain, western, and
southern bark beetles. In order to do this we will construct
cDNA libraries of all three beetles and screen the libraries with
heterologous probes derived from Drosophila and lepidopteran
cDNAs; alternatively, GST clones may be identified using
lepidopteran-derived GST antibodies. Positive GST isolates
from the libraries will provide us with homologous probes.
Homologous probes are necessary to attain the sensitivity necessary to detect allelic variations of genes and to identify (for
example) evidence of gene amplification in relationship to
insect population dynamics. Presently, our work on this project
includes developing methodology for purification of RNA and
DNA from bark beetles. In addition, we are conducting hybridization studies between a Drosophila GST cDNA probe and
bark beetle RNA and DNA. These preliminary studies provide
us with information useful for establishing experimental conditions for screening beetle cDNA libraries.
1
An abbreviated version of this paper was presented at the Bark Beetle
Genetics Workshop, May 11-18, 1992, Berkeley, California.
2
Research Molecular Geneticist and Supervisory Research Entomologist,
respectively, Pacific Southwest Research Station, USDA Forest Service, 800
Buchanan Street, Albany, CA 94710.
4
USDA Forest Service Gen. Tech. Rep. PSW-138. 1992.
Spatial Analysis in Bark Beetle Research1
Haiganoush K. Preisler2
Genetic and ecological information about bark beetle populations most often consists of data collected over space and
time. However, when these data are analyzed, little attention is
usually given to relationships operating in the spatial dimension. One of the reasons for the small number of studies that
have examined the spatial aspects of bark beetle populations is
the lack of statistical tools for the analysis of spatial-temporal
data.
One statistical technique that can be adapted for the analysis of spatial data is the methodology of generalized linear
regression models. This method was used in a series of articles
by Mitchell and Preisler (1991, Preisler [in press], Preisler and
Mitchell [unpublished]). By fitting an auto-logistic model, we
were able to study the factors affecting the probabilities of trees
being attacked by mountain pine beetle (MPB). Those factors
included characteristics of individual trees (such as tree size
and vigor), in addition to the locations of each tree within a
stand. We have found that the framework of generalized linear
models is quite flexible and very helpful as an aid for understanding spatial patterns of discrete data points and for relating
those patterns to specific environmental factors or tree characteristics.
The second technique that might be useful for the analysis
of spatial data is the methodology of Geographic Information
Systems (GIS). GIS can be used to store spatial and temporal
information on insect populations. Genetic information can
then be added to the system and used to analyze changes over
space and time; interactions among and within various factors
can be investigated. Although GIS allows entomologists to
compile spatial data, deriving inferences from these data is still
extremely difficult because appropriate statistical techniques
are lacking. A few references that discuss statistical techniques
for analyzing geographic information and gene-frequency maps
are Brillinger (1990), Kemp and others (1989), Liebhold and
Elkinton (1989), Piazza and others (1981). Further statistical
research is needed in this area.
1
An abbreviated version of this paper was presented at the Bark Beetle
Genetics Workshop, May 12-18, 1992, Berkeley, California
2
Research Statistician, Pacific Southwest Research Station, USDA Forest
Service, 800 Buchanan Street, Albany, CA 94710.
USDA Forest Service Gen. Tech. Rep. PSW-138. 1992.
5
Bark Beetle Genetics1
Wyatt W. Anderson and C. Wayne Berisford2
1. Previous work
A. Protein electrophoresis the basic technique for most
work.
B. Genetic variability has been characterized.
C. Population differences demonstrated and analyzed. D. Genetic population structure assessed. E. Basic molecular systematics explored; phylogenetic trees developed.
F. Attempts to link protein loci to phenotypic characters
such as behavior.
2. Our work at University of Georgia
A. Starch gel electrophoresis of Dendroctonus and Ips
populations, in terms of genetic variability, population structure, and population differences.
B. mtDNA studies, using restriction site analysis and DNA
sequencing.
(1) Cloned part of mitochondrial DNA
(2) Completed restriction site analysis of SPB populations
(3) Begun sequencing of several mtDNA segments
3. Work to be done
A. Long-term study to follow genetic variability and population structure in relation to population cycles of SPB. Electrophoresis and perhaps restriction mapping techniques of choice.
B. Studies using mtDNA as molecular markers of population history in relation to geographic location―phylogeography.
Restriction mapping and sequencing of PCR-amplified sequences
techniques here.
C. Molecular systematics of bark beetle taxa such as
Dendroctonus species, using mtDNA and nuclear loci. Restriction mapping and sequencing of PCR-amplified sequences
techniques here.
1
In lieu of a full narrative paper, the authors submitted this outline for
publication. Any questions should be directed to the authors.
2
Professor of Genetics, Dept. of Genetics, 101B Life Sciences Bldg.,
University of Georgia, Athens, GA 30602 and Professor of Forest Protection,
University of Georgia, Athens, GA 30602
6
USDA Forest Service Gen. Tech. Rep. PSW-138. 1992.
Isozyme Studies of Bark Beetle Population Genetics and Systematics1
Molly W. Stock, Gene D. Amman, and Barbara J. Bentz2
Electrophoretic analysis was used extensively in the 1970's
to estimate genetic relationships among populations, species,
and closely related genera of many different plants and animals, including many insect groups. Some of the first electrophoretic work on bark beetles was done by Anderson and
others (1979), Anderson and others (1983), Florence and Kulhavy
(1981), Florence and others (1982), and Namkoong and others
(1979). These studies demonstrated that bark beetles are highly
polymorphic and amenable to population studies using this
technique. The first isozyme study of the mountain pine beetle
(MPB) Dendroctonus ponderosae (Stock and Guenther 1979)
showed differentiation among geographically separated populations. A subsequent study of MPB from four locations (Stock
and Amman 1980) suggested that there might also be genetic
differences at individual gene loci between MPB from lodgepole and ponderosa pine, and greater overall heterozygosity in
beetles from ponderosa pine. Some preliminary data also showed
differences between beetles emerging from trees with thin and
thick phloem and between early- and late-emerging beetles
from the same tree.
Inferences that could be made from these observations of
differences related to host species were, however, confounded
by geographic distance among sites. Sturgeon (1980) and Sturgeon and Mitton (1986) studied sympatric groups of MPB
attacking different hosts in mixed pine stands and found significant differences related to host tree species. Further evidence of
differentiation of MPB by host was found in a comparison of
isozyme frequencies of MPB collected from lodgepole and
ponderosa pine at a single site in Utah (Stock and Amman
1985). Deviations in genotype frequencies from Hardy-Weinberg
(random mating) expectations occurred when isozyme data
from all beetles were pooled (Wahlund effect); frequencies
were much closer to random mating expectations when separate analyses were conducted for beetles from each of the host
species. Once again, greater heterozygosity was observed in
beetles from ponderosa pine. Overall, beetles from thin-phloem
lodgepole were more heterozygous than those from thick phloem, though these differences were not detected in beetles
from ponderosa pine.
1
An abbreviated version of this paper was presented at the Bark Beetle
Genetics Workshop, May 17-18, 1992, Berkeley, California.
2
Professor of Forest Resources, Dept. of Forest Resources, College of
Forestry, University of Idaho, Moscow, ID 83843; and Supervisory Research
Entomologist and Research Entomologist, respectively, Intermountain Research Station, USDA Forest Service, Ogden, Utah.
USDA Forest Service Gen. Tech. Rep. PSW-138. 1992.
Heterozygosity and Environmental Stress
Males were more differentiated than females among sites
and numbers of emerging males more variable in thin-phloem
trees (Stock and Amman 1980). In a later study (Stock and
Amman 1985), males emerging from thin-phloem lodgepole
pine were found to be more heterozygous and more variable
from tree to tree, and fewer and more variable males (relative to
females) emerged from thin phloem. We suggested that genetic
diversity of MPB (as measured by average heterozygosity)
might increase in response to increased severity of environmental conditions (i.e., stress), a response observed in numerous other organisms by other workers (e.g., Samollow and
Soule 1983). Male MPB are generally thought to be more
sensitive to microenvironmental conditions than females, and
therefore would be expected to have greater genetic diversity if
a generalized population-level response to increased selective
pressures was an increased level of heterozygosity. Further
evidence to support this hypothesis was provided by a study of
isozyme variation observed over six consecutive generations of
MPB laboratory-reared in thick and thin phloem; heterozygosity increased in both groups over the generations, but more
rapidly in the beetles in thin phloem (Amman and Stock unpublished).
In Ips pini (Gast and Stock unpublished), genetic diversity
was significantly higher in overwintered than in non-overwintered beetles. However, Langor and Spence (1991) observed
changes in gene frequencies but no consistent increase in heterozygosity in MPB with overwintering. These authors attribute the genetic differentiation they observed to differential
survival in the hosts, rather than to differential host preference
of beetle genotypes. Thus, one might see, as did Gast and Stock
(unpublished), greater differences between beetles that have
lived for a longer time in different hosts or in the same host
species under more severe environmental conditions. Other
evidence supports this hypothesis. For example, Gast (1987)
reported that overwintered I. pini responding to pheromone
were significantly more heterozygous than overwintered beetles
responding to host volatiles. Heterozygosity levels in hostresponding, pheromone-responding, and nonresponding beetles
that had not overwintered were very similar across response
categories.
Time of collection can therefore have an important influence on the type of genetic variation observed and interpretations that can be made from it. Langor and Spence (1991) and
the work of Gast (1987) suggest that laboratory rearing of
beetles collected from logs cut before winter reduces the differential selective influence of host tree species. This possibility
could help explain the fact that Sturgeon (1980) and Sturgeon
and Mitton (1986) found genetic differences between MPB
7
from lodgepole and limber pine, whereas Langor and Spence
(1991) did not. Sturgeon's (1980) beetles were collected in
summer, just before emergence from their host trees; they
represented survivors of within-tree mortality estimated to average 85 to 99 percent of the brood which had been established
the previous fall. Langor and Spence (1991) used beetles reared
in the laboratory and collected in the fall and suggested that
data collected from laboratory-reared beetles better reflect possible effects of host selection behavior and the genetic makeup
of colonizing beetles. We compared heterozygosity of earlyand late-emerging MPB reared from fall-collected lodgepole
logs from six sites (Stock and Amman unpublished). In beetles
from four of the six sites, heterozygosity went up over the
emergence period. It is possible that such trends would be even
more apparent in samples of beetles collected as they emerged
in the field the following year.
Heterozygosity and Species Distribution
Langor and Spence (1991) attributed the low levels of
heterozygosity (10.0-11.2 percent) they observed in MPB from
Alberta and British Columbia (compared to heterozygosity in
MPB from U.S. studies) to the location of their populations
near range margins. In contrast, our studies suggest that populations near the margins of species distribution (presumably living under suboptimal conditions) could be expected to be more
heterozygous. The highest level of polymorphism that we observed in a MPB population (more than 17 percent) occurred in
a population from California, another area that might be considered near the range margin for this species (Higby and Stock
1982). Heterozygosity levels of Langor and Spence's (1991)
Canadian populations were lower than in this California population, but approximately equal to heterozygosities in almost all
other populations we studied―including a population from the
Black Hills (South Dakota), a third area that could be considered marginal for the species and which had an average heterozygosity of 11.1 percent (Stock and others 1984).
Genetic Indicator of Population Phase
Some very early electrophoretic work with the Douglas-fir
tussock moth (Stock and Robertson 1977) revealed an esterase
allozyme that appeared only in low-density populations. Populations sampled two or more times during 1976 and 1977
showed changing frequencies of this allozyme (relative to the
other two allozymes at that locus) that seemed to parallel
changes in population density. At that time, we hypothesized
that relative frequencies of these three allozymes might be used
as a genetic indicator of future population trends in the Douglas-fir tussock moth. Hayes and Robertson (these proceedings) describe allozyme variation at an esterase locus showing
similar correlation with population status in the MPB.
Average heterozygosity also appears to be related to population phase, reaching highest levels as epidemic populations
reach peak numbers and begin to decline (Stock and others
8
unpublished). Thus, there may be a correlation between factors
of increased environmental stress that act to end an outbreak
and the higher levels of heterozygosity observed at this time.
One might infer a relationship between heterozygosity and
population size, but we need to keep in mind that the processes
underlying observed genetic variation are very complex and
that factors that influence genetic variation under one set of
conditions (e.g., during outbreaks or epidemics) can be very
different under another set of conditions (e.g., during the lowdensity or endemic population phase). For example, inbreeding
in low-density populations might act to reduce heterozygosity
and counterbalance any tendencies toward increased heterozygosity related to environmental stressors that might be keeping
the population at a low level. In the field, heterozygosity appears to be lower in endemic than in epidemic populations
(Stock and others unpublished), and laboratory studies (Amman
and Stock unpublished) showed that small numbers of parent
beetles per log resulted in an F1 population with a much lower
level of heterozygosity than the parental population.
On the basis of these ideas, Stock and others (unpublished)
hypothesized that heterozygosity increasing over time might
indicate that a population was approaching the end of an epidemic; conversely, a stable or decreasing heterozygosity in a
high-density population could indicate that the population was
unstressed and that population numbers would increase or stay
high. Predictions of population trend based on these hypotheses
for six MPB populations were good in four of six cases.
Systematics of Dendroctonus Species
Using electrophoretic techniques, Higby and Stock (1982)
identified diagnostic isozymes for sympatric populations of the
sibling species D. ponderosae and D. jeffreyi in California. An
earlier study (Stock and others 1979) showed very similar
levels of differentiation between D. pseudotsugae populations
from Idaho and Oregon, but the wide geographic separation
between these two populations precluded any definitive statements about species status.
Wood (1963) used anatomical and biological characters to
rank Dendroctonus species in order of increasing evolutionary
specialization relative to other Scolytidae. Lanier (1981) also
suggested an order of specialization based on cytogenetic characteristics. Although Wood and Lanier placed the species into
nearly identical species groups, their postulated order of specialization of these groups were nearly opposite. Bentz and
Stock (1986) used electrophoretic data from 22 populations,
representing 10 Dendroctonus species, in an attempt to unravel
the discrepancies in phylogenetic interpretations. Although the
species groups identified using electrophoretic data were similar to those previously identified by both Wood (1963) and
Lanier (1981), the implied phylogeny was independent of the
species groupings. Some members of a species group were
more specialized than other members in the same group. On the
basis of these data, it appears that no single species group can
be said to be more advanced or more primitive than another.
USDA Forest Service Gen. Tech. Rep. PSW-138. 1992.
The phylogenetic tree produced using electrophoretic data
suggests that D. rufipennis, D. adjunctus, and D. approximatus
were the most primitive species, followed by D. ponderosae
and D. brevicomis; D. valens, D. terebrans, D. simplex, D.
pseudotsugae, and D. frontalis are among the more evolutionarily advanced species in the genus. There were no discernible
trends in level of heterozygosity (by species) based on evolutionary position in the hypothesized phylogenetic tree. These
interpretations of the phylogenetic relationships among
Dendroctonus species could be useful in analysis of behavioral
strategies used in scolytid genera, such as production and response to semiochemicals, mating strategies, and host tree
associations.
Stock and others (1987) compared the great European
spruce bark beetle, D. micans, to the 10 North American
Dendroctonus species studied by Bentz and Stock (1986). Average heterozygosity was 5.3 percent in D. micans; North
American species ranged from 11.4 to 22.6 percent. Cluster
analysis suggests that D. micans is most closely related to D.
terebrans and D. valens, species with which it shares the characteristic of gregarious larval feeding in the phloem of living
hosts. More recently, Gast and Furniss (unpublished) compared
isozyme characteristics of D. micans with its morphologically
similar, spruce-infesting, North American counterpart, D.
punctatus.
USDA Forest Service Gen. Tech. Rep. PSW-138. 1992.
Research Needs
(1) Test the heterozygosity model of Stock, Schmitz,
and Amman for predicting phase (endemic, building, epidemic,
postepidemic) of MPB populations.
(2) Investigate further the change from endemic to
building phase: What are the influencing factors associated
with it (population size, mixing of demes, synchrony of emergence and attack, host characteristics, etc.)?
(3) Continue to investigate differences in MPB response to verbenone (and possibly other pheromones): Are
they genetically based and, if so, what factors are causing
selection?
(4) Dispersal: Are there physiological and/or genetic
differences among individuals with different flight behavior
(long vs. short fliers) and do these relate to population phase?
(5) The effect of temperature on MPB is very important in Rocky Mountain populations. Is cold tolerance genetically based, possibly showing up as geographic differences
associated with elevation and latitude?
(6) Continue study of bark beetle systematics using
electrophoresis and other, newer, molecular genetic techniques.
9
Biosystematics of Ips mexicanus and Ips plastographus (Coleoptera:
Scolytidae) and Their Fungal Symbionts1
David L. Wood, Andrew J. Storer, Thomas R. Gordon, Steven J. Seybold, and Marion Page2
Ips mexicanus (Hopk.) (concinnus group of S.L. Wood
1982) and I. plastographus (plastographus group) share common coniferous hosts throughout western North America (S.L.
Wood 1982). In California their distribution in the coastal pines
(Pinus radiata, P. muricata, P, contorta contorta, and P.
contorta bolanderi) and in the high elevation P. contorta
murrayana of the Sierra Nevada Mountains (and Cascade Mountains) is especially interesting because of their wide separation
by the Sacramento and San Joaquin Valleys. This separation is
2-400 km from Redding in the north to Bakersfield in the south,
a latitudinal distance of about 1200 km. Thus these populations
have been noninterbreeding populations for thousands of years.
Furthermore, these coastal and montane populations probably
come together where P. contorta and P. contorta murrayana or
P. c. contorta hybridize. Native Monterey pine (Pinus radiata)
is now represented by only three small separated coastal populations. Other hosts of I. mexicanus and I. plastographus occur
on small islands off the southern California coast and into the
mountains of Mexico (Bright and Stark 1973, S.L. Wood 1982,
Seybold and others 1992a).
On the basis of morphological criteria and reduced hatching in the F1, Lanier (1970b) defined subspecies of I.
plastographus: I. plastographus maritimus Lanier for the
coastal form and I. plastographus plastographus (Leconte) for
the montane form. In limited cross-mating studies, he showed
that both populations were interfertiles at the F1 and at the
parental backcrosses. Similar limited studies were conducted
with the cohabiting I. mexicanus (Lanier 1967). However, in
recent studies of the distribution of ipsenol (2-methyl-6methylene-7-octen-4-ol) and ipsdienol (2-methyl-6-methylene2.7-octadien-4-ol), pheromone components in Ips spp.,
Seybold and others (1992b) found both components in
montane I. mexicanus but only ipsdienol in coastal I.
mexicanus. The enantiomeric com-position of ipsdienol is ~90
percent-(-) for both populations. Although the status of these
compounds as pheromone compo-nents for these species has
not been established, they have been shown to be pheromone
components in all species studied to date (D.L. Wood 1982).
Nevertheless, the presence of ipsenol in only the montane
population offers considerable opportunity to investigate the
genetics of these two populations.
1
An abbreviated version of this paper was presented at the Bark Beetle
Genetics Workshop, May 17-18, 1992, Berkeley, California.
2
Professor of Entomology and Post-Doctoral Researcher, respectively,
Dept. of Entomological Sciences, University of California, 201 Wellman Hall,
Berkeley, CA 94720; Assistant Professor, University of California, Berkeley,
CA; Post Doctoral Scholar and Research Entomologist, Pacific Southwest
Research Station, USDA Forest Service, 800 Buchanan Street, Albany, CA
94710.
10
Very little is known about the fungal symbionts of these
species. Recently Parmeter and others (unpublished data) have
isolated unidentified species in the genera Ophistoma,
Leptographium, and Graphium from I.p.. maritimus and coastal
populations of I. mexicanus. These fungi were inoculated into
living trees. Through dye studies (Parmeter and others 1989)
some isolates were shown to interrupt water transport. The role
of symbiotic fungi in the biology of bark beetles has been
studied extensively, especially in the Ophistoma spp./
Dendroctonus spp. system (reviewed exhaustively in Schowalter
and Filip 1992; see for example, Owen and others 1987, Parmeter
and others 1989, 1992). In Ips spp./fungi associations,
Ophistoma spp. have been shown to inhibit oviposition but not
development (Yearian and others 1972) and to interrupt water
transport (Parmeter unpublished data, Homvelt and oth-ers
1983). Although no specialized morphological structures are
apparent in Ips spp. for carrying these fungi [such as the
pronotal mycangium of Dendroctonus brevicomis and D. frontalis
and maxillary mycangia of D. jeffreyi and D. ponderosae
(Whitney 1982)], there is a consistent and ubiquitous association between bluestain fungal invasion of the sapwood and
phloem and the gallery systems of these bark beetles. Therefore, studies of relatedness and divergence in I. mexicanus and
I. plastographus would be enhanced by similar analyses of
these phoretic symbionts. Evidence of divergence with either
or both taxa would be of great value in understanding the
evolution of these species.
Studies of relatedness in Ips spp. have focused on I. pini
(eastern/western populations) and on the grandicollis group
sibling species (paraconfusus, confusus, and hoppingi). In I.
pini, no post-mating barrier has evolved between eastern populations (Ontario, Canada) crossed with western populations
(California and Arizona) (Lanier 1972). However, a premating
barrier has evolved between these two populations. There pheromone component, ipsdienol, is produced by both populations.
However, in the California population (as well as in most
western North American populations―see Seybold and others
1992d), ipsdienol is produced as a mixture of 98 percent (-)and 2 percent (+-)-ipsdienol. The (+)- isomer is an interruptant,
whereas the (-)- isomer is a powerful aggregation attractant
(Birch and others 1980; Seybold and others 1992d). In the
eastern population a mixture of about 40 percent (-) and 60
percent (+)-ipsdienol is produced. Here both isomers function
as an attractant. We have found only a few beetles responding
to the racemate in California (Seybold and others 1992d).
Eastern populations respond at low levels to 98 percent (-)ipsdienol (Teale 1990). In addition to ipsdienol, lanierone (2hydroxy-4,4,6-trimethyl-2.5-cyclohexadien-l-ol) has recently
USDA Forest Service Gen. Tech. Rep. PSW-138. 1992.
been isolated and shown to be an important component of the
pheromone of the eastern population (Teale and others 1991).
Although this compound caused an increased response to
ipsdienol in California, it has not been found in California
populations (Seybold and others 1992c). As with ipsenol in the
montane population of I. mexicanus, the presence or absence of
lanierone and the differences in enantiomeric composition of
ipsdienol offer opportunities to investigate the genetics of these
two populations of I. pini.
In the work with the grandicollis group, Lanier (1970a and
b) and Merrill (1991) established the presence of postmating
barriers among I. paraconfusus, I. hoppingi, and I. confusus.
Premating barriers are weakly developed or not developed at all
(Cane and others 1990c, Fox and others 1991 a). Although there
is pheromonal cross-attraction among these species (Lanier and
Wood 1975; Cane and others 1990a, 1990c), we have found
nonreciprocal divergence in these pheromone-elicited behaviors, i.e., I. paraconfusus females do not discriminate between
conspecific and I. confusus males, whereas I. confusus females
prefer conspecific males in the host pine species (Cane and
others 1990c). Furthermore, females join heterospecific males
in host and nonhost pines (Fox and others 1991 a). Morphometric, electrophoretic (Cane and others 1990b), and cuticular
hydrocarbon (Page and others 1992) analyses support the species status of these sibling 5-spined Ips described by Lanier
(1970b).
Thus we see the evolution of postmating and weak-tononexistent premating barriers in the sibling grandicollis group
and the reciprocal scenario in I. pini. The studies we propose
will add to the knowledge base developed over the past 20
years on the biosystematics of these two Ips spp. complexes. At
the same time, expanding our investigation to include the symbiotic fungi should give us greater insights into the evolution of
this group of organisms.
Also of importance to the proposed study is the recent
discovery of the pitch canker fungus, Fusarium subglutinans,
in Monterey pine (P. radiata) plantings in the vicinity of Santa
Cruz, Calif. Our studies have shown that I. paraconfusus and I.
mexicanus adults carry propagules of this fungus (Fox and
others 1990, 1991b). Furthermore we have demonstrated that
attraction of I. mexicanus to Monterey pines results in pitch
USDA Forest Service Gen. Tech. Rep. PSW-138. 1992.
canker infections associated with attacking adults. This fungus,
which is native to pines of the southern U.S. (Correll and others
1992), causes extensive tip dieback followed by numerous
resinous bole-cankers. We believe that this fungus further weakens trees so that they are more easily killed by these Ips species.
Our studies suggest that progress of this disease from
urban plantings of Monterey pine in the Santa Cruz area into
our native forests will likely be a consequence of propagule
transmission via I. plastographus maritimus and I. mexicanus.
We predict transmission from I. mexicanus to I. plastographus
in the northernmost native stands of Monterey pine (Ano Nuevo
State Park) to urban plantings of Monterey and bishop pine (P.
muricata), to native stands of bishop and shore pine (P. contorta
contorta) on the coast and ultimately inland to the Cascade and
Sierra Nevada Mountains, perhaps via P. ponderosa. Ips
mexicanus and I. plastographus cohabit these coastal pine
species as well as P. contorta murrayana (I.p. plastographus)
in the above mountain ranges. Thus our studies on the coastal
and montane populations of these Ips spp. will be important in
predicting the ultimate distribution of this newly introduced
fungal pathogen. At the same time we are in a unique position
to investigate the coevolved system of these bark beetle species
with their symbiotic fungi. Such studies will provide baseline
information which can be used to understand the new symbiotic relationship that we expect will develop when F. subglutinans
becomes a part of this community.
Research Objectives
1. Determine the extent of postmating barriers to gene flow
between coastal and montane populations of I. mexicanus.
2. Determine the extent of premating barriers to gene flow
between the above populations.
3. Determine the presence or absence of ipsenol in the
above crosses of I. mexicanus and in the backcrosses.
4. Compare the genetic constitution of the above populations using mitochondrial DNA and cuticularhydrocarbon analyses.
5. Compare the genetic constitution of the fungal symbionts commonly associated with the above populations using
mitochondrial DNA analyses.
11
The Status of Scolytid Genetics―A Reductionist's View1
Steven J. Seybold2
As biologists considering studies of scolytid genetics we
are fortunate that an insect model exists that has been the target
of genetic analyses on both the individual and population levels
for most of the twentieth century. The knowledge base associated with Drosophila melanogaster3 and congenerics is staggering, and it is a tribute to scientific inquiry that so much is
known about organisms of such little economic importance.
The goal of the classical genetic crosses of D. melanogaster
begun in the lab of T.H. Morgan in 1909 was to infer the
genetic basis of the phenotype (phenotype →a gene). The development of P-element-mediated transformation (Rubin and
Spradling 1982) allows "reverse genetic" studies where the
gene of choice is placed into the organism and the resulting
individual is assayed to infer the phenotypic product of the
gene (gene → phenotype). During the period between the
advent of each of these approaches with Drosophila, serendipity and intensive screening procedures revealed a wide array of
morphological, developmental, and behavioral mutants; biochemical studies characterized nucleic acids and proteins; genetic studies led to the development of physical, cytological
maps of polytene chromosomes; and, soon, the entire genome
will be sequenced (Merriam and others 1991). The phenomenon of transposition ("jumping genes") has been actively
investigated in D. melanogaster (Green 1980, Engels 1983),
and the transposons themselves have been the targets of studies
of population genetics (Charlesworth and Langley 1989). In
addition to the biochemical and molecular work, the drosophilids
have been the subject of studies of population biology, phylogeny, and pheromone production (Bartlet and others 1986,
Kaneshiro 1988, Singh 1989).
In the Scolytidae, no mutants have been isolated, prospects
for long-term culture have not been developed, most crosses
have been performed to determine species status of wild populations rather than to assay for intraspecific phenotypic variants, and few biochemical reactions have been characterized.
When evaluated against the background of Drosophila genetics, the status of scolytid genetics can be described as primitive
at best. Thus, it is in an area of enormous opportunity, where,
1
An abbreviated version of this paper was presented at the Bark Beetle
Genetics Workshop, May 17-18, 1992, Berkeley, California.
2
Post Doctoral Scholar, Pacific Southwest Research Station, USDA
Forest Service, 800 Buchanan Street, Albany, CA 94710.
3
An organization called the Drosophila Information Service catalogs and
communicates biological (mostly, genetic) data about D. melanogaster and
related species. In addition, there is an encyclopedia of Drosophila mutations
authored by Lindsley and Grell, and various stock cultures of wild type and
mutant strains are maintained around the world.
12
because of the stature of the insects in forest management,
minor scientific gains may have large economic impacts.
However, rather than repeat the past 80 years of Drosophila genetics with the Scolytidae, we should apply the Drosophila knowledge base to scolytids as often as possible. No
scolytid is likely to become a model for eukaryotic biology, but
by applying the tools used with Drosophila, we may discover
some interesting and perhaps unique biological phenomena in
the Scolytidae.
For the purposes of organization, it is useful to divide
scolytid genetics into the reductional and holistic approaches.
By reductional I mean the analysis of biochemical, morphological, or behavioral traits of individuals and the interpretation
of the results from a molecular or mechanistic perspective. By
holistic I mean the analysis of the same traits from pooled
samples of individuals and the interpretation of the results from
an ecological or population perspective. The following are
several reductional areas of study that seem relevant to scolytid
genetics.
1) Genetic analysis of morpho- and chemosystematic
characters: A variety of useful morphological characters have
been identified for distinguishing scolytid taxa (Bright 1976,
Bright 1992, Bright and Stock 1982, S.L. Wood 1982). However, mutations in these characters have not been studied in a
systematic fashion [e.g., red vs. white eye color, vestigial vs.
wild-type (normal) wing morphology in Drosophila]. Recently
there has been considerable interest in establishing chemical
phenotypes for some species in the Scolytidae, and using the
newly discovered chemosystematic characters in constructing
phylogenetic relationships (cuticular hydrocarbons―Page and
others 1990a, b, 1992; Bright 1992; chirality of pheromone
components―Seybold and others 1992b; and migration of
metabolic enzymes in starch gels―Cane and others 1990b, and
other studies).
However, while typical chemical phenotypes have been
characterized, nothing is known about the genetic regulation
leading to these phenotypes and possible mutations that might
lead to altered chemical phenotypes. Furthermore, because of
the insensitivity of the chemical assays, the chemical phenotypes have often been determined on pooled samples of individuals, whereas we ought to evaluate the phenotypes of individuals. It will be important to standardize laboratory and
statistical procedures used to evaluate a range of individual
normal chemotypes so that mutants can be recognized when
they are encountered. This is not an easy problem. For example,
the cuticular hydrocarbon profile from a species typically contains 60 to 80 hydrocarbon components which vary in presence,
USDA Forest Service Gen. Tech. Rep. PSW-138. 1992.
absence, and quantity. Here are two interesting problems that
seem appropriate to this topic.
(a) Chirality of ipsdienol as a heritable trait of individual male Ips pini. Ips pini occurs in two basic pheromone
types (Birch and others 1980, Lanier and others 1972, Lanier
and others 1980, Miller 1990, Teale 1990, Miller and others
1989). The eastern form produces and responds maximally to
ipsdienol with an enantiomeric composition of 30 percent to 60
percent-(4R)-(-), whereas the western form produces and responds maximally to ipsdienol with an enantiomeric composition of 93 percent to 100 percent-(4R)-(-). The eastern form
produces and responds strongly to lanierone (Teale and others
1991); the response of the western form is enhanced by lanierone,
but males from this population do not appear to produce lanierone
(Seybold and others 1992c). Populations from North America
have been surveyed (Miller 1990, Seybold and others 1992b),
and a zone of sympatry between the forms appears to occur in
British Columbia. Teale (1990) has crossed eastern (New York)
and western (Arizona) forms and demonstrated heritable patterns to the chirality of male-produced ipsdienol. He has also
found a correlation between the response to enantiomeric blends
by males and production in the eastern form of I. pini. We are
pursuing similar studies with the western population.
(b) Ipsenol as chemotypic marker for two populations
of Ips mexicanus. Ips mexicanus occurs as three distinct populations in western North America: coastal in Pinus radiata, P.
muricata, and P. contorta; montane in P. contorta murrayana
and P. contorta latifolia; and interior in other Pinus spp. The I.
plastographus and I.integer complex can be broken down into
three similar entities. In a survey of the enantiomeric composition of ipsenol and ipsdienol produced by male Ips, we have
found that the coastal and montane populations of I. mexicanus
differ in their ipsenol-ipsdienol phenotype. Coastal I. mexicanus
produce ~90 percent-(4R)-(-)-ipsdienol, whereas montane I.
mexicanus produce the same ipsdienol and >99 percent-(4S)-(-)ipsenol. We have not studied an interior population of I.
mexicanus. Thus, interbreeding studies of coastal and montane
populations of I. mexicanus could be followed by traditional
traits such as larval: egg niche ratios (Lanier 1967), as well as by
ipsenol production. Coastal I. mexicanus could be treated as a
USDA Forest Service Gen. Tech. Rep. PSW-138. 1992.
strain of I. mexicanus that is deficient in the ability to produce
ipsenol.
2) Characterization of biochemical pathways: To accompany classical genetic analyses, we need to expand our
knowledge of bark beetle biochemistry. Because of a series of
pheromone biosynthesis studies with I. pini and I. paraconfusus
using labelled precursors (Fish and others 1979, 1984, Hendry
and others 1980, Vanderwel and Oelschlager 1987, Vanderwel
1991), these two species make particularly good candidates for
initial studies on the enzymes involved in pheromone biosynthesis.
3) Sex ratio condition: One intriguing genetic problem
that bears further study is the sex ratio condition discovered in
Ips latidens and other species (Lanier and Oliver 1966). In this
phenomenon individuals from certain populations carried a
maternally transmitted factor that resulted in selective mortality
of male embryos. A similar sex ratio condition also occurs in
Drosophila spp. (Sakaguchi and others 1965, Ikeda 1965,
Novitski and others 1965), and in recent studies the causative
microorganisms have been isolated and cultured (Hackett and
others 1986). Other sex ratio distortions in Drosophila spp. are
caused by chromosomal and extrachromosomal factors. Molecular studies of a distortion in the latter group (hybrid
dysgenesis) led to the discovery of the P-element transposon
(Rubin and Spradling 1982). Application of modern biochemical and molecular biological techniques to the sex ratio condition in Ips may lead to similar discoveries. Lanier never published the fact that the condition also occurs in another species
of Ips [confusus (LeConte)] and in the scolytid genera
Pityophthorus and Dendroctonus.
4) Construct gene libraries: With the cooperation of
Carol Alosi (Pacific Southwest Research Station, USDA Forest
Service), we have begun to construct a c-DNA library of genes
for I. pini. Large numbers of male and female western I. pini
have been frozen for this purpose. Genomic libraries for the
major scolytid species should be constructed and made centrally available to all researchers (the beginnings of a Scolytid
Information Service?).
13
Genetic Characters for Bark Beetle Phylogenies1
James H. Cane2
The current taxonomic classification of scolytid bark beetles
is based largely upon external morphological characters. This
will remain the status quo into the foreseeable future for rea­
sons of cost, time, convenience, and reproducibility. For genera
of particular economic interest, notably Dendroctonus and Ips,
additional taxonomic characters have been sought more or less
successfully through karyological and electrophoretic studies
and applied to teasing apart sibling species complexes and
circumscribing species groups (e.g., Lanier 1967, 1970a, 1970b,
1972, 1981; Cane and others 1990b). As noted by Bright (this
workshop), this task of describing species of the North Ameri­
can scolytid fauna is virtually complete, thanks to the works of
S. L. Wood, D. E. Bright, the late G. N. Lanier and G. R.
Hopping and many others (Bright 1992). Before us lies the task
of developing supportable phylogenetic hypotheses for key
bark beetle taxa, essentially converting lists of species and
higher taxonomic groups into ancestor-descendant hierarchies
or genealogical lineages.
How can we benefit from having rigorous phylogenetic
hypotheses for bark beetles? Certain bark beetle taxa offer us
unusual opportunities for insights into the evolution of po­
lygyny, mate choice, pheromonal and stridulatory courtship
communication, host association, and predator-prey interac­
tions. For instance, does pheromonal specificity result from
ecological interaction of co-occurring species, or does it reflect
the intervening time since common ancestry? For species of
the grandicollis group of Ips, at least, only the latter hypothesis
is supported (Cane and others 1990a, 1990c, Lewis and Cane
1990b).
Furthermore, it appears that divergence in specificity of
stridulatory communication parallels that of pheromone speci­
ficity (Lewis and Cane 1992) despite its role in defense (Lewis
and Cane 1990a). Evolutionary biologists and ecologists are
coming to appreciate the value of a proper phylogeny for the
testable evolutionary scenarios that it can offer to comparative
or evolutionary questions in chemical ecology, plant-herbivore
interactions, parasitology, and studies of symbioses (Cane 1983,
Gittleman and Kot 1990, Mitter and others 1991, Wanntorp and
others 1990).
Rigorous, testable evolutionary hypotheses for bark beetle
behavior and ecology require equally rigorous phylogenetic
statements that are defendable on their scientific merits. Unfor­
tunately, mere character similarity is insufficient evidence for
quantifying such phylogenetic relationships, as it confounds
the possible sources of similarity. Similarity may be the result
1
An abbreviated version of this paper was presented at the Bark Beetle
Genetics Workshop, May 17-18, 1992, Berkeley, California.
2
Associate Professor Entomology, Dept. Entomology, 301 Funchess
Hall, Auburn Univ., Auburn, AL 36849-5413.
14
of evolutionary convergence, as in the stridulatory capabilities
of taxonomically disparate bark beetle taxa. Even if characters
shared by two taxa are indeed homologous, such as some of the
saturated straight chain cuticular hydrocarbons of Dendroctonus
spp. (Page and others 1990b), they are shared with many other
insect genera and so are uninformative in describing phyloge­
netic relationships within any one lesser taxonomic unit. This
dilemma is best resolved using cladistics, an approach which
has emerged from the fires of heated debate over the past 20
years as the most appropriate method for reconstructing phylo­
genetic relationships (Wiley 1981). Cladistic algorithms hierar­
chically arrange species into ancestor-descendant lineages (a
genealogical tree) supported by shared, derived characters, thus
formalizing what good taxonomists have done all along. Recent experimental work with a virus has shown that a known
ancestral genealogy can be fully recovered using a cladistic
analysis of nucleotide sequences of the terminal viral families
(Hillis and others 1992).
Bark beetles had been too wee for molecular methods
using DNA until the advent of the polymerase chain reaction,
or PCR. Now, isolated mitochondrial, ribosomal or nuclear
DNA can, with skill, determination and some luck, be itera­
tively amplified to provide enough reliably replicated DNA for
RFLP's (restriction fragment length polymorphisms) or actual
nucleotide sequencing. These data can have advantages over
electrophoretic and morphometric data in that they provide
discrete characters suitable for binary, presence-absence cod­
ing. Because the cladistic approach works with patterns of
shared, derived characters and not clustering based upon overall similarity, character states must be discrete rather than
continuous so that they can be polarized as either ancestral or
derived at every juncture or branch in the phylogenetic tree. It is
comparatively easy to imagine an ancestral Ips either possess­
ing or lacking a third, hooked spine on its elytra. However, for
a continuous measure such as size, is a length of 4.6 mm primi­
tive or derived? The same problem can be encountered with
karyological, electrophoretic or biochemical data. Nucleotide
sequences may provide us with the supplemental discrete, heri­
table characters needed to establish trees of phylogenetic rela­
tionships for our evolutionary questions (Gittleman and Kot
1990, Cracraft and Helm-Bychowski 1991).
I have emphasized the application of genetic information
to the solution of phylogenetic and evolutionary questions for
bark beetles because of the topic's familiarity to me and rel­
evance to this workshop. Obviously, there are a number of
other prominent and perhaps more pressing genetic questions
regarding bark beetles that demand our attention as well.
1. What is the genetic structuring of select scolytid popula­
tions? In particular, what is the size of a deme for a given
species during eruptive and quiescent periods of population
USDA Forest Service Gen. Tech. Rep. PSW-138. 1992.
cycles? What are the respective rates of gene flow during these
periods? Do allele frequencies shift as a result of natural or
sexual selection during either part of these population cycles?
If there is an oscillation in selective regimes, is its effect
roughly reciprocal, such that selection in one phase cancels that
in the next? Or do populations gradually change genetically as
a result of such cyclical selection? Or is it not selection at all
but drift during the phase of population crash that fixes genetic
change in populations, the so-called "flush-crash" hypothesis
of Hampton Carson?
2. Over what distances do some individuals of a species
migrate? Are outbreaks intensified by an influx of beetles from
neighboring populations, or do they result from expansion of
the local population? Do outbreaks spawn greater proportions
of dispersing individuals?
USDA Forest Service Gen. Tech. Rep. PSW-138. 1992.
3. It is crucial that we gain an understanding of the relative
contributions of environment and inheritance to the variance
we see in bark beetle phenotypes, particularly in their produc­
tion, perception and response to semiochemicals. Teale and
Hager's work (this workshop) with heritability and covariance
in pheromone production and response, and its responsiveness
to imposed selection, is some of the first work in this exciting
and much-needed line of investigation.
4. We must not allow ourselves to become complacent or
flip about the methodological hurdles that lie in wait of bark
beetle biologists wishing to make use of the tools of molecular
biology and genetics. As noted by Seybold, by Alosi, and by
Anderson (this workshop), we can benefit from the genetic and
molecular knowledge already accrued for other insect taxa in
our hunt for methods to answer our own genetic questions
about scolytids.
15
Cuticular Hydrocarbon Research1
Marion Page2
We have been studying existing taxonomies of forest insects that are based on morphological, genetic and/or behav­
ioral characteristics to evaluate the utility of cuticular (surface)
hydrocarbons as taxonomic characters (Haverty and others
1988, 1989, Page and others 1990a, 1990b). Cuticular hydrocarbons are relatively stable metabolic end products that appear
to be genetically fixed. Because the insects studied so far
synthesize all or most of their hydrocarbon components, hydrocarbon composition and taxonomic grouping should be related.
Ideally, we would use these chemical characters much as clas­
sical taxonomists use morphology, behavior or genetics, i.e., to
sort the groups of insects on the basis of surface chemical
characters first, rather than after groups have already been
sorted on the basis of existing (nonchemical) character criteria.
However, by comparing our taxonomic separations on the basis
of cuticular hydrocarbons with existing taxonomic divisions
based on other characteristics, we are broadening the data base
of cuticular hydrocarbons as potential taxonomic characters for
forest insects.
We initially started our studies on the dampwood termites,
Zootermopsis, while trying to understand a synonymy of two
species of scolytid cone beetles, Conophthorus ponderosae and
C. lambertianae. Since these beetles have few useful diagnostic
morphological characters, we decided to examine cuticular
hydrocarbons as taxonomic characters. To test our understand­
ing of the current methodology, we repeated the results of
Blomquist and others (1979) on Z. angusticollis. Our initial
investigation produced hydrocarbon profiles that differed from
those they had published. Had we made an error in methodol­
ogy? Was their hypothesis about species and caste-specific
hydrocarbons correct? Had we observed population or colony
variation unreported by them?
On the basis of morphological characters, our termite speci­
mens were identified as Z. nevadensis, not Z. angusticollis.
Blomquist (personal communication) suggested that both our
laboratories reevaluate our termite collections using identical
methods and gas chromatography parameters. Surprisingly,
Blomquist's and our laboratory data were identical to those in
our first trial but different than his published data. Had we
discovered a sibling species? Additional collections and analy­
ses led us to identify an "extra" hydrocarbon phenotype of
Zootermopsis and to find a morphological character for un­
equivocal identification of the three described species of
Zootermopsis (Thome and Haverty 1989). We have separated
all species of the dampwood termite by hydrocarbon pheno-
types. Two phenotypes within a species were so different that
we used these data as a basis for proposing two subspecies, Z.
nevadensis nevadensis and Z. nevadensis nuttingi.
Confident that we were capable of proceeding with our
original purpose involving cone beetles, we embarked on stud­
ies of Conophthorus spp. and [testing hypothesis on] select
species of forest insects. We conducted a study to determine the
degree of similarity or diversity or both among eight of the 15
described species of Conophthorus (Page and others 1990a).
From these species we identified 140 hydrocarbons occurring
as individual and isomeric mixtures. Many hydrocarbons were
species specific. We discovered that the relatedness of hydrocarbon profiles for each species parallels existing morphologi­
cal keys. These data support the synonymy of C. monticolae
with C. ponderosae. Conophthorus from sugar pine could com­
prise a sibling species. Hydrocarbon mixtures of two eastern
species, C. resinosae and C. banksianae, are identical, support­
ing the suspicion that C. banksianae may not be a valid species.
Closely related pinyon cone beetles, C. cembroides and C.
edulis, have similar combinations of hydrocarbons except for a
unique and abundant alkene (C27:1) in C. edulis and two
dimethylalkanes in C. cembroides.
To date we have published the only studies on the cuticular
hydrocarbons of scolytid beetles (Page and others 1990a, 1990b).
We examined four species of Dendroctonus that comprise two
sibling species and one pair of morphologically similar species
(Page and others 1990b). Each of these four species has an
abundance of information on behavioral classifications (i.e.,
location and patterns of larval and adult galleries), host associa­
tions, and taxonomic classification based on host-finding and
mating behaviors, pheromone chemistry, and classical mor­
phological traits. The mountain pine beetle and Jeffrey pine
beetle are sibling species and are difficult to separate on the
basis of morphological characters. The western pine beetle can
be distinguished from its close relative, the southern pine beetle,
by a few morphological characters, such as larger body size,
and on the basis of geographical distribution. We were able to
ascertain that the cuticular hydrocarbon mixtures in these four
species are species specific. The hydrocarbon patterns of the
sibling species corroborate their similarity, yet a few hydrocar­
bon components are unique enough to allow separation. Western pine beetle and southern pine beetle have hydrocarbon
mixtures that are not qualitatively identical. However, they are
similar enough to each other to confirm that these species are
closely related, as suggested by electrophoresis analyses.
1
An abbreviated version of this japer was presented at the Bark Beetle
Genetics Workshop, May 17-18, 1992, Berkeley, California.
2
Research Entomologist, Pacific Southwest Research Station, USDA
Forest Service, 800 Buchanan Street, Albany, CA 94710.
16
USDA Forest Service Gen. Tech. Rep. PSW-138. 1992.
Bark Beetle Genetics―an Overview1
Stephen Teale and Barbara Hager2
Our understanding of bark beetle genetics is fairly rudi­
mentary. A few studies have used enzyme variation to docu­
ment population structure of Ips and Dendroctonus species in
relation to geographic separation (Anderson and others 1979,
1983, Namkoong and others 1979), host trees (Langor and
Spence 1991, Sturgeon and Milton 1986), dispersal (Florence
and others 1982), and phenology (Gast 1987). Electrophoretic
data have also been used to look at distance relationships of Ips
species (Cane and others 1990b). Preliminary studies have
investigated genetics of morphological (Linton and others 1984)
and pheromonal (Piston and Lanier 1974) traits.
The three main priority areas that we see in need of par­
ticular attention (not necessarily in this order) are areas that,
until recently, have not received attention:
1. Heritable variation in traits affecting bark beetle-natural
enemy interactions, in particular, the use of bark beetle phero­
mones by predators and parasitoids in prey location and how it
affects heritable qualities of bark beetle populations as well as
population size. The basis for this research need is reports by
Raffa and Klepzig (1989) and Herms and others (1991) show-
ing that the pheromone systems of Ips pini is significantly
altered by predator eavesdropping. An understanding of the
genetic control of pheromone production and response is essen­
tial to understanding these reciprocal predator-prey adapta­
tions.
2. Heritable variation in host selection traits of bark beetles
and how it can influence population dynamics through interac­
tive selection between host trees and beetles under varying
population densities. The groundwork in this area was laid by
Raffa and Berryman (1987). The genetic component involved
in interactions both among and within trophic levels has gener­
ally not been considered in bark beetle population biology.
3. The use of DNA techniques, particularly sequencing, in
phylogenetic reconstruction. With an unparalleled array of mating
systems, the family Scolytidae begs to become a model for the
study of the evolution of mating systems as well as the evolu­
tion of host selection (i.e., which beetle taxa feed on which tree
taxa). This can be done only in a phylogenetic context, and
DNA sequences yield the most robust phylogenies.
1
An abbreviated version of this paper was presented at the Bark Beetle
Genetics Workshop, May 17-18, 1992, Berkeley, California.
2
Assistant Professor and Post Doctoral Associate, respectively, Dept. of
Environmental and Forest Biology, Syracuse University of New York, College
of Environmental Science and Forestry, Syracuse, NY 13210.
USDA Forest Service Gen. Tech. Rep. PSW-138. 1992.
17
Current Status and Critical Research Needs in Scolytid Genetics1
Kenneth F. Raffa2
During the past two decades, our knowledge of bark beetle
behavior and ecology has greatly expanded. In particular, stud­
ies of pheromone communication, host plant relations, and
population dynamics have increased our basic knowledge of
bark beetle biology, improved our ability to manage pest spe­
cies, and provided strong models for research on other insect
groups. However, bark beetles remain our most damaging
forest pests nationwide. In addition, the various components of
scolytid biology are usually studied in isolation, and some key
gaps in our knowledge preclude effective synthesis. In my
opinion, a major reason for these difficulties is that insufficient
attention has been given to bark beetle genetics. Increased
knowledge in this area could serve as a powerful unifying
theme among the various subdisciplines, and improve our man­
agement capabilities.
The lack of attention to scolytid genetics is well evidenced
by the major books and review articles on this family. Most
texts and chapters make no, or relatively brief, mention of
genetics or the role genetics could play in population behavior.
Some areas of scolytid genetics have received well-focused
attention, in particular isozyme variation within and between
species (Langor and Spence 1991, Stock and Guenther 1979,
Stock and others 1984). These studies established polymor­
phism, heterozygosity, and population structuring across geo­
graphic regions and sometimes host species. Some basic work
has been done on cytology (e.g., Lanier 1981), before the
development of modern methods. However, a number of criti­
cal areas, ranging from the molecular to population levels,
remain unexplored.
A number of new tools are now available for studying
insect genetics. However, I feel the most critical need is to
generate testable hypotheses regarding how heritable aspects of
scolytid biology relate to their behavior and population dynam­
ics. Milton and Schneider (1987) summarized two basic ap­
proaches to the role of genetics in insect outbreaks: (1) start
with a heritable marker, and conduct a broad search for correla­
tions between gene frequencies and population fluctuation, and
(2) identify life history traits whose variation would greatly
influence population behavior, and determine heritable varia­
tion. Both approaches are valid, and both must be pursued to
make relevant gains. As a population ecologist, I will approach
the issue from the latter perspective. Hopefully, this perspec­
tive will complement equivalent suggestions arising from the
marker-up, and more molecular approaches. From an ecologi­
cal/population management view, I think that three critical
genetic issues must be addressed.
1
An abbreviated version of this paper was presented at the Bark Beetle
Genetics Workshop, May 17-18, 1992, Berkeley, California.
2
Forest Entomologist, Dept. of Entomology, University of Wisconsin,
Madison, WI 53706
18
Host Selection and the Genetics
of Outbreak Behavior
Genetic traits associated with host selection may be useful
in predicting the onset of bark beetle outbreaks. Low density
populations are typically concentrated within severely stressed
trees. Yet during outbreaks, almost all trees are attacked and
killed, regardless of their vigor. The mechanisms by which
high beetle numbers kill trees is fairly well understood: aggre­
gation pheromones coordinate rapid mass attacks on entered
trees, and the tree's resistance capabilities are exhausted. More
beetles are required to kill vigorous trees than stressed trees.
However, this tells us nothing about how beetles' decisions to
enter trees could be influenced by their population densities.
Each beetle undergoes a strict behavioral repertoire, which
culminates in either host entry or resumed flight (Raffa 1988).
This suggests that different selective pressures may operate on
individual beetles at different beetle densities. At low popula­
tions, the likelihood of an entered beetle being joined by enough
recruits to successfully colonize healthy trees is low, and so
relatively discriminating behavior may be favored. Conversely,
at high densities, beetles that enter healthy trees are more likely
to be joined by sufficient recruits to kill the host, and so less
discriminating behavior is adaptive (also, discriminating types
may never find acceptable hosts, as the weakest trees are
colonized by generations leading to the outbreak). Therefore,
changes in gene frequencies associated with host acceptance
could both result from, and partially cause, bark beetle popula­
tion increases (Raffa and Berryman 1983, Raffa 1988). The
relatively high levels of within-tree, compared to spatial and
temporal, genetic (electrophoretic) variation in Dendroctonus
frontalis (Florence and Kulhavy 1981), and the high levels of
genetic diversity that can be maintained in dispersed popula­
tions (Florence and others 1982) could lead to such directional
selections. Observed behavioral differences between outbreak
and nonoutbreak Ips typographus japonicus (Futura 1989) are
consistent with such a pattern.
There are alternative explanations of how host selection
could change through time, such as individuals becoming less
discriminating as their stores deplete, or pheromones eliciting
host entry in addition to attraction [although Hynum (1978)
observed the proportion of beetles resuming flight after landing
does not decrease during mass attack]. However, the genetics
of host selection should be considered a high-priority area. This
can be approached by: (1) quantifying for heritable variation in
beetle selectivity, (2) quantifying genetic diversity throughout
the endemic and epidemic phases within an area, and between
endemic and epidemic populations at one time, using polymor­
phic DNA markers such as RFLP's or RAPD's, and (3) testing
alternative hypotheses.
USDA Forest Service Gen. Tech. Rep. PSW-138. 1992.
Role of Natural Enemies
Pheromone Synthesis and Attraction
Biological control of bark beetles has received only scant
attention, largely because the major pests are native species.
However, geographic and local variation in predator behavior
may provide some avenues for better exploiting natural en­
emies. These include introducing predators from distant popu­
lations, and biasing against natural enemy removal during
semiochemically based population control measures.
The major insect predators locate prey by responding to
bark beetle aggregation pheromones. Prey mortality is often
extremely high. Thus, selective pressures exerted by predators
may favor alterations in prey semiochemistry that reduce their
attractiveness to predators, yet retain intraspecific functional­
ity. Several lines of evidence support this view: (1) predacious
beetles, although attracted to scolytid pheromones, may show
greater affinity for stereoisomers that are relatively less attrac­
tive to (and produced by) the prey (Herms and others 1991,
Payne and others 1984, Raffa and Klepzig 1989); (2) minor
pheromone components can have markedly different effects on
predator and prey species (Seybold and others 1992c, Miller
and others personal communication); (3) local predator guilds
may be more attracted to distant than local populations of the
same prey species (Lanier and others 1972, Raffa and Dahlsten
personal communication).
The underlying genetics of predator-prey interactions are
completely unknown. No studies on the heritability of predator
responses to bark beetle pheromones have been conducted,
and only recently has individual variation in bark beetle
semiochemistry been explored (see #3 below). Studies should
be aimed at: (1) conducting classical hybridization studies of
divergent perception phenotypes within predator species, (2)
identifying DNA markers associated with pheromone percep­
tion among both predators and prey, and (3) quantifying selec­
tive pressures imposed by predators on beetle reproductive
success (at both the colonizing cohort and individual beetle
levels) and on predators to locate a particular prey guild.
Pioneering work by Lanier (1970c) demonstrated Mende­
lian control over Ips pheromone production and response. Such
inheritance patterns can contribute to demic structuring (Cane
and others 1990c). Recent studies of scolytid pheromone com­
munication have shown strong inter- and intrapopulation dif­
ferences within species (Berisford and others 1990, Birgersson
and others 1988, Miller and others 1989, Seybold and others
1992c). These results could have strong implications for bark
beetle biology and management, particularly in guiding
semiochemically based management strategies and biological
control (see above).
Several factors have been proposed as possible contribu­
tors to heritable genetic variation in scolytid semiochemical
synthesis and response. For example, it has been proposed that
some beetles do not actively engage in host searching, but only
react to pheromone plumes indicative of others' success. The
frequency of such types may change with beetle density
(Birgersson and others 1988), in a fashion similar to the host
selection model proposed in #1. If so, accompanying genetic or
phenotypic markers may be useful in predicting outbreak be­
havior. Genetically based variation in pheromone chemistry
may also be important in maintaining the structure of scolytid
mating populations (Teale personal communication). Studies
should (1) conduct classical hybridization tests among beetles,
putatively divergent types (based on arrival time in aggregation
sequence and relative responsiveness to host cues in laboratory
assays), (2) identify DNA markers associated with putatively
different behavioral types, and (3) determine the level of gene
flow and demic integrity among putatively different types.
USDA Forest Service Gen. Tech. Rep. PSW-138. 1992.
19
Genetic Basis of Semiochemically Mediated Bark Beetle-Predator Coevolution:
Implications for Developing Mass-Trapping Technologies1
Robert A. Haack. Daniel A. Herms, Bruce D. Ayres, Matthew P. Ayres, and M.L. Doozie Snider2
Background
The pheromone chemistry of Ips pini (Coleoptera:
Scolytidae) has been extensively studied in recent years. In
eastern North America, I. pini populations respond most strongly
to ipsdienol with an enantiomeric composition of about 40
percent to 75 percent-(S)-(+) (Herms and others 1991, Lanier
and others 1980, Raffa and Klepzig 1989, Teale 1990). In
western populations, I. pini typically responds maximally to
ipsdienol with an enantiomeric composition of 93 percent to
100 percent-(R)-(-) (Lanier and others 1980, Miller and others
1989, Seybold and others 1992d).
In eastern populations of I. pini, both isomers of ipsdienol
function as attractants (Teale 1990), whereas in most western
populations the (-)-isomer serves as an attractant and the (+)isomer functions as an interruptant (Birch and others 1980,
Seybold and others 1992d). Recently, lanierone was demon­
strated to be an important component of the aggregation phero­
mone of eastern populations of I. pini (Teale and others 1991),
but it was not found in western populations (Seybold and others
1992c).
Teale (1990) demonstrated in I. pini that the chirality of
male-produced ipsdienol was heritable. In addition, Teale and
others (personal communication) have shown a strong, positive
correlation between male response to and production of enan­
tiomeric blends of ipsdienol. If positive assortive mating were
common among individuals with similar ipsdienol preference/
production, then genetic substructuring of I. pini populations
could easily occur. Moreover, if movement by Ips adults were
limited between habitat patches (e.g., isolated pine woodlots),
then further genetic divergence could result on a spatial scale.
Thanasimus dubius (Coleoptera: Cleridae) is a specialist
predator of Ips and other bark beetles, and can cause significant
bark-beetle mortality (Riley and Goyer 1986, and references
therein). Thanasimus dubius uses the pheromone of I. pini as a
kairomone in prey location, and like I. pini, demonstrates prefer­
ences among different blends of ipsdienol (Herms and others
1
An abbreviated version of this paper was presented at the Bark Beetle
Genetics Workshop, May 17-18, 1992, Berkeley, California.
2
Supervisory Research Entomologist, North Central Forest Experiment
Station, USDA Forest Service, Stephen S. Nisbet Bldg., 1407 S. Harrison Road,
East Lansing, MI 48823; Staff Entomologist, The Dow Gardens, Midland,
Mich.; President, Woods Run Forest Products, Colfax, Wis.; Research Ento­
mologist, Southern Forest Experiment Station, USDA Forest Service, Pineville,
La.; Research Technician, Michigan State University, East Lansing, Mich.
20
1991, Raffa and Klepzig 1989). Such variation in the kairomone
response of Thanasimus could lead to differential mortality of
Ips adults that respond/produce different enantiomeric blends
of ipsdienol. Ips that respond/produce low-risk ipsdienol blends
theoretically could escape intense predation and increase in
numbers. This change in Ips pheromone production would,
eventually exert a reciprocal selective force on its predators,
resulting in a coevolutionary feedback system.
Similar to the selective pressures described above for
Thanasimus, mass-trapping with a single enantiomeric blend of
ipsdienol may also cause differential mortality of Ips popula­
tions and lead to an increase in Ips that respond/produce blends
of ipsdienol that differ from the pheromone bait being used. In
other words, there is potential for Ips populations to evolve
resistance to mass-trapping programs that utilize a single blend
of ipsdienol.
Current Research Objectives
We are interested in developing mass-trapping technology
for managing I. pini populations in midwestern pine woodlots.
This will require an understanding of the relative genetic and
environmental components to variation in the pheromone re­
sponses of I. pini and its specialist predators (Herms and others
1991). Heritable variation in beetle responses to different enan­
tiomeric blends of the aggregation pheromone ipsdienol im­
plies the potential for Ips populations to evolve resistance to
mass-trapping programs that use individual blends of ipsdienol.
This risk is especially great if Ips populations are genetically
substructured on the basis of pheromone preferences or if gene
flow between pine woodlots is rare. The risks are complicated
by secondary impacts on predators that exploit ipsdienol as a
kairomone for prey location. Mass-trapping has potential as an
economically viable, ecologically sound control measure, but
its sustained effectiveness may be compromised by disregard­
ing the above risks. We will develop models to predict evolu­
tionary responses of Ips and their predators to different masstrapping scenarios. The expected product is an economical
resistance management program, probably based on sequential
or simultaneous use of different ipsdienol blends, and including
considerations of predator-prey dynamics.
Current Research Question
To what extent are populations of I. pini and T. dubius
genetically substructured within and between habitat patches
(pine woodlots)?
USDA Forest Service Gen. Tech. Rep. PSW-138. 1992.
Experiments in Progress
Future Research Plans
1. Electrophoretic studies to test for genetic differentiation
within and among populations of I. pini (and its clerid predator,
T. dubius) based on ipsdienol-blend preferences.
2. Mark-recapture studies to test pheromone-response specificity of I. pini.
3. Comparison of pheromone-response profiles of I. pini
populations from several isolated red-pine stands in Michigan.
Assuming a genetic component exists to the observed intrapopulation variation, then genetic drift or differential selection
pressures could result in stand-to-stand differences on a local
scale.
We plan to initiate replicated mass-trapping studies in
several isolated red pine woodlots in Michigan in 1992. If there
is a genetic component to the observed variation in pheromone
response, we should be able to manipulate pheromone response
profiles by exerting selection through mass-trapping. Directional selection (e.g., using one pheromone blend) should change
the response profile, whereas stabilizing selection (e.g., using
several blends) should maintain it. If the genetic component to
pheromone response is weak relative to environmental influences, or if there is substantial gene flow within and among
woodlots, then mass-trapping should have little effect on response profiles of local I. pini populations.
USDA Forest Service Gen. Tech. Rep. PSW-138. 1992.
21
Population Genetics of Spruce Bark Beetle Ips typographus (Col., Scolytidae)
and Related Ips Species1
Christian Stauffer, Renate Leitinger, Erwin Fuehrer2
Enzyme electrophoresis proved to be a useful method to
study population genetic aspects in Ips typographus (L.) because of three highly polymorphic loci. Aspartate aminotransferase locus-2 (Aat-2), physiologically involved in the transfer
of aminogroups, has six alleles per population. Amylase locus1 (Amy-1), physiologically involved in the hydrolysis of starch,
has 10 alleles per population, and esterase locus-2 (Est-2),
physiologically involved in processes of detoxification and the
metabolism of juvenile hormone, has 16 alleles per population.
The allozymes, pattern, and the mode of inheritance of nearly
all alleles were genetically proved. The different amount of
alleles among allozymes could reflect the physiological role of
the enzymes (Johnson 1974); therefore most studies were calculated individually (Stauffer and others 1992b).
A related but distinctly different species is the spruce bark
beetle Ips amitinus. This relationship could be also demonstrated by enzyme electrophoresis. The existence of I. amitinus
and I. amitinus var. montana (Fuchs 1913), which appeared to
be doubtful, was investigated by behavioral, morphological
and biochemical (cuticular analysis and enzyme electrophoresis) techniques. Because no obvious differences between the
two races could be found, the distinction of a var. montana
becomes very unprobable (Zuber and others 1992).
Efficiency of a commercial pheromone product, which is
commonly used in central Europe for trapping I. typographus,
proved to be regionally different. A genetic basis of these
differences was assumed. Seventeen European populations of
this species, collected from infested logs, were studied. The
dendrograms and an Alscal analysis of Aat-1 and Est-2 clustered three groups―the northern (Scandinavian) populations,
the southern (alpine) populations, and the middle European
(prealpine/Czechoslovakian) populations. Distribution of these
three genetic groups corresponds to the three parts of natural
distribution of the host tree, Picea abies, in Europe, each of
them originating from different glacial refuge areas (Central
Russia, Carpathian Mountains, Dinaric Alps, respectively). Thus
it seems likely that I. typographus populations had been subject
to genetic differentiation by long-term isolation during the
glacial period and postglacial remigration to middle and north
Europe, parallel to its host tree (Leitinger and others 1992).
Preliminary results indicate, moreover, that within
I. typographus populations the genetic variance of samples
collected from pheromone traps differs more or less from those
which had been collected from infested logs. As to the genetic
divergence between log-trapped and pheromone-trapped samples,
a slight trend from middle European lowland populations toward upland and Scandinavian populations, respectively, was
ascertained (Leitinger 1991). Because subpopulations of
I. typographus, which are colonizing different spruce logs, also
differ genetically, a genetically determined variance of orientation patterns within this bark beetle species is assumed. This
phenomenon may concern the pioneer problem too. Voltinism
is another property with epidemiological significance, which
seems to be genetically determined within populations of
I. typographus. Depending on different altitudes, the proportion of obligately and facultatively diapausing beetles is different. First attempts to biochemically identify both diapausing
types revealed promising results.
From the epidemiological point of view, work will be
focused on functional and ecological population genetics. Using the allozyme method for identification of different "types,"
the functionality of enzymes as well as enzyme activity must be
studied more in detail. We proposed to begin with the juvenile
hormone esterase (Stauffer and others 1992a), the
monooxygenase (Sturgeon and Robertson 1985), and the general esterases.
1
An abbreviated version of this paper was presented at the Bark Beetle
Genetic Workshop, May 17-18, 1992, Berkeley, CA.
2
Institute of Forest Entomology, Forest Pathology and Forest Protection,
Universität für Bodenkultur, A-1190 Vienna, Austria.
22
USDA Forest Service Gen. Tech. Rep. PSW-138. 1992.
Novel Bark Beetle Research Possible with New Genetic Techniques1
Ken Hobson and Owain Edwards2
Prospects
The new molecular techniques in genetics offer solutions
to questions about bark beetle biology in two principal areas.
The first of these is dominated by questions of phylogenetic
interest. For example, the unity of Dendroctonus valens as a
species (e.g., Pajares and Lanier 1990) could be explored with
precision. Geographic substructuring of other scolytid taxa,
including regional differences in pheromone production and
response, partial mating barriers in sibling Ips and so on, share
this phylogenetic approach. Much of this work is well underway.
The second area, which is wide open for new ideas, uses
the ability of the new techniques to discriminate individuals
and populations to address questions from population dynamics and behavior (Slatkin 1987). It is possible now with mammalian and bird DNA to characterize individuals and verify
genetic contributions of parents to their offspring. Paternity
analyses of this sort have recently provided some surprising
insights in ornithological literature where extra-pair copulations have been found to be much more common than was
previously recognized. Little is known about bark beetle behavior under the bark―e.g., whether supposedly monogamous
Dendroctonus females sometimes mate with more than one
male before ovipositing their first clutch of eggs, whether there
is selection by females for males with particular characteristics,
and so on. The ability to identify individuals and their descendants would allow paternity analyses of eggs from scolytid
galleries. Male-male competition may be occurring in D. valens
galleries where there are three adults present (Hobson 1992).
Paternity analysis of egg clutches of even single pairs of beetles
could provide insights into the frequency of pre-emergence
mating.
The ability to identify beetles from subpopulations and
track their descendants may allow us to answer questions such
as whether patch kills of trees by D. brevicomis are caused by
beetles which are the descendants of an earlier nearby infestation or instead are beetles which are drawn from the general
population of all dispersing adults.
Similarly, typical bark beetle dispersal distances can be
assessed by looking at variability along transects through beetle
populations. The number of migrants per generation between
populations can be estimated from FST statistics as estimated
using the allele frequencies of several different genes. Studies
employing electrophoresis have had difficulty in the past find1
An abbreviated version of this paper was presented at the Bark Beetle
Genetics Workshop, May 17-18, 1992, Berkeley, California.
2
Graduate Students, Dept. of Entomology, Univ. of California, 201
Wellman Hall, Berkeley, CA 94720.
USDA Forest Service Gen. Tech. Rep. PSW-138. 1992.
ing enough polymorphism to conduct very fine-grained analyses of this sort. Now several of the new molecular techniques
permit rapid analysis of DNA with enough resulting information to carry out gene flow and dispersal analyses with dozens
of markers resulting in more precise estimates.
Contrasts that exist between beetles in an endemic or early
infestation population and an epidemic or late infestation population may involve genetic drift or perhaps repeating cycles of
shifting gene frequencies.
Methods
Sequencing of both mitochondrial and genomic DNA has
provided information previously unattainable to systematists.
These data, in conjunction with other chemical phenotype
data, such as cuticular hydrocarbon analysis (e.g., Page and others
1990b), will provide answers to many bark beetle phylogenetic
questions.
However, discrimination of bark beetle populations requires molecular techniques that detect higher levels of polymorphism. DNA sequencing of highly polymorphic regions of
DNA can be useful, but these regions must first be located.
Sequencing is costly and labor-intensive, so its application in
population genetics can only be limited. Restriction Fragment
Length Polymorphism (RFLP) has also been used for the detection of DNA polymorphism. However, RFLP analysis requires
that knowledge of a polymorphic region to target exists, and its
sequence must be known so that a DNA probe can be constructed. RFLP analysis is much less labor-intensive than sequencing, but it is still quite costly. Radioactivity is also necessary for both sequencing and RFLP. Because of the drawbacks
of these DNA techniques, protein allozyme analysis has been
the most popular method for performing genetic polymorphism
studies. Allozyme analysis is much less expensive, but it is
often difficult to find polymorphic loci.
The polymerase chain reaction (PCR) has made it much
simpler and more cost-effective to perform molecular analyses
at the level of the DNA (Arnheim and others 1990). Because
PCR dramatically increases the copy number of specific DNA
sequences, analyses using PCR can be performed with very
little starting DNA. DNA sequencing of PCR-amplified DNA
eliminates the need for cloning, which dramatically reduces the
cost and effort. RFLP analysis using PCR-amplified DNA
eliminates the need for a probe, so less sequence information is
required, and the need for radioactivity is eliminated. However,
the level of polymorphism detected using these techniques may
still limit their usefulness in answering some population genetics questions.
23
In the last two years, a PCR-based technique called Ran­
domly Amplified Polymorphic DNA (RAPD) has proven very
successful for performing genetic analyses in plants (Williams
and others 1990). Its advantages are that it detects higher levels
of polymorphism than either isozyme or RFLP analyses, and
that no prior DNA sequence knowledge is necessary. A disad­
vantage to this technique is that the electrophoretic bands it
produces are inherited as completely dominant traits as op­
posed to semi-dominant traits as in RFLP or isozyme analysis,
which can make genetic analyses somewhat more complicated.
It has also been reported that some RAPD bands are not inher­
ited in a Mendelian fashion (Riedy and others 1992). Success
with this technique has been reported with some insects (Edwards,
unpublished results), including haplodiploid species where the
dominance problem can be avoided by performing the analysis
on males.
The characterization of individuals, as needed for paternity
analysis, requires an analysis that targets highly variable re­
24
gions of DNA. In humans, this is accomplished by analyzing
Variable Number Tandem Repeats (VNTR's), which are highly
mutable (10-3-10-4) regions of repetitive DNA (Jeffreys and
others 1985). The major disadvantage of this technique is that
few VNTR's have been found in insects (Bigot and others
1990, Blanchetot 1991, Moritz and others 1991), so it is likely
that the application of this analysis to the study of bark beetles
would first require significant time and effort to locate the
repetitive DNA. Once this was accomplished a very high de­
gree of polymorphism would be available for free grain dis­
persal studies and behavioral investigations requiring charac­
terization of an individual and its progeny.
Given the diversity of techniques for assessing polymor­
phism and discriminating groups under study, one of our first
goals might be to survey the level of polymorphism that is
found in the groups of interest. Then the appropriate technique
for a particular problem can be chosen.
USDA Forest Service Gen. Tech. Rep. PSW-138. 1992.
Systematics Research1
Donald E. Bright2
The following comments about bark beetle genetics reflect
my current thoughts on the status of bark beetle systematics and
present some of my ideas concerning future needs.
The basic alpha taxonomy of North American bark beetles
is essentially finished. All of the species occurring in North
America are, more or less, well characterized at the alpha level
and their taxonomy is well established. An authoritative name
can now be consistently given to almost any specimen or series
of specimens. Keys and descriptions to all species in all genera
of North American scolytids are now available. A catalog of the
world fauna will soon be available. It is extremely unlikely that
very many morphologically distinct, unnamed species of North
American Scolytidae remain to be discovered. Any newly dis­
covered species will probably be found in either host plants of
very restricted distributions (e.g., Santa Lucia fir), in the vari­
ous desert shrubs or deciduous trees, or as currently unrecog­
nized sibling species. I would estimate that fewer than 10
morphologically distinct, unnamed species remain undiscov­
ered in North America.
This, however, does not mean that systematic investiga­
tions are no longer needed. Indeed, we are now able to embark
on the most exciting aspect of systematic research. With all of
this basic knowledge, we are now left with the task of resolving
a number of species groups, subspecies, host/geographic races,
sibling species, and so on. Unresolved species groups are known
to exist in Dendroctonus, Ips, Pityophthorus, probably Scolytus,
and undoubtedly in a number of less studied genera. Also, on a
more fundamental level, there is a need to develop information
that will improve understanding of phylogeny, evolution, ge­
neric and tribal relationships, and other such basic systematic
questions.
Morphological studies have been the backbone of taxo­
nomic research for decades and will probably remain of pri­
mary importance for decades to come. However, we now have
an exciting array of other methods that may prove to be of
immense importance. Isozyme analysis, cuticular hydrocarbon
analysis and PCR techniques will be increasingly used in the
future to help resolve some of the questions mentioned above.
A molecular systematics laboratory is being established at
the Biological Resources Division, Canada Department of Ag­
riculture, in Ottawa, Ontario. I hope it will be able to address
some of the questions raised above.
One concern for the future of scolytid systematics is that in
less than 10 years the currently working North American
systemists will have all retired. I am not aware that any current
graduate student is working on scolytid systematics, nor are
any young, currently employed researchers working on this
group. I urge university faculty to seek out qualified individuals
and guide them into the systematic study of the Scolytidae.
Forest entomology has been fortunate during the past three
decades in having several active individuals interested in sys­
tematic studies of bark beetles, and this interest needs to be
continued.
1
An abbreviated version of this paper was presented at the Bark Beetle
Genetics Workshop, May 17-18, 1992, Berkeley, California.
2
Research Scientist, Canada Dept. of Agriculture, Systematics Research,
Centre for Land and Biological Resources Research, Ottawa, Ontario K1A 0C6
USDA Forest Service Gen. Tech. Rep. PSW-138. 1992.
25
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USDA Forest Service Gen. Tech. Rep. PSW-138. 1992.
Workshop Participants
Alosi, M. Carol, Pacific Southwest Research Station, USDA Forest Service,
1027 Trenton Ave., Bend, OR 97701; (503) 388-5422, FAX: (503) 3835733.
Amman, Gene D., Intermountain Research Station, USDA Forest Service,
507 25th Street, Ogden, UT 84401; (801) 625-5393, FAX: (801) 6255723. (retired)
Anderson, Wyatt W., Department of Genetics, 101B Life Sciences Bldg.,
University of Georgia, Athens, GA 30602; (706) 542-3326, FAX: (706)
542-3910.
Bentz, Barbara J., Intermountain Research Station, USDA Forest Service,
Forestry Sciences Lab, 860 No. 1200 East, Logan, UT 84321; (801) 7521311, FAX: (801) 753-8625.
Bright, Donald E., Canada Dept. of Agriculture, Systematics Research, Cen­
tre for Land and Biological Resources Research, Ottawa, Ontario, Canada
K1A OC6; (613) 996-1665, FAX: (613) 995-1823 or 7283.
Cane, James H., Dept. of Entomology, 301 Funchess Hall, Auburn Univ.,
Auburn, AL 36849-5413; (205) 844-5006, FAX: (205) 844-3005.
Gordon, Thomas R., Dept. of Entomological Sciences, University of Califor­
nia, Berkeley, CA 94720; (510) 642-3874, FAX: (510) 642-3845.
Mitchell, Russ, 20454 Steamboat, Bend, OR 97702, (503) 388-2869. (retired)
Page, Marion, Pacific Southwest Research Station, USDA Forest Service,
800 Buchanan Street, Albany, CA 94710; (510) 559-6300, FAX: (510)
559-6440.
Preisler, Haiganoush K., Pacific Southwest Research Station, USDA Forest
Service, 800 Buchanan Street, Albany, CA 94710; (510) 559-6300, FAX:
(510) 559-6440.
Raffa, Kenneth F., Dept. of Entomology, University of Wisconsin, Madison,
WI 53706; (608) 262-1125, FAX: (608) 262-3322.
Robertson, Jacqueline L., Pacific Southwest Research Station, USDA Forest
Service, 800 Buchanan Street, Albany, CA 94710; (510) 559-6300, FAX:
(510) 559-6440.
Seybold, Steven J., Pacific Southwest Research Station USDA Forest Ser­
vice, 800 Buchanan Street, Albany, CA 94710; (510) 559-6477, FAX:
(510) 559-6440.
Stanton, Maureen, Department of Botany, Univ. of California, Davis, CA
95616; (916) 752-2405, FAX: (916) 752-5410.
Greene, Lula E., Pacific Southwest Research Station, USDA Forest Service,
800 Buchanan Street, Albany, CA 94710; (510) 559-6300, FAX: (510)
559-6440.
Stauffer, Christian, Institute of Forest Entomology, Forest Pathology and
Forest Protection, Universität für Bodenkultur, A-1190 Vienna, Austria.
Present address: Dept. of Entomology, Univ. of California, Davis, CA
95616; (916) 752-0492.
Haack, Robert, North Central Research Station, 1407 S. Harrison Road, East
Lansing, MI 48823; (517) 355-7740, FAX: (517) 335-5121.
Stock, Molly W., Dept. of Forest Resources, College of Forestry, University
of Idaho, Moscow, ID 83843; (208) 885-7033, FAX: (208) 885-6226.
Hager, Barbara, Dept. of Environmental and Forest Biology, State Univ. of
NY, College of Environmental Science and Forestry, Syracuse, NY
13210; (315) 470-6758, FAX: (315) 470-6934.
Storer, Andrew J., Dept. of Entomological Sciences, University of Califor­
nia, 201 Wellman Hall, Berkeley, CA 94720; (510) 643-5806, FAX:
(510) 642-7428.
Hayes, Jane L., Southern Forest Experiment Station, USDA Forest Service,
2500 Shreveport Highway, Pineville, LA 71360; (318) 473-7232, FAX:
(318) 473-7222.
Teale, Stephen, Dept. of Environmental and Forest Biology, State Univ. of
NY, College of Environmental Science and Forestry, Syracuse, NY
13210; (315) 470-6758, FAX: (315) 470-6934.
Hobson, Ken, Intermountain Research Station, USDA Forest Service, For­
estry Sciences Lab, 860 No. 1200 East, Logan, UT 84321; (801) 7521311, FAX: (801) 753-8625.
Wood, David L., Dept. of Entomological Sciences, University of California,
201 Wellman Hall, Berkeley, CA 94720; (510) 642-5538, FAX: (510)
642-7428.
USDA Forest Service Gen. Tech. Rep. PSW-138. 1992.
31
The Forest Service, U.S. Department of Agriculture, is responsible for Federal leadership in forestry.
It carries out this role through four main activities:
• Protection and management of resources on 191 million acres of National Forest System lands
• Cooperation with State and local governments, forest industries, and private landowners to help
protect and manage non-Federal forest and associated range and watershed lands
• Participation with other agencies in human resource and community assistance programs to
improve living conditions in rural areas
• Research on all aspects of forestry, rangeland management, and forest resources utilization.
The Pacific Southwest Research Station
• Represents the research branch of the Forest Service in California, Hawaii, American Samoa
and the western Pacific.
Persons of any race, color, national origin, sex, age, religion, or
with any handicapping conditions are welcome to use and enjoy
all facilities, programs, and services of the U.S. Department of
Agriculture. Discrimination in any form is strictly against agency
policy, and should be reported to the Secretary of Agriculture,
Washington, DC 20250.
United States
Department of
Agriculture
Forest Service
Pacific Southwest
Research Station
General Technical
Report PSW-GTR-138
Proceedings of a Workshop on Bark Beetle Genetics: Current Status of Research
May 17-18,1992, Berkeley, California