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W187: Interactions Among Bark Beetles, Pathogens, and Conifers in North American Forests

Duration:
October 01, 1999 to September 30, 2004
Administrative Advisor(s):
David Thawley (NEV)
NIFA Reps:

Statement of Issue(s) and Justification:

STATEMENT OF THE PROBLEM:

Bark beetles and pathogens interact to cause extensive losses in the forests of North America. The vast areas affected, the hidden nature of root diseases, the episodic nature of bark beetle infestations and the declining emphasis on extracting timber contribute to a lack of current, regional estimates of the impacts of these pests. Yet these losses are significant. For example, in 1997 an aerial survey of National Forests in California found groups of mortality on more than 90,000 acres (table 1). While the exact cause of mortality was not determined, bark beetles and root diseases are the most likely candidates. These numbers are very low compared with most years, as moisture was above normal for several years prior to 1997.

Pitch canker provides yet another example of the extent of insect-pathogen problems. This introduced pest was found in California in 1986. After just 14 years, an active infestation zone of 23.1 million acres was declared (http://frap.cdf.ca.gov/pitch_canker/zone_infestation.html). Movement of potential host materials out of this zone is discouraged.

There are far too many insect-pathogen-host systems for entomologists and pathologists to study each one. W-187 and its predecessor W-110 have made considerable progress by developing a general model of potential interactions, and studying the specific interaction, and then fitting the results into the context of the larger model. This has permitted us to extrapolate our findings to other insect-pathogen systems, and has greatly increased our understanding of all of our pest systems.

JUSTIFICATION:

Regional research project W-110, and its successor W-187 have solved problems facing our forests, but the changing management objectives from wood products to other resource uses (including wildlife and resource protection), the dramatic increase of urbanization in our native forests over the last two decades, and the increased threat of introduced pests require new approaches. New scientific technologies enable us to address old problems that were previously intractable. As outlined below, regional research project W-187 should continue because of the values at stake, the intrinsic nature of the problems, the need for cooperative work, the benefits that accrue from the proposed research, the relationship to current regional and national priorities, and this project's impact on science. An important value of the collaborative research proposed here is that the ecological mechanisms associated with interactions among bark beetles, pathogens and conifers are similar among different taxa found throughout North America. The investigators participating in this regional project present strengths through a diversity in research approaches, individual subject species, and by research projects conducted under different climatic regimes.

Values: The commodity value of western forests has supported the economy of many parts of western North America. However, society also values forests for watershed, wilderness, recreation, and habitat for threatened and endangered fish and wildlife species. These ecological and aesthetic values can exceed those of forest products such as timber, and their consideration has caused the removal of many forests from the timber-producing base. Traditional methods of pest management often rely on silvicultural options - most of which involve some form of timber harvest or regeneration techniques to modify the environment. When forest lands are reserved, these silvicultural options are restricted or severely limited. Yet these valuable lands are what must be managed to prevent ecological imbalances that can cause devastating outbreaks of pests and loss of the resource, as well as the quality life for the largely rural communities dependant on the forest. As forests are removed from the timber-producing base, the value of the remaining timber-producing forests will increase. Pest management in these forests will thus become increasingly important. Also, with Ecosystem Management, we need to more fully understand the role insects and pathogens play as disturbance agents in healthy, functioning ecosystems, and the effect of management on these processes.

Bark beetles and root pathogens are among the most important agents that destroy forests. In 1991, the Southern pine beetle was at outbreak levels in over 10.7 million acres in eight mid-Atlantic and southern states (Hofacker, et al., 1992). In the same year, over 2.2 million trees were killed by the mountain pine beetle in 11 western states. Losses due to the Douglas fir beetle, western pine beetle, spruce beetle and the fir engraver beetle occurred on 735,000 acres in Oregon and Washington. Root diseases are responsible for approximately 18% of the total tree mortality in the western United States, causing losses of up to 1.5 billion dollars annually (including stumpage worth) (Smith, 1984). These losses are greatly underestimated since they include only losses due to tree mortality, while losses caused by growth reduction are probably much greater, but are very difficult to measure. Perhaps more important, these are losses only in timber value; watershed, wilderness, recreation, and habitat values are not considered. Importance and Extent of the Problem: Losses due to pests are extensive. Reducing these losses can provide a more stable supply of timber and additional social benefits. Native insects and diseases severely debilitate forests, leading to catastrophes, such as the Yellowstone fires which burned forests that were heavily infested by mountain pine beetle, dwarf mistletoes, and root diseases. Unfortunately, much of our understanding of insects and diseases comes from studies of pests in outbreak status. We must know much more about the ecological interrelations among insects, diseases, symbiotic organisms, and trees in non-epidemic situations. W-187 is focused on the essential components of complex tree-beetle-fungal interactions so that we can better understand forest and ecosystem level processes.

Need for Cooperative Research: The task facing this group is large and too complex for any one group or discipline. Multiple disciplinary cooperative research is needed to assemble the expertise required to attack this problem. Entomologists, pathologists, systematists, and physiologists working under the aegis of W-187 have already made significant progress toward understanding the role of bark beetle-carried fungi in triggering host wound response and tree decline. We developed a conceptual model of host tree - bark beetle - fungus interactions (Fig. 1). This model, in addition to visualizing our concepts, serves as an operational framework for our collaborative research. Work on one process or interaction, without consideration of other processes, has applicability only to that population at that time. For example, studies of bark beetle brood success under various conditions provide little useful information unless they consider the taxonomy and biology of beetle fungal associates. By fitting our work within the context of this conceptual framework, and sharing our techniques, research results can be extrapolated to fill gaps in our knowledge. For example, we are studying the response of Clerid predators to the pheromones of their host bark beetles in three different geographic settings, and in three beetle-tree systems. These predators are more abundant the year following the peak of the bark beetle population. This ultimately leads to development of principles which can be further evaluated to determine their broad application, for example that predator populations depend on host density. Similarly, cooperative research will also help workers overcome the taxonomic barriers that often prevent them from correctly identifying subtle differences among fungal taxa. By fitting our research into this conceptual framework, we eliminate fragmented research and wasteful duplication and broaden the expertise and methods available to solve the problems.

Benefits: W-187 has benefitted forestry throughout the country in many ways. Resource managers already use knowledge provided by W-187 scientists regarding strain variation and the correlated host specificity in Heterobasidion annosum, Armillaria ostoyae and Leptographium wageneri in the management of these root diseases. W-187 scientists have produced the research that lead to the major reclassification of Ophiostomatoid fungi associated with bark beetles. Similar recognition of strain differences among fungi and determination of the complex taxonomic relationships between fungi associated with bark beetles have provided insights into the differences in host colonization and geographic distinctions in the biologies of broadly distributed species. W-187 scientists have also begun to unravel the complex interaction between bark beetles, their semiochemicals, fungal associates, and natural enemies. This understanding will help resource managers to minimize pest populations, especially in areas where harvesting is not an option.

The results from the research efforts have provided both management tools and fundamental understanding of complex relationships among pathogenic fungi, bark beetles, and host trees. The biotic interactions have been studied within the context of the abiotic environment which facilitates extrapolation of the conclusions to different systems in different regions. The ability to extend both the fundamental and applied information across different systems is one of the key benefits of the a regional research program of this breadth.

Related, Current, and Previous Work:

A search of the CRIS database turned up 126 projects with the keywords forest and insect or forest and disease. Of these, 17 were relevant to W-187, and all but three institutions were members of W-187. These people will be contacted shortly. Two projects which are not part of W-187 are seeking to understand the biochemistry of host defenses. Both P?s are biochemists, and may be unaware of W-187. Studies of host biochemistry are a missing component of W-187 efforts, but for results to be meaningful, they must be placed in the larger context of host stress, host volatile production and potential host-induced resistance.

Only one regional project, W-189 Natural Products Chemistry as a Resource for Biorational Methods of Insect Control, is even closely related to W-187. Their objectives focus on insect and host plant physiology. The CRIS report for this project was missing, so we could not identify the host plants of interest. We will contact this group to determine if there is any common ground.

Most of the work in North America on interactions among bark beetles, pathogens and conifer hosts is done by members of our group. This work was extensively reviewed in a 1993 book entitled Beetle-Pathogen Interactions in Conifer Forests (Schowalter and Filip 1993.) This book remains the key reference in this area. The Critical Review appended to this proposal discussed our more recent publications dealing with specific host-insect-fungus systems. The interaction between entomologists and pathologists continues to provide a unique perspective and approach for solving problems and synthesizing progress. W-187 members Paine, Raffa and Harrington collaborated on an annual review article entitled Interactions among scolytid bark beetles, their associated fungi, and host conifers (Paine et al 1997). They cite three key critical areas in bark beetle-fungus-host tree relationships where better understanding is needed:

? characterization of the multiplicity of potential interactions among organisms ? description of the dynamic rate of interactions at the biochemical level; and ? examination of a broader taxonomic range of associated microorganisms.

Each of these needs is covered under at least one of W-187's objectives. W-187 brings together the scientists working in this area, provides a framework for fitting their research results into a larger conceptual model, and facilitates the collaboration necessary to understand the complex problem of bark beetle-pathogen-conifer interactions.

Objectives

  1. Characterize the role of biotic and abiotic factors in predisposing trees to bark beetle attack and subsequent mortality
  2. Characterize interactions among conifer hosts, bark beetles, their natural enemies, and vectored fungi
  3. Characterize the taxonomic diversity and genetic structure of key fungal pathogens and symbiotic fungi associated with insects on North American conifers

Methods:


Included with the procedures for each objective are the cooperative studies to be undertaken in this regional project. Since many of the interactions are shared in objective 1 and Objective 2 (see Fig. 2 and 3), these shared interactions are discussed together. The procedures for objective 3 are discussed separately. It should be noted that other research planned by the participants often extends beyond the cooperative research outlined in this regional project description.

Objectives 1. Characterize the role of biotic and abiotic factors in predisposing trees to bark beetle attack and subsequent mortality; and 2. Characterize interactions among conifer hosts, bark beetles, their natural enemies, and vectored fungi.

Interaction 1: Effects of the abiotic environment on bark beetles.

Studies are proposed to determine the effects of abiotic conditions, particularly temperature influence on bark beetle flight period activity, host colonization, developmental success. Previous studies have examined the survival of larvae under winter conditions and how aspect and snow cover affect the developmental rates and mortality. These field and laboratory studies on cold hardening will be continued and expanded to other host tree systems. The results can be incorporated into beetle phenology and population growth models to refine their predictive values.

Interaction 2: Effects of the abiotic environment on host trees.

Water availability, temperature, ozone, fire, and soil characteristics may change tree resistance to bark beetle colonization, susceptibility to fungal infections, or the quality of the tree as a resource for beetle development. Characteristics of host resistance or suitability, including photosynthetic rate, stomatal conductance, terminal and radial growth, resin flow, carbohydrates, phloem thickness, resin composition, and volatile emission, will be evaluated following experimental manipulation of the critical abiotic factors. Drought, in particular, may lower the inducible defense systems that some plants possess. Atmospheric pollutants, including ozone and dry nitrogen deposition, may alter the source-sink relationships within trees and change the allocation of photosynthate between growth and defense within trees.

Interaction 3: Effects of the abiotic environment on fungal pathogens and fungal associates.

Fungal pathogen populations are greater in environments that are stressful to their hosts. The extent of Heterobasidion annosum, Armillaria ostoyae, and Leptographium wageneri pathogenicity across managed and unmanaged forests will be related to habitat type that incorporates detailed site information, such as soil type, fertility, drainage, drought, defoliation history, and other diseases. The research will provide information to explain how host stress operates to increase the incidence and rate of infection and mortality caused by A. ostovae. In addition to studies on root pathogens, similar research will be initiated to examine temperature dependent growth rates of the fungi associated with bark beetles in the host tissue.

Interaction 4: Biotic environmental effects on the natural enemies of bark beetles.

Research will continue in several bark beetle- natural enemy systems to determine the influence of various biotic factors such as other beetle species as alternate hosts or nutrition on natural enemy populations. Studies are planned to evaluate the effect of predators on populations of different beetle species, the response of natural enemies to components of the beetle host pheromones, how these responses change across broad geographic areas, the influence of the fungi associated with the beetles on the natural enemies, and the influence of host trees on the interactions between natural enemies and their bark beetle prey.

Interaction 5: Effects of the biotic environment on host trees.

Biotic factors such as defoliators, mistletoes, rusts, and intraspecific competition among trees can affect the suitability of host trees for bark beetles and associated fungi. Defoliators, parasites, pathogens, and plant competition can affect photosynthetic rates, stomatal conductance, terminal and radial growth, secondary metabolism, resin flow, and phloem chemistry (especially carbohydrates, nitrogen, and the capacity for inducible defenses). When entire stands are affected, as in spruce budworm outbreaks or thinning practices, there is great potential for landscape level effects on bark beetle population dynamics.

Interaction 6: Effects of fungal associates on bark beetles.

Fungal associates specifically vectored within elaborate mycangia, commonly associated with the external surface of the insect body, or on phoretic mites appear to influence bark beetle success by improving nutritional quality of the host, limiting the growth of antagonistic fungi, or by limiting the impact of host defenses on attacking adults. The nature of these interactions are of critical ecological interest but not well understood. The almost universal association of Dendroctonus spp. with fungi and the apparent coevolution of the specific mycangial relationships suggest that these associations are critical for both beetle and fungal success.

Interaction 7: Effects of bark beetles on fungal associates.

Bark beetles may affect the success of their fungal associates through the process of transmission into the host trees, by influencing the growth of the fungi in the tree, or by fostering the growth of the fungi within the mycangia. The fungi are acquired by the adult beetles prior to emergence from their natal host and must be maintained between hosts. This is accomplished by many Dendroctonus spp. within a mycangium. Secretory cells lining these structures in many beetle species apparently foster the growth of the fungi such that there are fungal propagules present during the colonization process. In addition, there may be by-products of insect metabolism (e.g. nitrogenous waste) that stimulate fungal growth and sporulation. Because these conidia may be important sources of insect nutrition,the interactions of the beetle on the fungi may be critical for insect success.

Interaction 8: Effects of host trees on bark beetles.

Bark beetle reproductive success is affected by host tree condition. The ability of the tree to resist colonization (especially as influenced by resin flow and lesion formation) may result in increased adult mortality and limit the fecundity of surviving bark beetle adults. In addition, phloem characteristics such as thickness, moisture content, carbohydrate levels, and nitrogen content, may impact the survival, development time, and adult size of bark beetle progeny. Correlating components of beetle success (number of successful and failed attacks; number of galleries; gallery length; number of eggs; number of larvae, pupae, and emerging adults) with tree characteristics will clarify our understanding of host effects on beetle population dynamics.

Interaction 9: Effects of bark beetle populations on host trees.

The effects of the host tree on bark beetle populations is the culmination of the entire conceptual model. All the other interactions describe factors which lead to successful bark beetle colonization and subsequent tree mortality, whereas this interaction is the final result. All the other interactions ultimately have an effect on this one. Colonization by the beetles results in tree debilitation either through exhausting nutritional reserves or primary resin defenses (resin flow rate and volume of flow), through transmission of pathogenic fungi or through a combination of these mechanisms. A consequence of tree mortality is resource availability for many species of beetles which may result in a population increase or a population refuge. Area wide population levels can be evaluated through permanent plots or periodic surveys and spot growth models.

Host trees partially infested with less aggressive bark beetle populations provide a refuge for endemic populations of the more aggressive bark beetles. Secondary bark beetles can predispose host trees to the more aggressive bark beetles. For example, at endemic mountain pine beetle population levels, trees infested with less aggressive secondary beetle species provide a refuge for the mountain pine beetle, thereby allowing them to maintain an endemic state. Diseased host trees play a role in providing a refuge for endsemic beetle populations.

Interaction 10: Effects of pathogens and fungal associates on host trees.

Colonization of root systems by pathogens is an important contributing factor in successful bark beetle colonization. The degree of infestation by root pathogens for successful and unsuccessful colonization remains unclear and will be the subject of continued investigations to assess individual tree and stand risk. Percent infection can be related to changes in the physiological processes that directly impact bark beetle survival immediately following initial attack. Preliminary evidence suggests that Armillaria root disease and infection by Leptographium spp. can alter the composition of host resins and induce moisture stress, both processes of which are important in the preformed defense system of trees. Similarly, the fungi vectored by bark beetles may reduce tree vigor, reduce the host resistance, and may result in tree mortality. The fungi may alter water conduction through the stem or affect the preformed (resin flow or volume of flow) or induced components of host resistance. The impacts of the fungi on the tree may directly affect beetle colonization success. In addition, the level of twig and branch mortality caused by the pitch canker fungus and Pityophthorus spp. is related to the occurrence of bole cankers and subsequent tree mortality caused by Ips.

Interaction 11: Host tree influences on fungal associates of bark beetles and tree pathogens.

The host tree species may influence the species composition of the fungal associates of bark beetles as well as the pathogen species. In addition, there may be seasonal effects expressed through tree phenology and tree chemistry on this interaction. Other interactions with the host tree that may influence fungi are drought stress, shade stress, elevated temperature and elevated ozone levels as well as other factors that may affect host tree condition.

Interaction 12: Bark beetle influences on natural enemies.

Natural enemies of bark beetles show a density dependent response to bark beetle populations. Some aspects to be considered in this interaction are the influence of bark beetle species and the density of natural enemies, i.e. some beetle species are more heavily attacked by natural enemies. In addition, natural enemy complexes may vary due to bark beetle species or to geographic variations within a species. This will be tested through both surveys and responses to pheromone lures.

Interaction 13: Natural enemy influences on bark beetles.

Although there have only been a few attempts to demonstrate the impact of natural enemies on bark beetles, many researchers feel that parasitoids and predators regulate bark beetle populations. The assumptions can be tested in many different systems in laboratory cages, on artificially infested bolts, or by exposure of beetle brood in cut trees. Beetle densities will be recorded by rearing and final peeling and counting the life-stages still beneath the bark. Natural enemies will be evaluated in the same way.

Interaction 14: Influences of host trees on natural enemies of bark beetle.

Host tree species, as well as host tree condition, may affect the natural enemy complexes of a bark beetle. The natural enemy complex of bark beetle species that infest different hosts may be compared by rearing cut infested bolts. Both predators and parasitoids are collected from specialized rearing containers, identified, sexed and counted.

Interaction 15: Bark beetle fungal associates and tree pathogen interactions with bark beetle natural enemies.

Studies in Georgia and California have indicated that bark beetle parasitoids may be using volatiles from logs infected with bark beetle fungal symbionts to locate their prey. However, the question of whether bark beetle natural enemies are capable of vectoring tree pathogens remains unclear. For example, there are a large number of cortical-feeding insects that have been demonstrated to vector the pitch canker fungus, but further research is required to determine whether natural enemies emerge from infested material carrying fungal spores and whether the natural enemies are capable of fungal transmission.

Interaction 16: Effects of biotic environment on pathogens and fungal associates.

The biotic environment, which includes the population dynamics of living organisms, can affect the epidemiology of pathogens/fungal associates and therefore affect disease severity in forest ecosystems. Pathogen populations and disease severity may be greater in unmanaged forests or improperly managed forests. In addition, living organisms such as bark beetles, defoliating insects, and fungi affect their host trees and indirectly affect pathogens and fungal associates. For example, the incidence and severity of Armillaria spp. may be greater where the fungal anatagonist Trichoderma citrinoviride is absent. In addition, the interaction between the biotic (e.g., fungal pathogens, defoliators) and the abiotic factors (e.g., drought) may be critical in pathogen success. Biotic agents such as fungi or insects directly affect pathogens and fungal associates through direct attack or feeding (i.e., insect feeding on dwarf mistletoe plants).

Interaction 17: Effect of the biotic environment on bark beetle populations.

There is no question that the biotic environment influences the dynamics of bark beetle populations. The biotic environment continues to be represented in our model by many attributes, including landscape level processes and patterns, forest ecosystem processes (e.g. nutrient cycling, animal interactions), stand structure, management, and fire effects. Typical features of the ecosystem such as the ones listed are hypothesized to be important drivers of bark beetle populations in particular. Recent technological advancements in tools such as remote sensing and geographic information systems, as well as canopy patch dynamics techniques, are providing a means to study the response of beetle and pathogen populations in the landscape context. The influences of management on these interactions is being examined by comparing patch profiles in wilderness areas with previously harvested sites nearby. How the spatial/temporal aspects of bark beetle populations and host trees infested with the root diseases interact to predispose trees to mortality is also being explored.

Objective 3: Characterize the taxonomic diversity and genetic structure of key fungal pathogens and symbiotic fungi associated with insects on North American conifers.

Although significant progress has been made, the fungi associated with bark beetles have long presented taxonomic problems. These fungi all have morphological features, including evanescent asci and long necked perithecia that facilitate arthropod dispersal. Although genera such as Ophiostoma, Ceratocystis, Leptographium, and Ceratocystiopsis have traditionally been placed in a single taxonomic order or even family, there is increasing evidence that the group is polyphyletic. Species of Ceratocystis represent a separate convergent lineage. Other scolytid associates, species of Ambrosiella, do not even constitute a monophyletic genus, but rather have some species related to Ceratocystis and others to Ophiostoma. New molecular techniques and cladistic analysis provide a means to begin to determine relationships of fungi with convergent morphological characters.

Defining monophyletic groups is an essential first step toward the eventual goal of this part of the project, the development of rapid identification protocols for species identification and determination of population structure. Once conservative DNA regions have provided evidence of monophyletic groups at the ordinal and generic level, elucidation of inter- and intraspecies level problems can begin using more variable regions of the DNA. Identification of fungal taxa will be accomplished using standard morphological criteria, vegetative compatibility, and nucleotide sequence data where available. Monophyletic groupings of fungal taxa will be based on cladistic analysis of DNA sequence data from appropriate regions in the nuclear or mitochondrial genome. The identification of monophyletic groups by sequence analysis will facilitate the development of taxon-specific oligonucleotide probes, which in turn will enable rapid identification of difficult species. A similar approach has been used to identify ectomycorrhizal fungi. Virulence and host range variants will be circumscribed using standard greenhouse and field assays for pathogenicity. To characterize population structure, variants will be identified based on allozymes, restriction fragment length polymorphisms, randomly amplified DNA, or direct measures of DNA sequence divergence. These data will be analyzed using standard methods in population genetics to describe population structure and test for populations subdivision. A diagram of the four levels of activity and their relationship is shown in Fig. 5.

Measurement of Progress and Results:

Outputs:
Outcomes or projected Impacts:
Milestones:

Projected Participation:

Include a completed Appendix E form

Outreach Plan:

Organization and Governance:

The technical committee shall consist of the Administrative Advisor (non-voting), and at least one representative from each participating Experiment Station and Cooperating Agency. Each participating Experiment Station and Agency shall be entitled to one vote.

The officers of the Technical Committee will be a Chair, Past-Chair, and a Secretary. Terms of office will be one year. The Chair, Secretary, and the Past-Chair will constitute the Executive Committee, which will be responsible for the routine affairs of the Committee, and will pass on pressing matters which arise between annual meetings of the Technical Committee. The Secretary will become the Chair in the subsequent year and a new Secretary the elected each year at the annual meeting with all members of the Technical Committee eligible for office. In addition, three members-at-large will serve for a period of two years. These positions are staggered so that a member-at-large is elected by the Technical Committee each year. Along with the officers, this constitutes the Coordinating Committee for the Technical Committee.

The Chair, in consultation with the Administrative Advisor, will notify the Technical Committee members of the time and place of the annual meeting, prepare the agenda, and preside at the meeting. The Secretary will be responsible for the final preparation and submission of the annual report to the Administrative Advisor who, upon approval, will send it on to the Western Directors. The Secretary will also record the minutes and preform other duties assigned by the Technical Committee.

To facilitate communications with members, we will continue to support and update the W-187 web site used to develop this proposal. This site is located at http://www.usu.edu/~forestry/w187/. This site will allow members and non-members access to our reports, results and other useful information.

Literature Cited:

Attachments:

[Outline.pdf]

Internal Linkages:

External Linkages:

Signatures:


s:/David Thawley

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