Western Balsam Bark Beetle
Dryocoetes confusus
Key Wildlife Value:
Western balsam bark beetles create snags of all sizes in high elevation areas wherever subalpine fir is found. Trees killed by western balsam bark beetle provide foraging habitat for woodpeckers and eventually contribute to levels of down wood when they break or fall over. Small-scale canopy gaps and concentrations of down wood created when aggregations of trees are killed may increase structural diversity and enhance habitat for various cavity-dependent wildlife.
Distribution in Oregon and Washington:
Found throughout both states in high elevation stands.
Hosts:
Subalpine fir is the principal host; occasionally found in other true firs, Engelmann spruce, and lodgepole pine.
Diagnosis:
Recent attacks may be difficult to detect using external indicators. Most attacks occur on the bole above 2 m (6.5 ft). Typical bark beetle attack symptoms may be present, including boring dust and entrance holes on the lower boles of standing trees, pitch flow, branch flagging, topkilling, and overall foliage discoloration. Copious pitch flow often indicates a tree was successful in repelling an attack. Tree foliage changes from green to brick red during the year following a successful beetle attack, and may be retained for up to five years.
Larvae are white legless grubs with brown heads. Pupae are soft and white, with body forms somewhat resembling adults. Adults are very small, ranging from 3 to 5 mm long, with shiny dark brown bodies and clubbed antennae. The posterior is somewhat flattened instead of being evenly rounded, and lacks spines. The front of the female beetle’s head is densely covered with a brush of short, reddish-yellow hairs and the front of the male’s head is sparsely covered with longer, reddish-yellow hairs.
Life History:
Adult beetles begin to emerge in late May or June when temperatures reach 15°C (~60°F). Flight activity generally peaks in mid-July and lasts through the end of September. Males are to drawn host trees by tree volatiles, where they bore through the bark to excavate nuptial chambers before emitting a pheromone to attract females. Each male usually mates with three to four females. Each mated female begins to excavate a gallery outward from the nuptial chamber in which she deposits her fertilized eggs. Adults overwinter in the galleries. The following spring, females continue extending their galleries and laying eggs before emerging in June or early July. The adults reemerge and move to fresh material, where females deposit a third brood either in the same tree or in a new tree. The beetles usually require two years to complete their life cycle from egg to adult, though some evidence suggests that the life cycle may be completed in one year given the proper climatic conditions.
More than half of the mortality caused by western balsam bark beetle is believed to result from a lesion-causing fungus, Grosmania dryocoetidis, carried on the beetle. Initial beetle attacks, though often pitched out by the tree, may successfully introduce the fungus. Establishment of G. dryocoetidis in the phloem facilitates subsequent beetle attacks. Trees may die without further beetle activity when coalescing lesions caused by the fungus girdle the tree bole.
Important Habitats and Outbreak Dynamics:
Western balsam bark beetle depends upon stressed trees for its survival, frequently occurring in association with drought, winter injury, and a complex of other organisms including the fungus G. dryocoetidis, balsam woolly adelgid, root disease, defoliators, and other bark beetles. It preferentially attacks larger, older, and slower growing trees.
Trees may be repeatedly strip-attacked and die slowly over a period of several years, or quickly killed in one season. D. confusus mortality occurs in discreet aggregations and do not expand to coalesce like mountain pine beetle group-kills do during outbreaks. Though only a small percentage of a stand is attacked during normal years, usually 5 percent or less, in some areas high beetle populations may persist for many years until most of the older subalpine firs are dead. D. confusus readily colonizes windthrown trees and other fresh down material, but it is uncertain what role this material plays in the population dynamics of the beetle.
Opportunities for Manipulation to Increase Wildlife Habitat:
It may be possible to create snags using western bark beetle semiochemical attractants, but methodology has not been developed for this specific use. Semiochemical attractant methods present some advantages over mechanical methods for creating snags, such as topping or girdling, in that they are less expensive, safer to administer, and mimic a natural process. Snags created using semiochemical attractants would likely provide good foraging habitat for woodpeckers during the years following initial colonization.
Potential Adverse Effects:
Although less aggressive and slower acting than some other western bark beetles such as the mountain pine beetle, western balsam bark beetle may interact with other agents, particularly drought, balsam woolly adelgid, and root disease, over many years to cause extensive mortality in mature subalpine fir forests. High levels of mortality can result in the degradation of wildlife habitat and aesthetic quality in high elevation forest landscapes, alter water quality and quantity dynamics, and produce abundant fuel accumulations, increasing the risk for high-severity ground fire.
Trees killed in campgrounds and other developed areas can pose hazards to public safety and structures, and may negatively affect screening, shading, and aesthetic values.
How to Minimize the Risk of Adverse Effects:
There is little that can be done to minimize the risk of adverse effects in stands affected by multiple damage agents or occurring in remote or inaccessible areas. In campgrounds and other high-value areas, infested trees and windthrows should be promptly removed from the area. As part of an integrated strategy that includes removal of infested material, it may be possible to use endo-brevicomin, the antiaggregant pheromone for western balsam bark beetle, to protect individual trees or high-value sites. This strategy has shown promise in British Columbia, but has not been tested in Oregon or Washington. It is likely the antiaggregant would have to be applied annually until elevated beetle pressure subsides, in order to ensure successful resource protection.
References
Bleiker, K.P., B.S. Lindgren, and L.E. Maclauchlan. 2003. Characteristics of subalpine fir susceptible to attack by western balsam bark beetle (Coleoptera: Scolytidae). Can. J. For. Res. 33:1538-1543.
Goheen, E.M. and E.A. Willhite. 2006. Field guide to common diseases and insect pests of Oregon and Washington conifers. USDA Forest Service, Pacific Northwest Region, Portland, OR. R6-NR-FID-PR-01-06. 335 pp. http://www.biodiversitylibrary.org/bibliography/80321#/summary
Garbutt, R. 1992. Western balsam bark beetle. Canadian Forest Service, Pacific Forestry Centre, Forest Pest Leaflet 64. 4 pp. http://cfs.nrcan.gc.ca/pubwarehouse/pdfs/4288.pdf
McMillin, J.D., K.K. Allen, D.F. Long, J.L. Harris, and J.F. Negron. 2003. Effects of western balsam bark beetle on spruce-fir forests of north-central Wyoming. Western Journal of Applied Forestry 18(4):259-266.
Molnar, A.C. 1965. Pathogenic fungi associated with a bark beetle on alpine fir. Can. J. Botany 43:563-570.
Stock, A.J., J.H. Borden, T.L. Pratt, H.C. Pierce, JR., AND B.D. Johnston. 1990. Endo-brevicomin: An antiaggregation pheromone for the western balsam bark beetle, Dryocoetes confusus (Coleoptera: Scolytidae). Can. Entomol. 122:935–940.