Western Balsam Bark Beetle
Key Wildlife Value:Western balsam bark beetles create snags of all sizes in high elevation areas when they colonize subalpine fir trees. Trees infested or killed by western balsam bark beetle provide foraging habitat for woodpeckers. Trees killed by D. confusus eventually contribute to levels of down wood when they break or fall over. When compact groups of trees are killed, canopy gaps result. Understory trees and shrubs may be released in canopy gaps, increasing structural diversity, and, depending upon initial stand characteristics, at times also resulting in an increase in compositional diversity of the mid- and upper canopy layers.
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 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. Look under the bark for characteristic gallery starfish-like gallery patterns consisting of a central nuptial chamber with several curving egg galleries radiating outward in a random pattern. Egg galleries lightly etch the surface of the sapwood.
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). The flight period may last until the end of July. 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 3 to 4 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. They then deposit a third brood either in the same tree or in a new tree. The beetles usually require 2 years to complete their life cycle from egg to adult, though some evidence suggests that the life cycle may be completed in 1 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, Ceratocystis dryocoetidis, carried on the beetle. Initial beetle attacks, though often pitched out by the tree, may successfully introduce the fungus. Establishment of C. 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 C. dryocoetidis, balsam woolly adelgid, root disease, defoliators, and other bark beetles. Trees may be repeatedly strip-attacked and die slowly over a period of several years, or quickly killed in one season. 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.
Opportunities for Manipulation to Increase Wildlife Habitat:Few opportunities exist for manipulation of western balsam bark beetle to increase wildlife habitat. Recent research on aggregating and anti-aggregating pheromones indicate that it someday may be possible to use a “push-pull” technique to direct the beetles to selected groups of trees.
Potential Adverse Effects:Western balsam bark beetle may interact with other agents to cause extensive mortality in subalpine fir forests, indicating that these systems are under stress. High levels of mortality can result in abundant fuel accumulations, increasing the risk for stand-replacement wildfire.
How to Minimize the Risk of Adverse Effects:Most infested stands occur in inaccessible areas, and there is little that can be done to minimize risk.
ReferencesGoheen, E.M., and E.A. Willhite. In prep. Field guide to the common diseases and insect pests of Oregon and Washington conifers. USDA Forest Service, Pacific Northwest Region, Forest Health Protection.
Garbutt, R. 1992. Western balsam bark beetle. Canadian Forest Service, Pacific Forestry Centre, Forest Pest Leaflet 64. 4 pp.
Molnar, A.C. 1965. Pathogenic fungi associated with a bark beetle on alpine fir. Can. J. Botany 43:563-570.
Website linksWestern balsam bark beetle links, An Online Catalog of Western Forest Insects and Diseases
not yet available: Field Guide to the Common Diseases and Insect Pests of Oregon and Washington Conifers