Western Balsam Bark Beetle - Dryocoetes confusus Swaine
Forest Insect and Disease Identification and Management Training Manual, USDA, Forest Service, R-1, Timber, Coop. Forestry and Pest Management, Idaho Department of Lands, Bureau of Private Forestry - Insect and Disease Section, Montana Department of State Lands, Division of Forestry
The western balsam bark beetle is the most conspicuous of a complex of pests which are responsible for high amounts of tree mortality in sub-alpine fir stands throughout the Northern Region. infestations are chronic in some areas. In 1995, more than 49,000 acres were infested Regionwide. Nearly 58,000 subalpine fir were estimated to have been killed.
Hosts. - Sub-alpine fir. Occasionally grand fir, Engelmann spruce, and lodgepole pine may be attacked.
Damage. - Low populations maintain themselves in trees weakened by old age and root disease, storm-damaged trees, or slash. During periods of drought or other environmental stress, infestations can build and spread to less-susceptible stands. Groups of 100 to 1,000 trees, generally of larger diameters, may be killed. An estimated 35 percent of sub-alpine fir mortality is due directly to attack by beetles. The remainder is attributed to a beetle-introduced lesion-causing fungus, Ceratocystis dryocoetidis, and other unidentified fungi. Coalescing lesions may kill trees without further beetle activity. Often, other secondary bark beetls become a part of this tree-killing association.
Life history. - Little has been published on the biology of this species. It probably has a 2-year life cycle in much of our Region. They probably overwinter mostly as larvae under the bark the first year, continue development during spring and early summer, and overwinter the second year as nearly mature adults. Males bore into the phloem, excavate a nuptial chamber and mate with several females. Egg galleries radiate from the central nuptial chamber in a random pattern. Larvae extend their mines from the main egg galleries until freezing weather, then become dormant. Pheromone trapping conducted in northern Idaho and western Montana indicate initial flights take place in late June to early July. There may be more than one flight each year.
Identification. - External evidence of attack on the boles of standing trees is hard to detect. Entrance holes and boring dust on the bark may be visible. Pitch flow may be evident. Attacked trees generally turn yellowish-red within a year. Adults are shiny, dark brown, cylindrical beetles ranging from 3.4 to 4.3 mm long. Their thorax is evenly convex above; their posterior is abruptly rounded and without spines. The front of their head is covered with distinct bristles. Females have a denser patch of these 'hairs' than do males.
Management. - Because of the high elevation on which sub-alpine fir grows, silvicultural control is seldom possible. To keep epidemics from developing, logging slash should be destroyed. Weakened and beetle-infested trees or windthrown should be salvage logged when feasible. Aggregative pheromones have been developed which can help concentrate beetles into stands scheduled for harvest.
Doidge, D. F.
Stock, A.J., J.H. Borden and T.L Pratt.
[ Back ]