Western Balsam Bark Beetle
The western balsam bark beetle (Dryocoetes confusus) is the most destructive insect of subalpine fir in British Columbia, causing scattered mortality over large areas.
This beetle is having significant impact in these high-elevation ecosystems, though the effects are difficult to quantify due to its scattered attack pattern. It is the primary disturbance agent and the driving force of succession — the openings left by the dead fir are usually filled by regenerating younger fir that were waiting in the understory for the canopy to open.
Adults are 3.4 to 4.3 millimetres long, dark brown and covered with erect, red-brown hairs. They emerge in late May or June. The life cycle normally requires two years, but given the right climatic conditions, it could be completed in one year.
Tree Species Attacked
Primarily subalpine fir is attacked, and occasionally amabilis fir. Some attacks of white spruce and Engelmann spruce have been recorded. Mature trees are targeted.
The extent of an infestation is difficult to determine as a result of overlapping life cycles, a lack of telltale pitch tubes, and the fact that the majority of the attacks occur above two metres on the bole. Newly attacked trees show pitch streaming along the bole, and a mixture of boring dust and frass in bark fissures and at the base of the tree.
The adults construct egg galleries that have a central nuptial chamber with brood galleries radiating from the top and bottom.
The foliage of an attacked tree will change from green to a bright brick-red in the year following the attack, though the red needles may be retained for up to five years.
Given appropriate conditions, balsam bark beetles can be responsible for extensive tree mortality in stands containing a large percentage of the preferred host. Normally, however, less than five per cent of a stand is attacked in a single season, with the damage usually scattered throughout the stand.
The adult carries a lesion-causing fungus, Ceratocystis dryocoetidis, which is responsible for an estimated 65 per cent of the mortality associated with balsam bark beetles. The lesions caused by the fungus may girdle and kill a tree, or make the tree susceptible to further beetle attacks.