Illustrated Guide to Pests


 

Western balsam bark beetle, Dryocoetes confusus

 

The western balsam bark beetle, (Dryocoetes confusus) is the most destructive insect of subalpine fir in British Columbia, causing scattered mortality over large areas, which is having significant, but non-quantified, impacts in these high-elevation ecosystems. It is the primary disturbance agent and the driving force of succession in most subalpine fir forests. D. confusus has a confusing life history, which has taken many years to elucidate.

Host: Mature subalpine fir (>90 years and >20 cm dbh) is the primary host throughout B.C. Occasional attacks on amabilis fir and even more rarely, Engelmann spruce, white spruce and lodgepole pine have been noted.

Life Cycle and Description: The western balsam bark beetle generally requires two years to complete its life cycle.

Adults: Beetles are approximately 3.5 mm long with dark brown bodies covered in erect reddish-brown or yellow hairs. Females have a circular concentration of hairs on their heads. Males also have these hairs, but they fewer and are less prominent.

The main flight starts in late May or June (late June in southern B.C.) when in-stand temperatures reach 15C. A second smaller flight, dominated by females, occurs in August, with little flight between flight periods. Females typically join males in established nuptial chambers. The second flight may be absent or very small depending upon annual weather patterns.

Pioneering males initiate attack on susceptible subalpine fir, excavating a nuptial chamber under the bark, while producing pheromones that attract both males and females. Males are polygamous and mate with 3-7 females.

Eggs: Females excavate brittle star-shaped egg galleries that radiate from the central nuptial chamber. When the tree is full of beetles, females release anti-aggregation pheromones to discourage further attack. Typically parent beetles and early instar larvae overwinter in the host. The following spring, adult females (old adults) may extend their egg galleries and lay additional eggs, before emerging to potentially attack a new host.

Larvae and pupae: There are three distinct larval instars, with a fourth instar sometimes observed. Parasitism and predation are highest during late instar and pupal life stages.

Overwintered larvae continue to develop, pupate and eventually becoming callow (immature) adults. This stage will overwinter again and emerge the following summer as mature adults, thus completing the life cycle in two years. Dependent upon annual summer temperatures variations in this life cycle are observed.

D. confusus can become active when sub-alpine fir reaches 70-90 years of age, with the incidence of attack increasing as stands age. Although green windthrow is a preferred host for D. confusus it is a relatively scarce resource. The more common event in these high elevation stands is top breakage and blowdown of old beetle killed trees. This combination of events gradually creates stand gaps and openings that allow understory regeneration to be released. In this way, it is felt that western balsam bark beetle is responsible in large part for driving succession in high elevation ecosystem.


Last updated on January 28, 2008
The contact for this web page is: kevin.buxton@gov.bc.ca