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
The main flight starts in late May or June (late June in southern
B.C.) when in-stand temperatures reach 15°C. 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.