Larva: The western spruce budworm goes through 6
larval instars during its life cycle. Early instar larvae do not
feed, but overwinter as 2nd instars in silken webs (hibernaculae) on
tree branches under bark scales, and in flower and needle scars.
As buds swell in the spring (late May), larvae migrate and begin
to feed, initially mining the needles, and then moving onto the
developing buds and cones. With bud flush, larvae feed on the new
shoots, subsequently moving onto older foliage, spinning loose webs
around new foliage and feeding within them during the fifth and
sixth instars. Later instar larvae range from 16-30 mm long. The
upper body is yellow-brown with conspicuous paired ivory coloured
spots on its body segments. The head capsule is brown. Feeding
continues through late June to early July, at which time the larva
Pupa: The larva pupates from late June to
mid-July within the webbed foliage. The pupa is 12-16 mm long, being
broad at the head end, tapering to a point at the rear. It is
reddish brown in colour. The pupal stage lasts on average 2 weeks.
Adult: Adults emerge between mid-July and early
August, mate and lay eggs on host foliage. They die within two weeks
of emergence. They are inconspicuous looking insects, mottled grey
to rusty brown in colour, with a wingspan of 24-30 mm.
The greatest impact from budworm is among suppressed and
intermediate trees. Repeated budworm defoliation causes tree
mortality over large areas, reduction of growth rates and reduced
lumber quality. Sustained attack results in complete defoliation in
4 to 5 years. Once an infestation has subsided, defoliated trees
take several years to regain a full foliage complement, and
therefore radial growth rates require several years to attain normal
growth following defoliation by the budworm. Successive years of
defoliation in stands may predispose trees to other insects and
pathogens. Tree mortality may continue due to root disease, bark
beetles, loss of vigour and other causes.