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The western spruce budworm is an important native defoliator of
interior Douglas-fir. Outbreaks have been recorded in BC since the
early 1900's to present in interior BC forests. Periodically,
population levels reach outbreak proportions. In 1987, over 800,000
ha were under attack by the budworm, mostly in the southern interior
of the province. Budworm outbreaks may be sustained for 25 years or
more.
Host trees: Primarily Douglas-fir, with other
tree species such as the true firs, larch and to a lesser degree,
spruce, also impacted by the western spruce budworm.
Description and life cycle: The western spruce
budworm requires one year to complete its life cycle.
Egg: Budworm moths mate and lay eggs in late
July through August. The eggs are laid in masses (up to 150) on the
underside of needles, overlapping like fish scales. They are bright
green when laid, measuring slightly less than 1 cm in total length,
and become translucent when empty. They hatch within 12 days of
being laid.
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
pupates.
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.
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