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The Douglas-fir tussock moth is a destructive
native defoliator of Douglas-fir. Outbreaks of
tussock moth occur every ten to twelve years
causing significant damage and mortality to
Douglas-fir stands in the interior of the
province. These outbreaks tend to last up to
four years before natural controls such as
predators, parasites, pathogens, and starvation
lead to population collapse.
Host trees: Primarily
Douglas-fir, occasionally ponderosa pine and
western larch.
Description and life cycle:
The tussock moth has a one year life cycle.
Adults appear from late July to early September.
The adult female is stout bodied, wingless and
sedentary, usually remaining camouflaged on her
cocoon. Males are slender bodied with about a 30
mm wingspan. Males emerge before females and fly
in search of females. Females attract males by
emitting a sex pheromone and mating occurs on
the cocoon, typically, on the same day that the
female emerges. Each female lays approximately
200 eggs in a single mass on her empty cocoon.
The action of depositing her eggs dislodges hair
from her abdomen which mixes with a frothy
cement produced during oviposition. The egg
masses overwinter. Larvae hatch in late spring
and feed voraciously on the current year’s
foliage. As the larvae mature, they feed on both
old and new foliage. In late July the larvae
pupate in cocoons on the underside of foliage
and emerge two weeks later as adults to begin
the cycle again.
Damage symptoms: The upper
part of the crown and the branch tips are
defoliated first. The remainder of the foliage
is destroyed as the larvae migrate down the
crown. By July, defoliated trees appear
scorched. Trees may die after one or more years
of severe defoliation. Frequently, the top third
of the crown is completely defoliated, which
leads to damage in the form of top-kill and
branch dieback. Douglas-fir trees that have been
weakened by tussock moth defoliation may also be
susceptible to attack by other insect pests,
such as the Douglas fir beetle.
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