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
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.