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.
