Hosts: Douglas-fir, white fir and spruce
The Douglas-fir tussock moth caterpillar is grayish with brightly
colored tufts of hair and a shiny black head. There are also two
long horns of black hairs behind the head and another at the rear
of the body. Insect feeding results in needle damage and defoliation,
which is sometimes followed by top-kill. Small cocoons and egg mass
cases are attached to the underside of twigs and branches.
Biology: Eggs hatch from mid-May to early June and caterpillars
feed on the current year's developing foliage. Young larvae are
4 to 7 mm long and covered with long, thin body hairs that later
develop into tufts. Because the female moth is wingless, the primary
means of dispersal from tree to tree is by windblown larvae. Young
larvae congregate on the tops of defoliated trees and drop by silken
may be over 3 meters long. These threads eventually break from the
tree and give a ballooning effect to the larvae. Mature larvae are
about 20 to 30 mm long with a gray or brown body and a shiny black
head. Two long hair pencils project forward from behind the head
and another occurs at the rear of the body. Four brushes of brown
or cream-colored hairs and red spots occur on the first four and
last abdominal segments. Some people develop an itchy rash from
exposure to the frequently airborne caterpillar hairs. After the
caterpillars are about half-grown, they feed on all age classes
of needles throughout the crown. Pupation occurs inside a thin,
silk cocoon spun on the undersides of branches from late July to
the end of August. Adults emerge shortly afterwards and the flightless
female deposits egg masses on twigs and branches.
Severe outbreaks can cause significant mortality of both overstory
and understory trees relatively quickly (1 or 2 years). Top-kill
occurs in less severe outbreaks. Bark beetle epidemics can occur
in tussock moth defoliated areas.
Similar Insects and Diseases: Early damage
and webbing in trees may be similar to that caused by western
spruce budworm; however, the larvae are very distinct.