Forest tent caterpillar, Malacosoma disstria
 

Fig. 317   Adult forest tent caterpillar.	Fig. 318  Forest tent caterpillar larva on bur oak.	Fig. 319  Trees defoliated by forest tent caterpillar.
Fig. 320  Trees defoliated by forest tent caterpillar.	Fig. 321  Forest tent caterpillar cocoon on aspen

Distribution:  Throughout B.C. 

TREE SPECIES ATTACKED: Mainly trembling aspen is attacked, but also black cottonwood, balsam poplar, red alder, hybrid poplars, and paper birch. Willow and apple are also attacked.

WHAT TO LOOK FOR: Greyish egg bands on twigs and branches in winter; groups of larvae with characteristic keyhole-shaped dorsal spots feeding on leaves that are not webbed or rolled. 

INSECT DESCRIPTION & DAMAGE SYMPTOMS: Larvae are dark brown, hairy, and 4.5 to 5.5 cm long when mature. They have a prominent row of white keyhole-shaped dorsal spots, two fine orange lines on either sides of the white spots, and a broad, bluish lateral band below the lines. Moths are light to dark reddish-brown, stout, and hairy. The forewings have two dark oblique bands, and the wingspan is 2.5 to 4.5 cm. Brown or grey (when weathered) egg bands covered with silvery-brown foam can be found encircling small twigs, from late summer to early spring. Larvae emerge in early spring when the buds swell, and feed gregariously on opening buds and on leaves. Fully grown larvae congregate on the trunk or on large branches, and will migrate to other broadleaf trees and shrubs if all the foliage is consumed. Unlike northern tent caterpillars, forest tent caterpillars do not form a tent, but spin a trail of silk wherever they go. Mature larvae spin cocoons between leaves or in any sheltered place, and pupate in these cocoons.

DAMAGE: Infestations usually occur about every 10 years, and last 3 to 5 years. One or more years of severe defoliation may result in top-kill, branch mortality, reduced radial growth, and if the infestation persists, occasional mortality. Significant tree mortality has not been recorded. Severely defoliated trees usually refoliate in mid-summer.

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Contact Tim Ebata if you have comments on the presentation of this information.

BC Ministry of Forests
Forest Practices Branch
P.O. Box 9513 Stn. Prov. Gov.
Victoria, BC
V8W 9C2

Section phone: (250) 387-8739
Section fax: (250) 387-2136

Last updated March 04, 2002