Hosts: Aspen, willows, cottonwoods, and mountain mahogany

Symptoms/Signs: Western tent caterpillar is an early season defoliator with feeding damages typically occurring between May and June. Symptoms include moderate to complete defoliation of trees; large silken tents on branches; and presence of larvae in and around the tents. Trees repeatedly defoliated will have sparse foliage, minor branch dieback, and in some cases, tree mortality.

Mature larvae are 4 to 5 cm long and vary widely in coloration. Their heads are blue to black and body color patterns are mixtures of black, orange, and blue. Larvae are usually quite hairy.

Biology: Larvae overwinter as first instars inside egg masses glued around twigs. Larvae emerge from egg masses in spring and construct silken tents on branches that are used for shelter and molting during the daytime. At night, caterpillars feed outside of the tents. As the larvae mature, they disperse and become solitary feeders. Moths emerge from cocoons and glue egg masses to live twigs that are less than 2 cm in diameter.

Effects: Heavy defoliation of aspen for a number of years will cause growth loss and branch dieback. Some mortality may also occur during prolonged outbreaks. Outbreaks, however, are generally short lived, generally lasting 2 to 3 years.

Similar Insects and Diseases: See fall webworm and foliar diseases.

References: 3, 15, 23