Forest Insect Defoliators
Western Tent Caterpillar
Malacosoma californicum (Packard)

Hosts: Aspen, willows, cottonwoods, and mountain mahogany

Figure 46. Figure 46. Adult western tent caterpillar and egg mass.

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.

Figure 47. Figure 47. Larva of western tent caterpillar.

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 following mating glue egg masses to live twigs that are less than 2 cm in diameter.

Figure 48. Figure 48. Tents of western tent caterpillar on aspen.

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.

Figure 49. Figure 49. Western tent caterpillar larvae and defoliation of aspen on the Carson NF, New Mexico.

Similar Insects and Diseases: See fall webworm, large aspen tortrix and foliar diseases of aspen.

References: 3, 16, 24