Forest tent caterpillar (Malacosoma disstria)
Western tent caterpillar (Malacosoma californica)
Pest description and damage The western tent caterpillar attacks a wide variety of plants including alder, ash, birch, cottonwood, and willow as well as fruit trees and roses. The over 1-inch long adult moths are attracted to lights at night. Mature western tent caterpillars can reach a length of about 2.0 inches and are hairy, dull yellow-brown, with rows of blue and orange spots on the body. Mature forest tent caterpillars can be as long as 2.5 inches and are black and blue with dorsal white "footprints." Eggs of these moths are laid on twigs or buildings and may be especially numerous around lights. The egg masses are brown to gray in color, about 0.625 inch in length, and look like bits of gray, hardened foam. The larvae of both species construct unsightly nests or "tents" in the crotches and branches of host trees. The larvae leave the tent by day to feed on foliage of host plants and can do significant damage by defoliation. The larvae usually return to the tent in the evening. Tent caterpillars can defoliate small trees. Defoliation can hinder plant growth, make the plants more susceptible to competition, diseases or poor weather and defoliation over consecutive years may weaken or kill unthrifty hosts. Healthy trees usually will grow new leaves by midsummer.
For biology, life history, monitoring and management
Alder (Alnus)-Tent caterpillar
See Table 2 in: