Alder (Alnus)-Tent caterpillar

Forest tent caterpillar (Malacosoma disstria)
Western tent caterpillar (Malacosoma californicum pluviale)

Pest description and damage The western tent caterpillar attacks a wide variety of plants including alder, apple, ash, birch, cherry, cottonwood, and willow, as well as fruit trees and roses. The adult moths are stout, light to darker brown, and are active in early- to mid-summer. Adults are attracted to lights at night. Western tent caterpillars are hairy, dull yellow-brown, with rows of blue and orange spots on the body. Forest tent caterpillars are black and blue with dorsal white "footprints." Eggs of these moths are laid on twigs or buildings in masses and may be especially numerous around lights. The eggs are brown to gray in color, about 0.0625 inch long, 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 weak and unthrifty hosts.

Biology and life history Tent caterpillars overwinter as egg masses on twigs (or buildings). The eggs hatch as buds break in April or May. The young larvae feed in groups for 5 to 6 weeks, growing larger and molting (shedding skins) four times. As they mature, they split into smaller groups and move to new feeding sites in the tree returning to the tent most evenings. In mid-June, the mature larvae congregate in large clusters on dense mats of webbing on the tree trunk and then begin to migrate in search of sheltered sites where they can spin their cocoons and pupate. Adults emerge 7 to 10 days later and fly in large numbers around lights at night. They mate and females lay the overwintering egg masses. There is one generation per year.

Pest monitoring Watch for egg hatch and the glint of the first white webbing of the tents early in the spring. A few tents do not threaten the health of the tree. The earlier the intervention, the more environmentally friendly options can be employed.

Management-cultural control

Trees that have been defoliated need additional, but not excessive, irrigation during dry summers to help them refoliate. Healthy trees will withstand defoliation better than closely spaced, drought stressed trees. Remove egg masses from twigs or other sites. Cut out infested twigs and discard.

Management-biological control

Heavy infestations of tent caterpillars occur from time to time as the populations of the caterpillars and their predators rise and fall. Tent caterpillars have many natural enemies. Some birds eat the caterpillars, and small mammals and birds will consume the pupa inside the cocoons. A tachinid fly parasitizes the caterpillars by laying a white egg on the caterpillar; then the hatching fly larva burrows into and feeds within the caterpillar. A Trichogramma wasp will also parasitize eggs. When pruning out tent caterpillar egg masses, discard the masses in a protected area away from a host plant so these parasitoids can emerge naturally.

Management-chemical control

Spray in spring after overwintering eggs hatch, at about the time leaves are 0.75 to 1 inch long and the first small tents are noticed. Control is much more effective if pesticides are applied when larvae are small. Be vigilant as fluctuating spring temperatures may result in eggs hatching in waves corresponding with warmer temperatures.

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For more information

Collman, S.J. and A.L. Antonelli. 1996. "Biology and Control of Tent Caterpillars." http://gardening.wsu.edu/library/inse003/inse003.htm

Collman, S.J. 2010. "Tent Caterpillars of Spring." http://snohomish.wsu.edu/garden/documents/tentcaterpillars0410.PDF

Murray, T. "Western Tent Caterpillar Update." http://whatcom.wsu.edu/ag/homehort/pest/tent_caterpillar_biology.htm