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 StyrofoamÆ. The larvae feed in large groups on foliage of host plants and can do significant damage by defoliation. Larvae of western tent caterpillars build moderate silken tents over leaves, but leave the tent to feed in new areas. They usually return in the evening. Tent caterpillars can defoliate small trees and defoliation over consecutive years may weaken or kill them the host; defoliation can reduce plant growth and make the plants more susceptible to competition, diseases or poor weather. Healthy trees usually will grow new leaves by midsummer.
Biology and life history Tent caterpillars overwinter as egg masses on twigs (or buildings). The eggs hatch in spring 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 on the tree trunk, webbing and then begin to migrate in search of sitea to 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.
Scouting and thresholds Watch for egg hatch and the glint of the first white webbing of the tents when apples begin to bloom. A few tents do not threaten the health of the tree. The earlier the intervention, the more environmentally friendly options can be employed.
Trees that have been defoliated will need additional, but not excessive, irrigation during dry summers to help them refoliate. Trees that are kept healthy will withstand defoliation better than closely spaced, drought afflicted trees. Remove egg masses from twigs or other sites. Cut out infested twigs and discard away from the host to allow parasitoids to emerge naturally.
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 consume the pupa inside the cocoons. The larvae of a tachinid fly parasitizes the caterpillars by laying a white egg on the caterpillar; then the hatching larva burrows into the caterpillar. Consider the presence of white parasite eggs visible on the caterpillar, an underestimate of parasitism because eggs are shed with each molt. A trichogramma wasp parasitoid will also parasitize eggs. If pruning out eggs, discard in a protected place away from a host plant so parasitoids can emerge naturally. Birds and other insectivores prey on pupae within the cocoon.
For management options:
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: fluctuating spring temperatures may result in eggs hatching in waves corresponding with warm then cooler temperatures.
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For more information
Batzer, H.O. and R.C. Morris. 1978. "Forest tent caterpillar." (USFS) Forest Pest ID Leaflet 9. http://na.fs.fed.us/spfo/pubs/fidls/ftc/tentcat.htm
Ciesla, W.M. and I.R. Ragenovich. 2008. "Western tent caterpillar." (USFS) Forest Pest ID Leaflet 119. http://www.fs.fed.us/r6/nr/fid/fidls/fidl-119.pdf
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