Tent caterpillar
Malacosoma spp.
137 host plants
Last updated
Tent caterpillars construct large silken webs over foliage in spring, with larvae feeding on leaves inside and around the web. Look for grayish tents in branch crotches of alders, willows, and fruit trees by early spring. The caterpillars are dark with light-colored stripes and yellow heads. Complete defoliation of small trees is possible, though mature trees recover.
Prune out tents with larvae intact in late April or early May, and destroy them to prevent larval emergence. Since mature trees recover readily after one season of defoliation, this is cosmetic damage. Do not apply pesticides, which eliminate natural parasites that suppress populations the following year. Trichogramma wasps and Tachinid flies parasitize egg masses and larvae.
Quick Reference
Monitoring & Action
Scout for egg masses on dormant twigs in winter (dark, shiny bands encircling small branches). In spring, look for small silk tents at branch crotches beginning at bud break. Tents are most visible in early morning when larvae cluster inside. Monitor weekly from April through May.
Treatment is rarely necessary on established landscape trees. Intervention warranted when: (1) young or recently transplanted trees face >50% defoliation, (2) trees have been defoliated in consecutive years, or (3) aesthetic concerns justify action. Prune out tents when small for easiest control.
Larvae feed gregariously, defoliating branches and sometimes entire small trees. They spin conspicuous silken tents at branch forks that enlarge as the colony grows. Defoliation is primarily cosmetic on established trees, which typically releaf by midsummer. Repeated years of heavy defoliation can weaken trees, reducing growth and increasing susceptibility to secondary stresses. Young or recently transplanted trees are more vulnerable. Silk tents and frass (droppings) are the most visible signs. Larvae may migrate in large numbers when food runs short, crossing sidewalks, buildings, and other non-host surfaces.
Cultural Controls
- Remove egg masses from twigs during winter dormancy
- Prune out tents when small and destroy
- Maintain tree vigor through proper irrigation and fertilization
- Tolerate moderate defoliation on established trees
Both western and forest tent caterpillars occur in the Puget Sound lowlands. The western species (M. californicum) is more common on fruit trees and ornamental Rosaceae. Outbreak years bring highly visible populations, especially on wild cherry and crabapple along roadsides and in parks. Tents typically appear in mid-April in the Kent area. The cycling nature of populations means heavy years are usually followed by several light years.