Western Tent Caterpillar

Malacosoma californicum — the western tent caterpillar — is one of about 24 species in genus Malacosoma (the tent caterpillars) and the WESTERN US ANALOG of the eastern tent caterpillar (Malacosoma americanum — already in the Wild Files). The species is widespread across western North America from southern British Columbia south through the western US to northern Mexico, with multiple subspecies in different regional habitats. Larvae are 4-5 cm long when fully grown, gregarious, with the species' diagnostic markings: dark blue-and-black bodies with ORANGE-AND-BLUE DORSAL MARKINGS (the orange-and-blue markings distinguish western tent caterpillars from the closely-related forest tent caterpillar Malacosoma disstria — also in the Wild Files — which has cream-colored 'keyhole' or 'footprint' markings; and from the eastern tent caterpillar Malacosoma americanum which has white dorsal stripes). Adult moths are 3-4 cm wingspan, plain tan-brown coloration with two dark crossbands on the forewings, and short non-functional mouthparts (adults do not feed). The species' major ecological feature is the SILK TENT-BUILDING BEHAVIOR. Like the eastern tent caterpillar, gregarious larvae construct large WHITE SILK 'TENT' NESTS in the branch crotches of host plants — the tents are made of dense silk webbing constructed by the gregarious larvae as a daytime resting site (the larvae emerge from the tent at night and dawn/dusk to feed on host plant leaves, then return to the tent during midday). The tents are conspicuous in western NA shrublands and chaparral during outbreak years — large white silk masses up to 20-30 cm in diameter visible in the branch crotches of host plants. Larvae feed on a wide range of WESTERN NA HOST PLANTS — primarily BITTER CHERRY (Prunus emarginata), ANTELOPE BRUSH (Purshia tridentata), OCEANSPRAY (Holodiscus discolor), CHOKECHERRY (Prunus virginiana), and various other western NA shrubs and small trees. The species causes major economic damage during cyclic OUTBREAK YEARS — outbreak populations defoliate large areas of host plant vegetation across western NA chaparral, oak savanna, and shrubland habitats. Outbreak cycles are similar to the eastern forest tent caterpillar (10-12 year intervals) and similarly driven by the interaction between caterpillar populations, NUCLEOPOLYHEDROVIRUS (NPV) infections, and parasitoid wasps. Major historical outbreaks include 1980s outbreaks across British Columbia bitter cherry, 1990s outbreaks in California chaparral, and ongoing outbreaks across western NA. Defoliated host plants rarely die directly but suffer significant growth reduction. The species is the focus of major USDA Forest Service western NA forest entomology research and is a flagship species of western NA forest insect biology. The species is harmless to humans (no urticating hairs, no venom).