Forest Tent Caterpillar

Malacosoma disstria — the forest tent caterpillar — is one of the most economically important DECIDUOUS FOREST DEFOLIATORS in North America and one of about 24 species in genus Malacosoma (the tent caterpillars). The species is widespread across all of North America from southern Canada south through the eastern and central US, with outbreak populations occurring periodically at 10-12 year intervals across the continent. Larvae are 5-6 cm long when fully grown, gregarious, with the species' diagnostic markings: dark blue-and-black bodies with a row of CREAM-COLORED 'KEYHOLE' OR 'FOOTPRINT' SHAPED MARKINGS along the dorsal midline (the keyhole markings distinguish forest tent caterpillars from eastern tent caterpillars, which have continuous white dorsal stripes, and from western tent caterpillars, which have orange-and-blue dorsal markings). 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 CYCLIC OUTBREAK BEHAVIOR. Forest tent caterpillar populations rise and fall in dramatic 10-12 YEAR CYCLES — peak outbreak populations are 100-1,000x higher than between-outbreak densities, and outbreak populations defoliate massive areas of deciduous forest. Major historical outbreaks include the 1980 Quebec outbreak (defoliating ~20 million hectares of forest), the 1998-2002 Ontario outbreak, and the 2011-2013 Minnesota/Wisconsin outbreak — each causing tens-to-hundreds of millions of dollars in timber losses, sugar maple syrup production losses, and forest health damage. The cyclic outbreaks are driven by the interaction between caterpillar populations, NUCLEOPOLYHEDROVIRUS (NPV) infections (a baculovirus that causes outbreak collapse by killing infected caterpillars), and parasitoid wasp populations (which build up in response to outbreak host density and contribute to outbreak collapse). The cyclic outbreak dynamics are one of the most-studied insect-pathogen-parasitoid systems in NA forest entomology. DESPITE THE COMMON NAME, forest tent caterpillars do NOT BUILD TENTS like their close relatives the eastern tent caterpillar (Malacosoma americanum) and western tent caterpillar (Malacosoma californicum). Instead, the gregarious larvae construct silk MATS on tree trunks where the caterpillars cluster together to rest between feeding bouts. The mat-resting behavior is one of the species' diagnostic features and is the most common confusion in the field — observers familiar with the tent-building eastern tent caterpillar are surprised to find the silk-mat-building forest tent caterpillar. Larvae feed on a wide range of deciduous trees — especially sugar maple (the species causing the most economically significant damage), aspen, oak, ash, basswood, and birch. The species is harmless to humans (no urticating hairs, no venom).