Forest Tent Caterpillar

The forest tent caterpillar, Malacosoma disstria, is a lappet moth (family Lasiocampidae) native to North America and one of the continent's most widespread forest defoliators. Unlike its close relative the eastern tent caterpillar, it does not build an enclosed silken tent, instead resting on silk mats spun on trunks and branches.

The species is best known for periodic population outbreaks that can last several years, during which caterpillars defoliate large areas of deciduous forest, particularly stands of aspen, oak, and sugar maple. Most trees refoliate and recover the same season, though repeated years of heavy defoliation can stress trees.

Between outbreaks, populations remain at low, inconspicuous levels for years, kept in check by natural predators, parasitoids, and disease, illustrating a classic boom-and-bust population cycle common among forest Lepidoptera.

• Body is bluish-gray with a row of pale, keyhole- or footprint-shaped spots running down the center of the back (a key difference from the solid white stripe of the eastern tent caterpillar)
• Sides are lined with narrow blue and orange-brown stripes and sparse fine hairs
• Reaches about 5 cm (2 in) in length when mature
• Gregarious in early instars, often seen in groups along branches and trunks
• Does not build an enclosed tent; instead spins silk mats used as resting sites

Ranges across most of forested North America, from Canada south into the United States, wherever deciduous hardwoods such as aspen, oak, sugar maple, birch, and ash grow. Larvae are active in spring, coinciding with leaf-out of their host trees.

Caterpillars hatch in early spring and feed gregariously at first, following silk trails between feeding sites and resting together on silk mats on trunks or branches during the day. As they mature they become more solitary foragers. As folivores, they can, in outbreak years, defoliate extensive areas of forest; most host trees produce a second flush of leaves later in the season and survive, though repeated defoliation over consecutive years can weaken trees. Outbreak populations are eventually reduced by natural enemies including parasitic flies and wasps, predators, and a naturally occurring virus.

Eggs are laid in shiny, varnish-coated bands encircling small twigs in mid-summer and remain dormant through winter. Larvae hatch in early spring as host trees leaf out, passing through five or six instars over four to six weeks while feeding gregariously. Mature caterpillars spin pale yellow silk cocoons in curled leaves or bark crevices and pupate for one to two weeks before emerging as adult moths in early summer, which mate and lay the next generation's overwintering eggs. One generation is produced per year.