A full-grown larvae caterpillar is about two inches long, mostly blue and black, with a row of white, footprint shaped markings on their backs, and many hairs along the edge of the body. In June, older larvae become restless and move around trees and other vegetation to find food. Significant damage to nearby plants can occur at this time. Adult moths emerge from cocoons about two weeks later in mid-July. These tan moths are nocturnal and are attracted to lights at night.
Adults live for about five days.
During this time they deposit 100 to 350 eggs in gray cylindrical masses surrounding small twigs. The eggs overwinter and larvae hatch from them during the next spring. In Minnesota, the number of forest tent caterpillars changes in relatively predictable cycles. Some years they may be hard to find, but over a period of eight to thirteen years, their populations start to increase until they reach tremendously large numbers. These outbreaks usually last about three to four years.