But they aren't army worms. According to Larry Zilliox of the Douglas County Extension Service, Grant County, along with most of Minnesota, is suffering from an infestation of forest tent caterpillars this summer. True army worms are grass eaters. Forest tent caterpillars, Malacosoma disstria, occurs throughout most of the United States and Canada, wherever hardwood trees are found. Forest tent caterpillars larvae emerge from egg masses in early to mid-May, about the same time aspen leaves begin to open.
Caterpillars feed actively on broadleaf trees for five to six weeks.
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.