The egg masses of eastern tent caterpillar, Malacosoma americanum, are hatching in northern Illinois, and many of the eggs have hatched in central and southern Illinois. After hatching, the caterpillars create a white, silken tent in the branch crotches of crabapple, hawthorn, mountain ash, flowering cherry, and other trees and shrubs in the rose family. Eastern tent caterpillar is one of the earliest defoliators; and although the feeding damage may not initially kill a tree or shrub, it may reduce the plant’s ability to produce food, thus increasing susceptibility to secondary pests such as wood-boring insects.
It is now easy just to pull out or destroy the silk tent(s) by hand (using gloves), or—for those that are a bit squeamish—a rake or hard spray of water can be used. Dispose of the nest as far as possible from any susceptible trees. The caterpillars generally stay in the nest during the day to avoid predators such as birds. During the night, the caterpillars crawl out the silken tents to feed on leaves. Most of the caterpillars will be in the silk tent on cloudy or rainy days and at night.
Sprays of Bacillus thuringiensis kurstaki (Dipel or Thuricide) are effective when the caterpillars are young. Once caterpillars are mature (about 2 inches long), then the insecticide acephate (Orthene), carbaryl (Sevin), cyfluthrin (Tempo), or spinosad (Conserve) should be used. Research at Purdue University showed that spinosad (Conserve) is very effective at controlling mature caterpillars.