Although aspen and oak are their favorite tree species, they feed on many other hardwood tree species, such as sugar maple, birch, and ash. Department of Natural Resource s Entomologist Kyoko Shimizu expects that feeding will becomplete by mid- to late June. Adult moths will emerge in July. Shimizu said this native insect periodically causes heavy defoliation when populations are high. In northern Wisconsin, the last outbreak occurred from 1986-1992 defoliating thousands of acres of aspen and oak.
Hardwood trees can usually withstand several years of defoliation.
Trees in poor health or under drought stress, Shimizu said, may die after repeated years of severe infestation. Several species of wasps and flies are believed to control the outbreaks of the caterpillars by parasitizing them. In a yard, however, general use insecticides like Malathion or Orthene can be sprayed on trees to kill the caterpillars. Persons should read the instructions and warnings on any insecticides purchased. Bt is specific to caterpillars and won t kill their natural predators.