The gypsy moth has become, over the past century, a major pest in the northeastern United States and southeastern Canada.
Tree damage is caused by the insect larvae, or caterpillars, which emerge from their eggs beginning in early spring and continuing through mid-May. The larvae move to the leaves of trees and begin to eat, mostly at night. Feeding continues until mid-June or early July when the caterpillar emerges as a moth. Gypsy moths are seen only in mid-summer.
The caterpillars have a preference for the leaves of deciduous hardwood trees such as maple, elm, and particularly oak. Gypsy moths can also feed on apple, alder, birch, poplar and willow trees. As it grows it will also attack evergreens like pines and spruces. However, during heavy infestations, competition for food will drive the caterpillar to attack almost any tree or shrub.
Depending on the degree of infestation, tree damage ranges from light to almost complete defoliation. Most deciduous trees can survive a moderate degree of defoliation. Many can even survive a complete defoliation by the gypsy moth caterpillar. Unfortunately, continuing attacks can fatally weaken a tree or leave it vulnerable to other insects or disease.