Gypsy moths damage trees by feeding on leaves, partially or completely defoliating the tree. Feeding is done by the caterpillar stages, usually between early May and early July. Caterpillars feed on a wide variety of plant species, but prefer those in the oak group. The amount of defoliation in a stand is difficult to predict, but depends on variables such as the gypsy moth population level in the area and the tree species composition. Light defoliation will weaken but usually not kill a tree.
Weakened trees are more susceptible to secondary pests such as wood borers or root diseases.
Repeated heavy defoliation over several years will kill some trees. In forested rural areas, heavy defoliation can affect tree vitality and forest health, which can have an impact on timber quality and recreational use such as camping. In addition, some people are sensitive to the caterpillar's hairs and can experience an allergic reaction. Ohio's forested areas comprise thirty percent of Ohio's total land cover. The greatest impacts will probably be observed on sites with a high composition of oaks and other preferred hosts.