These insects periodically cause serious defoliation in the Northeast and range over most of the United States and southern Canada. There is a close resemblance between the two species in all stages. There is only one generation a year. The caterpillars of both species appear on trees in early spring. They destroy the young leaves and buds of a wide range of common deciduous trees, but prefer elm and apple.
Cankerworms are also abundant on oak in southeastern New York and Long Island.
By the time the larvae finish feeding, only the major veins and midribs of the older leaves remain. Trees may be completely defoliated. If this happens 2 or 3 years in succession, trees may die or be seriously weakened. This is an important leaf-feeding pest in areas where Dutch elm disease is present, because defoliation renders an elm very susceptible to attack by the bark beetles that spread the fungus causing the disease. The forewings are thin and silky with an ash gray color.