Pest: Fall Cankerworm (Alsophila pometaria (Harris)); Spring Cankerworm (Paleacrita vernata (Peck)
Cankerworms are caterpillars when immature and later develop into moths as adults. As immatures, they have elongate, cylindrical bodies with a well developed head capsule. In addition, they also have 3 pairs of thoracic legs. Further back on the body, they also have 2-3 pairs of abdominal legs known as prolegs. The Spring Cankerworm has 2 pairs while the Fall Cankerworm has 3 pairs.
The adult male cankerworms (of both species) are rather dull-colored, small moths while the females are wingless. They both get their names from the times of the year in which they are active as adults; the spring cankerworm adults are active in February into March while the fall cankerworm adults are active in late November into early December. They mate and lay eggs during these times.
Both the Spring and Fall cankerworm eggs hatch about the same time in the spring. In Massachusetts, hatch occurs about mid-May. Caterpillars of both species may be found feeding together in mixed populations on the same host plant. The larvae feed on the foliage of many deciduous trees including: oaks, cherry, elm, apple, maples, and others. Their feeding initially causes the foliage to become very tattered in appearance but later they may completely defoliate the entire host plant. The immature stage (caterpillar stage) lasts until about the end of June in Massachusetts.
Adult male moths of the Fall Cankerworm sometimes become nuisance pests in late November and into December because they are very attracted to night-time holiday lighting.
Bacillus thuringiensis, commonly known as B.t., is sold under many different trade names and is quite effective against the younger caterpillars. This product is relatively "safe" when compared to traditional chemicals.
Cankerworm populations tend to build up into large, damaging numbers over the course of several years and then almost entirely disappear due to natural causes in the environment.
Written by: Robert Childs