Cankerworms

There are numerous cankerworms, more commonly known as inchworms, that attack deciduous trees, including oaks, cherry, elm, apple, maples, and others. The spring  cankerworm and the fall cankerworm tend to be the most common. They both get their names from the times of the year in which they are active as adults: the spring cankerworm adults are active from February into March while the fall cankerworm adults are  active in late November and early December. They mate and lay eggs during these times. Adult male moths of the fall cankerworm sometimes become nuisance pests in because they are very attracted to night-time holiday lighting.

The adult male cankerworms (of both species) are rather dull-colored, small moths while the females are wingless. Fall cankerworm larvae grow to ¼ – 1 inch long  and are usually apple-green to brownish-green in color, with a dark middle stripe and three narrow white lines on each side. Spring cankerworm larvae reach the same size but are green to reddish-brown in color and have a single yellowish strip on each side.

Cankerworm larvae feed on tree leaves from late April to mid-June. Adult fall cankerworm females emerge to lay eggs in the late fall during warmer periods in October  through early December. The eggs overwinter and hatch in late April to early May. Spring cankerworms emerge as adults during warm spells in February or March. By late June to early July the larvae of both species have matured and they descend to the ground on silk  threads. The larvae then burrow into the ground, spin a silken cocoon, and pupate. The pupae remain in the soil until late fall or early spring.

Damage

Upon hatching, the young larvae of fall cankerworms rapidly feed on the fresh tender spring leaves of various trees. They chew small, irregular holes in young leaves. As  they mature the larvae eventually eat entire leaves, leaving only the major leaf veins,  skeletonizing the leaves. Although low populations do not damage healthy trees high populations can defoliate whole trees. 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.

Control

• Trap adult females by banding susceptible trees with sticky adhesives. Trapped females may remain attractive to the males, which also get stuck in the sticky band.

Spring Cankerworm Moth

• A 2-3% dormant horticultural oil spray is most effective against the eggs. Be sure to thoroughly wet the trunk bark if spring cankerworms are present. Reduce the oil rate, especially on maples, if the trees seem to be active in the spring.

• Spraying withBacillus thuringiensis (B.t.), a bacterium, is quite effective against young cankerworms. Wait until all the eggs have hatched but spray before the larvae grow to more than 1 inch in length. If insecticides must be used, best results are obtained if the spray is applied after all the eggs have hatched and the larvae are still small. Be sure to check for currently registered insecticides.

Spring Cankerworm

Fall Cankerworm

Adapted from: David J. Shetlar, Ohio State University Extension, 1999; UMass Extension