Cankerworm

Facts, Identification & Control

Scientific Name

Spring cankerworm Paleacrita vernata & Fall cankerworm Alsophila pometaria

Appearance

Spring and fall cankerworm are long-standing forest and shade-tree pests of forest. Both complete their life cycle by developing in four distinct phases: eggs, larvae (caterpillar), pupae (cocoon) and adults. Both species of cankerworm adults are very similar in appearance. Female adults are wingless, grayish-brown and about 3/8 inch long. Male adults have wings. Cankerworm larvae are about one inch long when fully grown. Fall cankerworm larvae range between light and dark green, while spring cankerworm larvae are brown or black. Another way to differentiate between the two species is the number of legs on the rear half of the abdomen. Fall cankerworms boast three pairs of legs, whereas spring cankerworms possess only two pairs of legs. Cankerworms are sometimes referred to by other common names, including inchworms, spanworms, loopers or measuring worms.

Behavior, Diet & Habits

The common name of each cankerworm species comes from the time of year when the adult moths appear, not when the larvae are present. Fall cankerworm adults emerge from their pupal stage in the fall months, while the spring cankerworm adults do not emerge until the early spring months.

Cankerworms are defoliating pests of many hardwood tree species, but their preferred host trees are ash, basswood, beech, black cherry, maples and oaks in both large, forested areas and shade trees located in urban and suburban environments.

Spring and fall varieties of cankerworm larvae feed at the same time of the year, regularly on the same trees, and cause similar damage. As the larvae feed on leaves, they create tiny holes resembling the size and shape of BBs on the leaves. As the larvae become larger, feeding continues until the only leaf tissue remaining is the leaf veins.

By mid-summer, the larvae of both species begin dropping to the ground from the host tree using a silk thread they produce. Once on the ground, they burrow into the soil, spin a cocoon and pupate. Fall cankerworm pupae remain in the soil until they develop into adults in the late fall. Spring cankerworms also pupate in the soil, but do not emerge as adults until the early spring of the subsequent year. Mating occurs as the emergent females crawl up tree trunks to lay eggs. Both cankerworm species have their egg hatch at about the same time in the spring. Each species normally completes one generation per year in most areas.

More Information

A non-chemical, but limited cankerworm control option available to homeowners is to place a band of sticky adhesive on the host tree trunk. The sticky substance captures the wingless female adults as they crawl up the tree trunk and before they have the chance to lay eggs. However, the properly timed use of insecticides is more effective if the application coincides with the presence of young larvae that are just beginning to feed on the host tree leaves. It always is a good idea to get your pest management professional’s advice and recommendations in order to maximize the chances for effective insecticide use for cankerworm control.