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There are numerous cankerworms, more commonly known as inchworms, which attack deciduous trees and shrubs.The spring cankerworm, Paleacrita vernata (Peck), and fall cankerworm, Alsophila pometaria (Harris), tend to be the most common cankerworms, but the elm spanworm, Ennomos subsignaria (Hubner), and linden looper, Erannis tiliaria (Harris), also have periodic population surges. Both fall and spring cankerworms feed on a wide variety of trees including apple, ash, beech, elm, hickory, linden, maples and oaks. The elm spanworm attacks elms, hickory, ash and oak, as well as a variety of other broadleaf trees. The linden looper is common on basswood, linden, apple, maple, oak and other trees. Damage: Young larvae chew small irregular holes in young leaves, skeletonizing the leaves. As they mature, the larvae begin eating larger irregular holes and finally entire leaves, leaving only the major leaf veins. Although low populations do not damage healthy trees, high populations can defoliate whole trees, causing them to expend considerable resources to refoliate. Many of the cankerworms and loopers spin down from the trees on a strand of silk when they are ready to pupate. These larvae drop onto people, cars and picnic tables and are sometimes considered nuisance pests, although they cause no harm to mammals. Description: Cankerworms move by arching the middle part of their bodies to pull the hind prolegs up to meet the anterior true legs. Many caterpillars move in a similar fashion. Fall cankerworm larvae grow to 19 to 25 mm (1/4 to 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. Fall cankerworms have three pairs of fleshy prolegs at the end of the abdomen. Spring cankerworm larvae reach the same size but are green to reddish-brown in color and have a single yellowish strip on each side. Spring cankerworms have only two fleshy prolegs at the end of the abdomen. Elm spanworms are generally gray-brown in color and are often confused with spring cankerworms. The adult spanworm is a completely white moth which flies in August. Linden loopers have a bright yellow band of color down each side, with brown to black lines running down the back.
Life Cycle: Cankerworm larvae feed on tree leaves from late April to mid-June. Adult female cankerworms are wingless and emerge to lay eggs in the late fall (fall cankerworms) or early spring (spring cankerworms). Fall cankerworms emerge as adults during warmer periods in October through early December. The wingless females are a dull gray color and crawl up on tree trunks to await a winged male. The males are about 25 mm (1 inch) long, dull gray in color and often have two light, wavy stripes on the forewings. After mating, the female lays a cluster of barrel-shaped eggs, often encircling small branches. The eggs overwinter and hatch in late April to early May. Adults occasionally emerge in March, especially in more northern areas. Spring cankerworms emerge as adults during warm spells in February or March. The wingless females are often mottled with grey or brown color and have a darker stripe down the back. The winged males are brownish-grey with three dark, irregular stripes across the front wings. The eggs are oval shaped; irregular clusters of about 50 eggs are attached under flaky bark or in cracks and crevices of tree trunks. Upon hatching in April and May, the young larvae rapidly feed on the fresh tender spring leaves of various trees. 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 to a depth of 2.5 to 10 cm (1 to 4 inches), spin a silken cocoon and pupate. The pupae remain in the soil until the late fall or early spring. Control: - Trapping the adults is an important strategy. The trunks of susceptible trees can be banded with sticky adhesives, such as tanglefoot, in order to trap females as they crawl up the trunk to mate and lay eggs. Trapped females may remain attractive to the males, which also get stuck in the sticky band. - Horticultural oil sprays can also be quite effective. The 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 with Bacillus thuringiensis (Bt), a biological control, is quite effective against young cankerworm and looper larvae. Wait until all the eggs have hatched but spray before the larvae grow to more than 25 mm (1 inch) in length. - If insecticides must be used to control cankerworms and loopers, 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.
Adapted from David J. Shetlar, Ohio State University Extension, 1999
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