The brightest summer days become richer with dense, dark maple tree shade to relieve them. As their lush greenery slowly thins and falls, maples inspire sad songs about the passage of time with leaves that rustle underfoot, and set sidewalks and forest floors ablaze with autumn carpets of scarlet, orange and gold. Where maple trees (Acer spp.) share their habitats with leaf-devouring inchworms in U.S. Department of Agriculture plant hardiness zones 3 through 9, however, the splendor of their cooling, color-changing foliage may never materialize.
Inchworms -- officially known as cankerworms -- are the caterpillars of several moth species, two of which account for most inchworm damage to maples across the United States. Spring cankerworms (Paleacrita vernata) are found from Maine to the Gulf Coast and in Oklahoma, Kansas, Nebraska and California. Their range partially overlaps that of fall cankerworms (Alsophila pometaria), but the fall-feeding pests also attack maples in Nevada, Montana and the upper Midwest.
The awkward way they arch their midsections to pull their hind legs and forelegs together as they advance identifies inchworms. The spring- and fall-hatching pests, offspring of wingless cankerworm moths as drab as maple leaves are colorful, measure 3/4 to 1 inch long. A yellow stripe adorns each of the reddish-brown to green spring-hatching inchworms' sides. Fall-emerging inchworms sport dark stripes on their bright-green to brownish-green backs, with three white lines on each side.
Fall cankerworm moths emerge from their cocoons from October to December to mate and lay eggs. During mild weather, the wingless gray females wait on maple trunks for winged males. They girdle slender branches with clusters of eggs after mating. Mottled, brown or gray spring cankerworm moths appear on warm, late-winter or early-spring days, laying batches of up to 50 eggs beneath peeling bark or in trunk crevices. Eggs of both species hatch in April or May. By July, the mature caterpillars have descended from the trees on silken filaments to pupate in the soil.
Young inchworm larvae riddle new maple leaves with tiny, irregularly shaped holes. Older ones chew progressively larger holes until only the major leaf veins remain. Vigorous, well-maintained maples handle small numbers of inchworms without ill effects. Trees defoliated from heavy feeding, however, exhaust much of their reserves producing replacement leaves. Inchworms also have the unpleasant habit of dropping onto passersby when descending from the trees.
Safeguard your maples from egg-laying cankerworm moths by wrapping their trunks with bands of adhesive-covered film between October and December, and replacing the bands in February and March. The sticky film traps female moths climbing the trees to mate. To kill hatching inchworms, spray the trees with a freshly mixed solution of Bacillus thuringiensis powder and water. Caterpillars that consume the Bacillus bacteria usually die within 72 hours. For best results, mix the spray at the label's recommended concentration and apply during cloudy weather. Repeat as necessary until all eggs have hatched.