Cankerworms belong to a family of moths whose larvae often are called measuringworms, spanworms, loopers, or inchworms due to their unique way of crawling. They have fewer legs than most caterpillars and walk with a “looping” motion, appearing to “inch” along. The caterpillar arches its back as it brings the rear portion of its body forward immediately behind the front legs. It then attaches itself by the hind legs, releases the front legs, stretches the body forward, and so forth.

Loopers are easily disturbed by the wind or by something brushing against an infested branch. They crawl rapidly or tumble to the leaf edge and drop on a silk thread anchored at one end to a leaf or twig. When a thread breaks, it acts like a sail and can transport the insect a significant distance in a wind. In species with wingless females, this is the primary means of dispersal.

Fall cankerworm is one of the longest known native defoliators of broad-leaved trees in Eastern North America. It is primarily a pest of oak, elm, and apple. However, cankerworms frequently defoliate northern hardwood stands, including sugarbushes, that contain high proportions of sugar maple.

Outbreaks usually appear suddenly and collapse quickly. They typically begin in small and scattered locations but can rapidly expand, coalesce, and encompass large areas. In 1984, more than 200,000 acres (81,000 ha) were defoliated in Maine, and defoliation was heavy throughout 30,000 acres (12,000 ha) in Wisconsin. Evaluation of a recent outbreak can result from cankerworm defoliation. In this instance, the most severely affected stands were on sites with poor soil and at a high elevation. Such sites are considered marginal for sugar maple.

Typical of many inchworms, female fall cankerworm moths are wingless (Fig. 28). They are rotund and approximately .03 inch (8 to 10mm) long, uniformly ash-gray, and glossy. The moths emerge from their pupal stage in the soil and ascend nearby trees to lay eggs. Then, the overwintering eggs are laid in a cluster on the bark of twigs or branches. The male, a typical moth, has a slender body and a wingspan of 1.0 or 1.2 inches (25 to 30 mm). The glossy forewings are a mottled, brownish-gray; the hindwings are uniformly light grayish-brown. Moths are active between October and December (hence the common name) normally after the first heavy frost in the northern range of sugar maple, but they may not emerge until January in more southern areas. The caterpillars are present in the spring. Fully developed caterpillars are 0.8 to 1.0 inch (20 to 25mm) long and are light green (Fig. 29) to dark blackish-green (Fig. 30). Light-colored forms have several distinct longitudial whitish lines, and dark forms have a black stripe down the back. Young caterpillars skeletonize the foliage. Older caterpillars consume most of the leaf blade except for the midrib and larger veins.