The Bruce spanworm is distributed over a wide area and had many hosts. It has recently defoliated sugarbushes across extensive parts of Eastern Canada and New England, and is an important sugarbush pest in Quebec. During a recent outbreak in Vermont, heavy defoliation was limited to understory sugar maple and the lower crowns of overstory trees. Outbreaks frequently involve other loopers, including fall cankerworms.

Female Bruce spanworms are wingless. The body is 0.2 to 0.3 inch (6 to 8 mm) long, grayish-brown, and marked with white spots. Males have a wingspan of 1.0 to 1.2 inches (25 to 30 mm). Their light brown forewings are slightly transparent and crossmarked with wavy brown and gray bands. The overwintering eggs hatch in the spring about the time that the sugar maple buds open. Caterpillars chew on the underside of leaves or feed on leaves they have bundled together using webs of silk. The larvae chew large holes between major veins f the leaf blade, giving the branch a tattered look. Most caterpillars are pale green but some may be a bright green to brown. Most have three distinct yellowish stripes along each side of the body. Full-grown caterpillars (Fig. 31) are 0.8 inch (18 mm) long.