Younger larvae are green to yellow and are hairy with many black spots. Full-grown larvae are approximately 1 inch long, are dark green to black, and have a light line running down the center of the back. Each body segment has three dark spots, each bearing one to three bristles. When disturbed, the larvae may retreat into tubular webs which they have spun, or they may drop to the ground. Webworms overwinter as larvae in the ground adjacent to their fall food host.
Moths emerge in early spring and lay eggs on leaves of host plants.
Larvae hatched from these eggs will feed for three to five weeks. Early feeding is beneath webbing on the undersides of the leaves. If numbers are abundant, this webbing will be clearly visible and will cover extensive areas of foliage. As the webworms grow larger they will venture out from the webbing, but will rapidly retreat into it when disturbed. We can have two - three generations per year in Wisconsin.