Introduction to Fall Webworm

The physical appearance and characteristics of the fall webworm change as it goes through its life cycle.

The egg masses are tiny and covered in white hairs, giving off an almost cottony appearance. The masses can contain hundreds of these eggs, which are light yellow in color.

Upon the initial stages of hatching, the larvae are a pale, almost bleary yellow with two distinct rows of black marks adorning their bodies. As these larvae mature, they display a variation of colors and patterns.

Once full-grown, the larval bodies are covered with fine, whitish hairs originating from distinct black and orange warts.

The coloring of these larvae varies, most often greenish sporting a broad, dusky stripe running along the back and a yellow stripe on their sides.

As the larvae move into pupation, they adopt a brownish color. Adults can vary considerably in their coloration. Some may remain pure white; others appear white with black spots. Adult moths have a wingspread of about 32 mm.

The lifecycle of a fall webworm begins as a pupa—a hibernating life stage that they assume during winter. During this stage, the pupae envelop themselves in a cocoon while concealed under ground litter, within cracks and crevices of trees, or in the soil.

As winter recedes, adults emerge from their cocoons in early summer, around mid-June. Adults continue to appear intermittently in small numbers through the summer months. Females begin depositing their eggs on the undersurface of leaves.

The hatching commences approximately seven days after the eggs have been laid. As soon as these larvae hatch, they spin small silken webs or nests over the foliage that is their food source. As they get larger, they expand their area, spinning increasingly large nests to enclose more foliage, and they can eventually grow to a 3-foot span. The remains of these webs can often persist till winter. Larvae sharing their foods and homes with other larvae until their last molt, when they may adopt independent feeding habits.

The maturation process of these larvae only takes roughly six weeks. Once mature, these full-grown larvae vacate their webs and pupate in or on the soil—slipping again into dormancy as the temperature decreases.

In our geographic location of Main Line, PA, there’s typically a single generation each year. However, a second generation might occur in certain years and under particular conditions.

During the larval stage, fall webworm consumes leaves inside the protection of their webs, leaving skeletonized leaves. Skeletonization is when only the leaf’s veins, or “skeleton,” remain intact, with the surrounding tissue removed. This prevents photosynthesis and can ultimately lead to the death of a plant.

As the fall webworms grow, they require additional food and will feed on more plants.