Forewings of adults may be either completely white, or white with varying amounts of grayish-brown or black spots. The spots, if present, are typically rectangular or wedge-shaped. The arrangement of the spots is diagnostic, too: at the basal half of the wing, they appear in loose rows; at the outer half of the wing, they appear either random or else collectively form a V shape. Hindwings are either all white or white with one or two black spots.
There is great variation in the extent of the dark markings on the wings; it’s not uncommon to find specimens that are totally white. Fall webworm moths in northern states usually have fewer or no spots, while those in southern states have more spots.
Viewed from below, the fore bodies of adults, and the femurs (thigh-like segments) of the forelegs have orange hairs.
The caterpillars are perhaps best recognized by their conspicuous tents formed around branch tips in late summer and fall. The caterpillars themselves reach about 1 inch in length; colors are variable, yellowish green to dark gray, thinly covered with pale hairs and scattered tufts of very long hairs. There are two distinct races, one in the northern states and one in the south.
- In northern states, the head is black, the body is yellow or greenish with a dark stripe running along the back, and tubercles on the sides of the body are orange and black; the tubercles give rise to clusters of long, whitish hairs, and each cluster has at least 1 hair that is approximately twice as long as the others.
- In southern states, the head is orange or reddish, the body is yellowish tan, and the tubercles along the side are orange to reddish, giving rise to brownish hairs; the hairs are extremely long, equal in length to 4 or 5 body segments.
Similar species: Eastern tent caterpillars also build conspicuous silken tents in trees, but they are usually only seen in spring and early summer (not late summer and fall), and their tents are constructed in the crotches of branches (not on the outer branch tips). Eastern tent caterpillars are in a different moth family and are not closely related to fall webworms.
A number of adult moths look confusingly similar to this species, including the salt marsh (acrea) moth (Estigmene acrea); Virginia tiger moth (Spilosoma virginica), which is much larger; and agreeable tiger moth (Spilosoma congrua). Specialists use abdomen coloration and the patterns of black and white markings on the legs and feet to verify their IDs of these moths. If you wish to learn the precise color patterns of the legs and other arcane characteristics, many Internet resources exist to help you with these fine points.