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.
Wingspan: 1–1¼ inches.
Statewide. It occurs throughout the Lower 48, northern Mexico, and southern Canada.
Habitat and Conservation
Fall webworms are a widespread species that may occur nearly anywhere there are deciduous trees and shrubs. Look for them from midsummer through fall.
Adults are nocturnal and are frequently found at lights at night.
The caterpillars eat the foliage of a wide variety of hardwood trees, shrubs, and low plants. Their silken tents cover the leaves at the branch tips of tree boughs, and the caterpillars can feed in relative safety. A colony of fall webworms typically strip the foliage from the branches covered by the tent. More than 100 species of trees, shrubs, and other plants have been recorded as food plants, including hickory, oak, maple, walnut, ash, cherry, elm, mulberry, willow, redbud, sweet gum, and more.
Common to abundant breeding resident.
The fall webworm moth is a multibrooded species in our state, and adults are found from early April into September. The sexes are similar in appearance. The female may deposit nearly 1,500 eggs on the underside of a host plant leaf; the female covers the egg mass with white hairs from her abdomen. This species is called the fall webworm because the larvae build their noticeable silken tents on branch tips from June through September. The tents protect them as they feed on leaves. As the larvae and their tents grow, the tents may cover an entire tree. The larvae may have as many as 11 molts before they leave the web. Full-grown larvae creep under flaps of bark and pupate in silken cocoons. They overwinter in the pupal stage.
When abundant, fall webworms can be pests in orchards or in ornamental trees. Usually, however, the damage is merely aesthetic and temporary.
We usually think about invasive species that came to America from somewhere else, but the fall webworm is one of our native insects that is invasive overseas. Fall webworms were accidentally introduced to eastern Europe after World War II in the 1940s, and the species spread invasively through Europe. It has also been introduced to China and has spread through east Asia. In attempts to control fall webworms in the Old World, people have tried to import this moth’s principal parasitoid enemies, but attempts to permanently establish those insects have not succeeded. Bacillus thuringiensis (Bt) spray is not the main defense. Fortunately, although fall webworm spread rapidly and caused much worry in the 1950s and 1960s, it seems to longer be spreading and causes only local damage.
Although fall webworms can completely defoliate a tree, they rarely cause severe damage to a tree’s health because the defoliation occurs late in the season, when many trees are preparing for fall leaf drop anyway. However, when trees — especially young or very old trees — experience repeated defoliations combined with other stresses, such as severe drought, flooding, or fungal or other illness, the combination may overcome the tree.
Birds and small mammals eat the larvae. Spiders and bats eat the adult moths.
Fall webworms are attacked by a wide range of parasitoid insects, especially tachinid flies, which deposit their eggs on or in the bodies of webworms; the hatching parasitoid larvae eat the webworms, killing them. Ichneumon wasps penetrate the webs to get to the larvae.
Wasps, ants, and beetle larvae, and other fly species eat the larvae, too. Some wasps tear right into the tents to get at the larvae, and spiders have been observed building their webs within the webworm tents.
Parasitic fungi also take a toll of webworm larvae.
With all their enemies, fall webworm larvae have several defensive behaviors. When threatened, they typically shake or jerk their bodies en masse. They might look like they’re dancing! They also secrete an obnoxious scent, and the body hairs may serve to repel some enemies.