Webworms are the larval stage of the Fall Webworm moth (Hyphantria cunea), a common species known primarily for the silken nests its caterpillars construct in trees during late summer and fall. While the webs are considered a nuisance, they are not typically a threat to the overall health of mature host trees. Identifying this species relies on recognizing the distinct visual markers of the caterpillar, the structure of its web, and the appearance of the adult moth.
The Larval Stage: Identifying the Caterpillar
The webworm caterpillar is highly variable in appearance, but shares several identifying features that become clear as it matures. Fully grown larvae reach a length of approximately one inch.
Body coloration is inconsistent, ranging from pale yellow or greenish-yellow to darker gray or brownish-black. A primary identifying trait is the dense covering of long, whitish-gray hairs (setae), which emerge from small bumps called tubercles. These tubercles can be black or orange, adding to the color variation across different populations.
The head capsule color is often reddish-orange or black, depending on the geographic race. Larvae with a black head often exhibit a dark stripe down the back, while those with a red head may have a more yellowish-tan body. The combination of the long, silky hairs and the one-inch size is characteristic of the mature Fall Webworm.
The Web and Nest Structure
The most noticeable feature of the webworm is the large, conspicuous silken nest. These nests are built communally by the larvae, who feed and live entirely within the structure for protection.
The web is a loose, messy structure that progressively expands as the caterpillars consume foliage and require fresh leaves. Over time, the nest accumulates caterpillar droppings (frass), dead leaves, and shed larval skins.
The nests are constructed over the ends or tips of branches and foliage. This location is a defining feature, as the larvae continuously expand the web to encompass new food sources. These nests are most visible from late summer through early fall.
Distinguishing Features of the Adult Moth
The adult webworm moth emerges after the pupal stage, typically in late spring or early summer. The moth is relatively small, with a wingspan measuring between one and one-and-a-half inches.
The wings are primarily white. Some individuals exhibit varying amounts of grayish-brown or black spots on the forewings, though the hindwings are usually all white.
The body is characteristically hairy and white. A detail for identification is the presence of orange hairs on the forelegs and forebodies when viewed from below. These moths are nocturnal and are strictly focused on reproduction.
Differentiation from Look-Alikes
The Fall Webworm is frequently confused with other common pest species, particularly the Eastern Tent Caterpillar and the Bagworm, but their visual differences are distinct. The Eastern Tent Caterpillar (Malacosoma americanum) is a springtime pest, appearing much earlier in the season than the late-summer webworm.
The Tent Caterpillar larva is also physically different, featuring a black body with a prominent white or yellowish-white stripe running down its back, with much sparser hair compared to the webworm. The Tent Caterpillar builds its web in the crotches or forks of tree branches, not at the tips, and the larvae leave the nest to feed.
Bagworms (Thyridopteryx ephemeraeformis) present an entirely different visual profile. The larva is rarely seen outside of its protective structure, which is a spindle-shaped, cone-like bag made of silk and camouflaged with pieces of host plant material, such as needles or leaves. These bags hang like small ornaments from branches and are most often found on evergreens, unlike the webworm which prefers deciduous trees.
The webworm’s messy, expansive web at the branch tip in the fall is the clearest visual distinction from the Tent Caterpillar’s tighter web in the branch crotch in the spring. The Bagworm’s individual, hanging, camouflaged case bears no resemblance to the webworm’s large communal silk tent.