Fall Webworm Moth

The fall webworm moth is a member of the family Erebidae, in the tiger moth group, and is native to North America though it has also become established as an introduced species in parts of Europe and Asia. While the adult moth itself is a rather unremarkable white insect, its larval stage is one of the most visually obvious caterpillars in late-summer landscapes due to the large, loose silk webs the caterpillars spin communally over foliage.

Unlike the similarly web-building eastern tent caterpillar, which builds its nest in branch crotches in spring, the fall webworm constructs its web at the tips of branches later in the growing season, gradually enclosing more foliage as the colony grows and consumes leaves within the silk enclosure.

The species is a generalist feeder on an extremely wide range of deciduous trees and shrubs, and while the communal webbing can look dramatic, defoliation from this species is generally limited to the outer canopy rather than affecting the whole tree.

• Adult: satiny white wings, sometimes with small black or brown speckles, a stout hairy white-to-orange body, and a wingspan of roughly 1.2 to 1.7 inches.
• Caterpillar: pale yellow-green to tan, covered in tufts of long, fine hairs arising from small dark or orange tubercles, varying somewhat in color across populations.
• Best identified by its behavior: caterpillars live and feed communally inside a loose, silk web spun over the tips of branches, expanding the web as they consume enclosed foliage.
• Distinguished from the eastern tent caterpillar by web placement (branch tips rather than crotches) and timing (late summer to fall rather than spring).

Native to North America and widely distributed across the continental United States, southern Canada, and Mexico, with introduced populations established in parts of Europe and Asia. Found on a very broad range of deciduous host trees in orchards, woodland edges, parks, and urban and suburban shade trees. Webs become conspicuous in late summer and persist into autumn.

Caterpillars feed gregariously within a communal silk web that they continually enlarge to enclose fresh foliage as they consume the leaves inside, a strategy that offers some protection from predators and weather. Because the species is a generalist able to feed on hundreds of different tree and shrub species, its webs are a common sight on a wide variety of hosts by late summer, and heavy infestations can make host trees look ragged, though damage is usually cosmetic. Adult moths are nocturnal and feed little, being primarily focused on reproduction during their short adult lifespan.

Eggs are laid in large masses on the underside of leaves in late spring to summer and hatch into caterpillars that immediately begin spinning a communal web, enlarging it as they grow through several instars over several weeks. Mature caterpillars leave the web to pupate in a cocoon in bark crevices, leaf litter, or the soil. There are one to two generations per year depending on climate, with the species overwintering as a pupa and adults emerging in late spring to summer to begin the cycle again.