The **Fall Webworm** (*Hyphantria cunea*) is a common North American moth whose larval stage (caterpillar) is distinguished by the large, unsightly silken tents it spins over the ends of tree branches in late summer and early fall. Unlike the Eastern Tent Caterpillar, which creates its tent in spring in a branch crotch, the Fall Webworm nests are built on the terminal ends of branches and are expanded to enclose more foliage as the larvae feed. The damage is considered largely **cosmetic** as it occurs late in the season after the tree has already completed most of its annual food production.
Taxonomy and Classification
The Fall Webworm belongs to the family Erebidae (formerly Arctiidae—tiger, lichen, and tussock moths) in the order Lepidoptera (moths and butterflies). It undergoes complete metamorphosis. It is highly **polyphagous**, meaning its larvae feed on over 100 species of deciduous hardwood trees and shrubs, including pecan, hickory, walnut, elm, and fruit trees.
Physical Description
The **adult moth** is typically snow-white with a wingspan of about $1$ to $1.5$ inches. Some regional races may have varying amounts of dark spots on their wings.
The **larvae (caterpillars)** are about $1$ inch long when mature and are densely covered with long, white to grayish hairs arising from black or orange bumps (**tubercles**). Larval color varies regionally from pale yellowish-green with a black head (northern race) to yellow-tan with an orange/red head (southern race).
The most conspicuous sign is the **nest**: a large, loose, silken web that can enclose entire branches and contains the entire colony of feeding larvae and their dark droppings (frass).
Distribution and Habitat
The Fall Webworm is native to North America and is widely distributed across the continent. It has also become an invasive pest in parts of Europe and Asia. Its habitat is primarily the foliage of deciduous trees, with the most noticeable infestations occurring on yard and roadside ornamentals, fruit trees, and nut trees in late summer.
Behavior and Life Cycle
This insect typically has one to two generations per year, depending on the climate (two or more generations in the south). It overwinters as a **pupa** inside a thin, debris-covered cocoon, usually in the soil, leaf litter, or under loose bark.
Adult moths emerge in mid-summer (July/August). The female lays hundreds of eggs in a cluster on the underside of a leaf, often covering them with white hairs. The larvae hatch in about a week and immediately begin spinning their communal tent. The caterpillars feed and grow entirely within the protection of this web for about 4–8 weeks, gradually expanding the tent to access new leaves.
Damage and Ecological Role
Damage is characterized by defoliated branch tips encased in silk webs.
- **Cosmetic Impact:** The feeding occurs so late in the growing season that it does **not** significantly impact the health of a large, established tree. The leaves are about to drop anyway.
- **Serious Damage:** Heavy, repeated infestations over several years, or defoliation of small or newly planted trees, can weaken the host and make it susceptible to secondary pests.
- **Ecological Role:** The larvae are an important food source for many natural enemies, including birds, parasitic wasps, and predatory insects, which often naturally control populations.
Management and Prevention
Control is generally unnecessary on large trees but is sought for aesthetic reasons on ornamentals or for protection of fruit and nut trees.
- **Manual Removal (Best Method):** The simplest and most effective control is to **prune out and destroy** the webs and the caterpillars inside when the tents are still small and reachable. Since the larvae never leave the web to feed, this removes the entire colony. **Do not burn** the webs while they are still on the tree, as this can severely damage the branch.
- **Biological Insecticides:** If the tent is too high to prune, apply **Bacillus thuringiensis (Bt)** or **spinosad** to the foliage when the larvae are young. A high-pressure sprayer or a wetting agent is needed to penetrate the silken webbing.
- **Chemical Sprays:** Broad-spectrum insecticides are rarely needed but can be used as a last resort. Application must be timed to coincide with the young, active larvae, as older larvae are less susceptible.