The fall webworm (Hyphantria cunea) is the caterpillar of a native moth that makes “unsightly webs” enveloping whole branches. The webs begin to appear in late summer and are active through early fall. They do not cause widespread defoliation and host trees typically survive an infestation.
Coloration of the webworm caterpillar varies from whitish to dark gray but is easily distinguished from tent caterpillars and other pest moths, by the coat of long white hairs covering the sides and back. The adult moths are pure white.
Adult webworm moths emerge in May and June and begin to lay eggs on the undersides of leaves at the tips of the branches of many species of hardwood trees. These include common roadside and garden trees such as cherries, crabapples, birches, and lilacs.
In mid-July the eggs hatch and the caterpillars begin to spin an extensive silken web. Unlike tent caterpillars however, which feed outside their web and retreat to it when not feeding, the webworm encloses whole clusters of leaves at the ends of branches and feeds on the leaves within their tent, expanding the web downward toward the trunk and capturing more leaves as the colony expands.
When the caterpillars are fully grown, they drop to the ground where they pupate in leaf litter and remain for the winter.
Situations & Solutions
Fall webworms can defoliate entire trees, but unless a tree has been stressed by other factors, it usually recovers, and the main negative effects of an infestation are aesthetic. Numbers and locations of webworm outbreaks differ from year-to-year based on environmental factors, so the same trees are not necessarily affected each year.
The best (simplest) remedy for a webworm outbreak is simply to remove the webs when they first appear in July, using a long stick or pole. If you wait until the caterpillars are larger and more abundant, opening the webs can provide an additional food source for nesting birds.Burning the webs will do more damage to the trees than the caterpillars will, and chemical and biological treatments also have downsides disproportionate to the problem.
When a webworm nest is disturbed, mature caterpillars will “wag” their bodies in unison, presumably as a distraction for predators. Also, the webs attract a great variety of parasitic wasps and flies, which use the caterpillars as hosts. These so-called “parasitoid” insects are key natural allies in controlling populations of pest species, and webworm colonies arguably perform a service by providing communal nurseries for these species.
For those who may be allergic to the hairs of webworm caterpillars, you should minimize contact with the insects by wearing long-sleeved shirts and gloves to protect exposed skin while removing the nests. Should you develop a rash, apply cold compresses and calamine lotion.