Natural Lowcountry: Webworms build their houses around their supper

Those large silken nests you may have noticed in the branch tips of many shade trees are probably the nests of fall webworms.

Webworms are hairy, yellowish caterpillars -- not worms, actually -- that spin communal webs and feast on the tree leaves trapped inside.

They're the larvae of a tiny white moth (Hyphantria cunea), a pest of more than 120 kinds of trees and shrubs across the United States. Here in the Lowcountry, webworms feed on pecan, redbud, sweetgum, persimmon, willow, and many other familiar trees.

As their food supply diminishes, the larvae enlarge the nest to envelop additional leaves. After six weeks of virtually nonstop feeding, the fully-grown caterpillars descend to the ground, spin cocoons, and overwinter in crevices or leaf litter.

Adult moths appear the following spring. Females lay eggs on tree leaves, and the life-cycle continues.

Fall webworms are sometimes confused with Eastern tent caterpillars (Malacosoma americanum), pests of cherry and other trees. Unlike webworms, tent caterpillars appear in spring and early summer and build their silken webs in the crotches of tree branches, not the ends. They leave the nest during the day to feed on young leaves, returning to it for shelter at night.

Although both may cause impressive defoliation, neither species causes permanent damage to healthy trees. If you find webworm nests unsightly, you can remove them by pruning. If they're too high to reach, you can break them apart with a stick or spray them with water.

Opening up the nests makes the caterpillars accessible to parasitic wasps and to natural predators such as stink bugs and birds.

Vicky McMillan, a retired biologist formerly at Colgate University, lives on Hilton Head Island.