Fall webworm

Caterpillars of the red-headed race of the fall webworm.

 

 You may be noticing webbing in trees and shrubs this time of year. In many cases these are the nests of the native caterpillar Hyphantria cunea, or the fall webworm. There are two or three generations a year in South Carolina and so we see the web nests into early fall, though they can be found in spring and summer, too.

Webbing constructed by caterpillars of the fall webworm.

 

For many years it was thought that there were two species of fall webworm; a black-headed species and a red-headed species. They do interbreed and so they are considered two races of the same species. The races are different in their coloration and their behavior. For instance, adults of the black-headed race tend to emerge almost a month earlier than the red-headed race. Both races lay their eggs in masses on the undersides of leaves, but the black-headed race lays a single layer of eggs, whereas the red-headed race lays two layers of eggs.

Black-headed form of the fall webworm. outside of its web nest.

Red-headed form of the fall webworm outside of its web nest.

 

 

The caterpillars spin the silk to make their nests and their nests are always at the terminals of branches, unlike the eastern tent caterpillar whose aggregation of webbing is always in the crotch of the host tree. The fall webworm is a polyphagous insect, attacking many hosts. In South Carolina we commonly see nests in persimmon and sweetgum, but they attack many other trees, including elms, hickories, walnuts, pecans, and maple, among many others.

The damage is mostly cosmetic. I have known landowners to mechanically remove the nests, pruning the affected terminals, but most trees can handle a certain level of defoliation, especially at the end of summer when leaves are getting ready to senesce anyway.

Young fall webworm larvae eating tissue of a sweetgum.

 

If you have a curious mind you can try rearing out parasitoids, wasps and flies that have laid their eggs on or in the caterpillars. More than fifty species have been reported from the fall webworm!