At first glance the nests of fall webworms (Hyphantria cunea, Drury) and tent caterpillars (Malacosoma americanum) might look identical, but the damage done to trees and shrubs by the residents of these nests is very different.
Fall webworms appear in early fall and build their large, grayish, silky nests on the ends of tree branches. Often smaller branches and leaves will be enclosed by the nest. The caterpillars don’t leave the nest; if they run out of food they just make the nest bigger and enclose more leaves. They feed on the leaves of many species of trees, but do very little damage because by the time they begin feeding most trees have stopped photosynthesizing and are heading into dormancy.
After about 6 weeks of feeding the larvae fall to the ground and pupate. Pupae usually over- winter in the ground but are occasionally found in old nests. Adults emerge in spring and lay their eggs on the undersides of leaves. After hatching in late summer or early fall the cycle begins again.
Fall webworms have long white, off-white, or yellow hairs. The black headed variety is light greenish-yellow to pale yellow with two rows of black bumps down its body and the red headed variety is tan with orange to reddish bumps.
Tent caterpillars appear in early spring as buds begin to open. They prefer fruit trees but can also be found on maples, hawthorn and others. Their nests are smaller and more compact than fall webworms and are found in the crotch of branches rather than at the ends. Often the caterpillars can be seen crawling over the outside surface of the nest. They feed in morning and early evening, and on warm nights. They do a lot of damage and can defoliate a tree in no time at all. Though the tree will usually grow new leaves it will have been severely weakened and may not bear fruit. As the larvae feed they will make the silky nest larger to enclose more foliage.
Tent caterpillars are full grown in just over a month and leave the nest to make individual cocoons. As they search for a suitable spot to build their cocoon they can often be seen crawling on walks, walls, driveways, and tree trunks in late spring. After about 3 weeks an adult moth emerges from the cocoon, mates, and the female lays eggs on smaller branches. The eggs hatch in the spring and the cycle begins again.
Tent caterpillars are black and hairy with a white stripe down the center of the back. They also have brown and yellow lines and a row of blue spots on their sides.
Nests of either tent caterpillars or fall webworms can be removed by hand, wound on the end of a long stick, or pruned out of trees. However they are removed, they should be destroyed afterwards.
Photo of fall webworm nest on crabapple by the University of Illinois Extension Service