Over the years many people have asked about the “tent caterpillars” that appear on hardwood trees in the late summer and fall. I’ve explained to them that these aren’t eastern tent caterpillars but are instead the webs of the fall webworm (Hyphantria cunea) that have both a very different life history and very different impact on the trees they inhabit than do tent caterpillars.
The webs we see on hardwood trees in the fall are formed by the caterpillars of an insect that as an adult is a non-nondescript white moth having a wingspan of about three-quarters of an inch. Fall webworm caterpillars hatch from eggs that the female moth lays on the underside of leaves in early summer. Newly hatched larvae begin to spin a web to enclose the branch-tip leaves on which they feed. As the caterpillars grow they enlarge the web to cover more and more leaves and eventually the webs become large enough to attract the attention of the people who ask about the “tent caterpillars”.
The webs are quite tough and easily defeat the efforts of most birds and small mammals that would otherwise prey on the caterpillars. If the web is breached, the caterpillars immediately drop from the web. Those that fall to the ground have little chance of survival, but if they merely fall to a lower branch they have a remote chance of returning to the web or creating a new web on the host tree.
As the days shorten and the caterpillars mature they emerge from the webs, drop to the ground and pupate over winter in the leaf litter on the ground or in the soil. The adults emerge in late spring or early summer, mate, lay eggs and die.
Because fall webworm caterpillars only feed in late summer and fall, their impact on host trees is minimal. The trees have already formed the leaf and flower buds for next year and have manufactured virtually all of the sugars they need to survive the winter and grow the following spring. However, the webs sure are ugly and often hang on the trees all winter – their impact on aesthetics can be significant.
Just as invasive exotic species are a problem in our forests, fall webworms have become invasive in Europe and Asia after their accidental introductions in Europe and Japan during the 1940s.
So how do fall webworms differ from the eastern tent caterpillars with which they are so often confused? Tent caterpillars build their tents in the spring; fall webworms, true to their name, are active in the fall. Tent caterpillars can defoliate entire trees and even kill small trees; fall webworms seldom do either. Tent caterpillars typically use their webs to hide from predators but leave their webs to feed; fall webworms feed within their webs. Tent caterpillar webs are in the crotches of trees –
Fall webworm webs are on the tips of branches –