Fall webworms

Ian Stewart

One of my favorite books is Arthur Conan Doyle’s ‘The Hound of the Baskervilles’, in which a mysterious beast pursues Sherlock Holmes through the eerie mist of an English moor. Driving the back roads of northern Delaware in the early morning reminds me of this gloomy setting because many of the trees are now shrouded with ghostly tents.

A typical scene in Delaware in late summer

These tents were constructed by the fall webworm, which is the caterpillar of an innocuous white moth (Hyphantria cunea). This native moth which lays hundreds of eggs on the underside of a leaf during the late spring and early summer then promptly dies. The eggs hatch out into tiny caterpillars which soon build extensive, multi-layered webs around groups of leaves for protection and perhaps insulation. The caterpillars start chomping away on the leaves within each web until all that remains is a skeleton of the indigestible frame of the leaf and a large pile of dark brown frass (feces).

The droppings pile up below a cluster of fall webworms

Once the caterpillars become large enough they chew their way out of the web and keep munching on leaves before overwintering as a pupa attached to the bark or in the leaf litter below the tree, and emerging as an adult moth the following spring. Unlike many butterflies and moths, fall webworms are not specific to a small range of trees or bushes, and have been recorded on almost 100 different host trees. My impression is that webs are most common on black walnuts and black cherries, perhaps because it is easier to build a web on these trees because they have long drooping branches with multiple leaflets. Or maybe it just reflects the fact that both these trees are common in our area? I also think the number of webworms is increasing each year and have seen several totally denuded trees this summer.

Skeletonized leaf of a hickory after the webworms have left

Contrary to popular opinion, fall webworms are not the same species as tent caterpillars, although they do share certain features. The eastern tent caterpillar (Malacosoma americanum) also builds extensive webs in which it chomps away at the leaves, and is also the larval form of an unremarkable dull moth (somewhat ironically given how conspicuous the caterpillars are!). However, the tent caterpillar is active in spring and usually builds its tents in the crotch of a branch whereas the fall webworm’s webs are usually at the end of a branch.

Interestingly, one would think that both tent caterpillars and fall webworms would be a magnet for hungry birds, especially because their appearance coincides with spring and fall migration respectively. The webs are easy to find and contain dense clusters of juicy caterpillars, which are a major food source for many small birds. Surprisingly however, very few birds seem to eat them apart from orioles and cuckoos, and it may not be a coincidence that these both have long thin beaks which they can poke through the layers of webbing without risking it sticking on their eyes. It may also be that the caterpillars are unpalatable because they have long hairs (especially the fall webworm, see the photo below) or contain toxic substances. Perhaps birds prefer to wait until they become moths the following year and eat them then. The caterpillars are not entirely safe within the web however, as several species of predatory invertebrates will try to penetrate it including wasps, ladybugs and most dramatically, the fiery searcher (Calosoma scrutator), a large and colorful beetle which climbs trees at night to feast on clusters of caterpillars.

Hairy caterpillar of the fall webworm
Fiery searcher photographed by Joe Sebastiani

Although many homeowners become concerned when a tree in their yard becomes coated with webs, there is no reason to be alarmed. Fall webworms usually do not do the tree any long-lasting damage because only a fraction of the leaves are eaten, so the tree can still photosynthesize for the rest of the summer and thereby store enough nutrients in the root to get it through the winter. Next year the new leaves will grow in just fine. Although trees with heavy infestations can appear almost leafless (like the one below), in the vast majority of cases fall webworms are nothing more than an unsightly pest and it’s not worth trying to remove them, especially as they are usually high in the tree. Instead, why not just admire these lowly creatures for building their own homes?

A heavy infestation of fall webworm

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