A Sign of Autumn: Fall Webworms

A predatory wheel bug, Arilus cristatus, feeds on a days-old fall webworm larvae, seen on the tip of its flexible-straw mouth,or proboscis. Photo: Andy Wood

HAMPSTEAD — A few days before the autumnal equinox, I happened upon a favorite bug of mine as it was dining on a family of hapless days-old fall webworms, Hyphantria cunea. The bug, Arilus cristatus, or wheel bug, was working a young dogwood sapling in our yard. By that I mean it was eating young caterpillars that were, in their turn, eating late summer dogwood leaves.

The wheel bug, so-named for a raised row of spines on its midsection, resembling spokes on a wheel, is a large predatory insect with a preferred taste for caterpillars, including several species that are injurious to our crops. Wheel bugs can be included in the list of so-called beneficial insects, but that’s a term I find disturbing because it implies some insects are not beneficial. But I digress.

Webworms are actually the larvae of a small white moth that we seldom see because they are active at night. Webworm moths, a native of North America from southern Canada to northern Mexico, can be attracted to porch lights, especially in spring after they have emerged from their winter cocoon, but unlike many of their cousins, adult webworms, with a wingspan of about one and a half inches, are not very glamorous and therefore easily overlooked.

Two Generations

Webworms in our region usually have two generations each summer, with the first starting out in spring as eggs placed on the underside of a deciduous tree leaf by an adult moth that was a larva the previous fall. The eggs, sometimes hundreds, are placed in a tight cluster on the leaf, and after incubating for several days they hatch into tiny caterpillars less than an eighth of an inch long and slender as a pin. The siblings cooperatively feed on the host plant by enclosing their leaf in a tangled web of silk to serve as a sheltering net that helps thwart the intrusions of predators, including wasps, spiders and our beloved wheel bug.

Within a few days, growing webworms double their size on a diet of soft leafy tissue; a process that effectively skeletonizes the leaf. As they grow, the caterpillars expand their webbing to incorporate more leaves and by the time they are half an inch or so, their jaws are strong enough to eat the entire leaf. By this time their protective tents are evident to even the casual observer. The first generation from last spring seldom gets noticed because there aren’t many of them, relatively speaking.

This inch-long fall webworm, Hyphantria cunea, is fully grown and days away from transforming into its pupa stage. Long, bristly hairs may repel some predators, but many webworms are eaten by songbirds, lizards and toads. If lucky, it will spend winter secreted in forest floor leaves as a pupa, and come spring it will transform to an adult webworm moth prepared to join others of its kind with intent to create a summer generation that will continue the species one more year. Photo: Andy Wood

After reaching about an inch long, the now hairy — not stinging — webworm caterpillar descends to the ground where, if it is not eaten by toads or other predators, it transforms into a brown pupa, wrapped in the loose hairs from its outer skeleton, and undergoes metamorphosis.

By late summer, adult webworm moths emerge from their cocoon, court, mate and the fertilized females continue the moth life cycle by placing her eggs on the underside of deciduous tree leaves, which hatch, in our area, sometime in early to mid-September, into the year’s second generation of larvae; a much larger generation I might add, with many more clusters of webs adorning many more trees. Hence the name fall webworm, not spring webworm.

In late summer or early autumn our deciduous trees are winding down for the year, getting ready to shed their leaves for winter. Webworms therefore, are feeding on leaves at a very opportune time because what the caterpillar eats, we don’t have to rake from our yards. In a sense, these caterpillars might be credited with saving people money by reducing yard maintenance costs. It’s conservation at caterpillar scale, especially considering that these insects are eating leaves that will be dropped by the tree anyway. In addition, the caterpillar’s waste droppings, called frass, provide available nutrients to the very trees the caterpillars eat.

While webworms may not be very glamorous, they are significant contributors to the flow of energy winging through our area right now. Each autumn many hundreds of millions of songbirds migrate from their northern summer breeding grounds to southern wintering areas; with stops along their path to feed from nature’s menus that include protein-rich insects, especially juicy caterpillars fattened on deciduous vegetation. In a very real sense, the flights of migrating warblers and other insect-eating songbirds are powered by the energy “stored” in the bodies of insects.

Webworms and Climate Change

In the fall, look for the cottony white webs enveloping leaves on a tree. The webworms encase themselves in these webs to protect themselves from predators. Photo: University of Florida

These half-grown fall webworms are partially shielded from predators by a latticework of silken threads they create to envelop leaves of their host dogwood tree. Photo: Andy Wood

Presently, fall webworms are regarded in landscape, forestry and agriculture circles as relatively benign creatures, requiring little need for chemical control. However, if fall webworms do adapt to a changing climate, by adding a generation to their summer numbers, could this otherwise benign species become something of note, especially to manufacturers of toxic pesticides? And if this were to happen, I wonder if we might expect to see greater advertising for fall webworm control? And if that occurs, what might that bode for the many animals that eat fall webworms?

These are not questions I ask lightly, because the insecticides we broadcast, along with herbicides and other so-called pesticides, are toxic compounds, especially the synthetic chemicals, many of which have had little or no testing to determine long-term impacts to target and non-target species, including us. Remember, we are part of the same flows of energy, via various food webs, that involve plants, eaters of plants, and eaters of eaters of plants, including wheel bugs, songbirds, and many other animals. This is why it is correct to say, “What goes around comes around.”

Knowledge Is Power.

So, here’s a webworm idea: This year I made a count of all the webworm tents I could find within a designated area of my property (18 and counting). Next year I plan to do the same. I may find more or less, in part because, in the space-time energy flow continuum, fall webworm populations fluctuate, with some years experiencing significantly larger outbreaks than others. This to say, a fall webworm population trend study cannot be completed with one or two years of data.

My objective with this idea is to gather information that tells me something more about the world in which we live, with maybe a little ulterior motive being a hope that, in the long term, we will think beyond chemical pesticides as a sensible management strategy for the habitats, including our yards, that provide ecosystem services to the benefit of living communities within which we live.

That said, now is as good a time as any to start counting webworm nests, noting any predators you observe dining on these scrumptious packets of nutrition; while also paying attention to insecticide advertising because, just as predatory insects and birds exploit webworms, so too can we expect others to do the same, albeit for a different form of currency—the kind that drives our contrived economy, operating within an otherwise earthly ecosystem.

About the Author

Andy Wood

Andy Wood is a coastal ecologist, lecturer and author of "Backyard Carolina: Two Decades of Public Radio Commentary." Andy’s focal interests include conserving rare and endangered species and their habitat. In fact, Andy is leading efforts in Pender County where he lives to prevent the extinction of an endemic freshwater snail and its habitat. Andy is the former education director for Audubon North Carolina and the former curator of education for the N.C. Aquarium at Fort Fisher. He is currently president of Andy Wood & Associates, a conservation consulting company.