Fall web worms create tents in trees

By Mark Ribble If you’ve driven through Point Pelee National Park in recent weeks, you have probably noticed the trees along the roadway appearing to have giant spider webs covering them.

While they are not spiders, they are caused by other insects — namely the Fall Webworm and the Tent Caterpillar.

While the two insects do things a little differently, people often get confused between the two. The Sun reached out to Parks Canada for some information on the caterpillars and their silky-woven nests that are quite visible.

Parks Canada Communications Officer Elizabeth Leblanc was able to gather some information and answer the following questions:

What is making the webs you can see in the trees along the main road at Point Pelee National Park?

“What you are seeing in the trees along the main road is Fall Webworm and inside each web is a small colony of caterpillars. They are eating the leaves they have tented within their web. The good news is they are a native species and have little to no impact on the health of the trees. And the webs are mainly present on Black Walnut trees which are very plentiful in the park.”

The Fall Webworm is the larva of a species of tiger moth. It is best known in its larval stage when it builds a communal web, tenting over the leaves it feeds on. These webs are seen in late summer and early fall.

Each tent contains many pale yellow, black-spotted, hairy caterpillars (each measuring about 2.5 cm long). As an adult they are a stout-bodied white moth, with black or brown spots and a wingspan of 5 cm. Do they normally appear at this time of year?

“Fall Webworms spend most of their lives as a pupa in a cocoon. In early summer, adult moths emerge from their cocoon and begin mating. Female moths will lay eggs in clusters on the food plants for the caterpillars (including tree species like Black Walnut, ash, cherry and willow) and then cover the cluster with hairs from their body.

When the eggs hatch, the caterpillars will begin to spin webs, tenting the leaves on a branch they will eat. Most of the web building happens during the evening and the caterpillars will happily munch on leaves during the day, protected from predation.”

The webs will continue to expand, sometimes covering entire branches, as the caterpillars progress through their six moults. This process can take anywhere from four to six weeks. At the end of this period, the caterpillar will pupate and spin a cocoon under tree bark or leaf litter to spend the winter, emerging as an adult in the spring and beginning a new life cycle.

Why do they build a tent?

“The tent offers protection from predators, like birds, during the early stages of the caterpillar’s life. As the caterpillars grow, predators will start to attack the nest. At Point Pelee, they are most predated by Yellow Warblers and wasps.”

If you take a closer look at a nest, you’ll be able to see many interesting things. Other than live caterpillars munching away on leaves, what you may notice next are the shed skins from caterpillars. They moult six times over their life span, leaving behind a shed of skin with each moult.

You’ll probably also see caterpillar droppings. Fall Webworms rarely leave the safety of the nest, so poop will build up inside.

What is the difference between Fall Webworms and Tent Caterpillars?

“These two species are not closely related, but are often confused. Fall Webworm caterpillars start building their webs about the time tent caterpillars are finishing their nests.”

The nests appear to be very similar, but the key feature to look for is where the nest is built. Tent Caterpillars build their nests in the forks of branches while Fall Webworms build their nest over an entire branch. You will rarely see a Fall Webworm outside the nest, usually only just before they are ready to pupate. Tent Caterpillars, on the other hand, will leave their nests to feed.

The caterpillars themselves are also different. Although both are hairy, Tent Caterpillars are brown, while Fall Webworm are yellow with black and brown spots.

These two species also have very different impacts on trees. Tent Caterpillars emerge in spring and will strip the trees of their young leaves. If the tree species cannot regrow its leaves, then Tent Caterpillars can have a detrimental effect on the tree’s health.

Fall Webworms, on the other hand, eat leaves in late summer and early fall, when many tree species are starting to drop their leaves. By this point in the growth cycle of the trees, they have obtained and stored enough energy in their roots to survive the winter and can afford to lose their leaves.

You will notice in years of severe drought that many trees will begin to drop their leaves at the same time Fall Webworms start building their tents.

Trees inside Point Pelee and other bush lots throughout the area are currently experiencing a fall web worm boom.