Webs in the trees? Meet the Fall Webworm
In late summer and early fall in the High Country, it’s hard to miss that many trees have what look like very large cobwebs. These are not actually cobwebs at all, but rather the work of insect engineers called fall webworms. Unlike many insects in our area that escape notice, the caterpillars of the moth Hyphantria cunea are easy to spot due to the large, webby homes that they build. The basic life cycle of moths is like butterflies: they change from egg, to caterpillar, to pupa, and then to the adult moth. Examining the timing of the stages in fall webworm helps us see how this fascinating insect gets its common name.
Fall Webworm nest- photo by Ray Williams
After hatching from eggs laid by the female moth, fall webworm caterpillars feed on many kinds of trees and shrubs, with persimmon, sourwood, hickory, and walnut especially prized in our area. As they eat over several weeks the caterpillars work as a group to make the web-like nest we see, building their home around all the leaves the group of caterpillars is eating. If you investigate a nest closely, you’ll see many cream or sometimes more darkly colored hairy caterpillars (along with their poop!) and tiny bits of leaves. Though it’s not a tidy home, this silky structure serves a useful purpose discussed below. Since the nest encloses all the leaves the caterpillars eat, and with lots of hungry caterpillars, it reaches considerable size. As the caterpillar stage comes to an end, they leave their tree-house home for the ground to form a pupa and spend the winter. The bright white (sometimes with spots) male and female adult moths emerge in the spring. After mating the female moth lays eggs on the underside of tree leaves to start the cycle over again.
Fall Webworm caterpillars. Phot by Milan Zubrik, Forest Research Institute - Slovakia. Bugwood.org
Questions that arise include: First, why build such big nests in the first place? Because insects are food for lots of animals, it’s not hard to see how the webworm’s home protects them. If you were a hungry bird, imagine how hard it would be to grab a caterpillar from the tangled web. Also, if the nest is disturbed, the whole caterpillar colony wiggles almost in unison, possibly causing an animal looking for a meal to get scared and go away. Another reason for the webby home is the enclosed nest helps prevent other insects from killing the fall webworm caterpillars by laying their eggs inside of them.
Another question is do the webworm’s large webs hurt our trees? The good news here is that since most feeding is done by the older caterpillars in late summer or early fall, which is late in the tree’s growing season, the impact on a tree is smaller than expected. By the time some of the leaves are eaten the trees can largely do without them. Though not common, one exception would be a small tree that is completely covered by webs with only a few leaves remaining. But most of the trees you see in our area are not in great danger from the fall webworm.
So as you notice big webs in our trees, think about another of the many interesting insects in the High Country as the fall webworms soon sneak down from their nests, wrap themselves and get ready for the winter that is coming for all of us.
References:
Steven Frank, James Baker and Stephen Bambara. Fall Webworm. NC State Extension Publication. 2019. https://content.ces.ncsu.edu/fall-webworm-1
Gregory A. Hoover, Sr. Fall Webworm. Penn State Extension. 2021. https://extension.psu.edu/fall-webworm
USDA Forest Service. Fall Webworm. https://www.fs.usda.gov/Internet/FSE_DOCUMENTS/stelprdb5341330.pdf
Portions of this article appeared in the Watauga Democrat (Boone, NC) on October 14, 2022.