While my wife and I were travelling south of Warren, Pa., the first week of September, we noticed among the tree-covered ridges, white, fluffy, cotton candy appearing objects scattered throughout the leaf canopy. The rising sun at 8 a.m. reflected off dew drops on the white spots, creating a beautiful natural scene. However, I realized these trees were likely infested with defoliating tent caterpillars living in the spots which were actually nests or tents.
I am familiar with tent caterpillars, the larval stage of a moth, since they have created silky nests in trees around my home and elsewhere in late spring. Could these nests visible near Warren be a second generation of the tent caterpillar moth? By the time I passed the same tree-lined ridges a week later, the trees with the white fluffy nests appeared brown amongst the other trees still holding green leaves. I was still bothered by the late-appearing nests, especially since my Peterson Field Guide on insects described tent caterpillar nests as appearing in late May and early June. I drove south of Warren to photograph the defoliated trees. I collected two caterpillars in a plastic jar after teasing the web-like nest apart. The nest had engulfed the terminal leaves of a sumac tree branch. These extremely hairy caterpillars, initially dark gray, later turned pale yellow in my jar. They had two black spots on the head. My field guide reported a yellow stripe down the back of the tent caterpillar; the caterpillars I collected did not have a yellow stripe.
I needed to discuss my findings with an expert on forest insects, so I consulted a well-known regional forestry service, because they treated an infestation of a related species, near Bradford, Pa., two years ago, salvaging hundreds of acres of trees. After a call to the expert, my answer was forthcoming from the person, who deals with forest pests regularly. He asked, "Did the nest (you) found enclose leaves at the end of branches?" I verified that was the case. He pointed out that tent caterpillars create nests in the crotch of tree branches exclusive of leaves; unlike the caterpillar I found. He also confirmed my impression that the tent caterpillar produces nests in the spring, while this particular caterpillar did so in the fall. He noted that the caterpillar in question can be ignored as a serious pest because it defoliates trees in the fall when leaves naturally fall off. Damage to trees is minimal since leaves produce glucose for growth all summer long.
Fall webworm caterpillars feed on leaves enclosed in their web nest. White fall webworm caterpillar nests appear on trees pictured above, south of Warren, Pa.
Photo by Robert M. Ungerer
At this point in our discussion, I eagerly awaited the name of the caterpillar I found. Apparently this caterpillar is extremely common but one I have never heard of. It is a fall webworm (hyphantria cunea-Latin name) known for its larval stage which creates web-like nests in the fall.
Armed with its identity, I conducted an Internet search, which revealed that more than 90 tree species in North America serve as hosts for the fall webworm, a native species. Numerous parasites and predators feed on the fall webworm. The caterpillars will drop to the ground later this fall and spin a cocoon. They will over winter in leaf litter on the forest floor emerging as a small white moth in late summer. The adult moths mate and lay eggs which become caterpillars. A web-nest is created by caterpillars that devour leaves, thereby starting the life cycle over again.
To my amazement, entomologists have described the life cycle of the fall webworm in a New York State Museum book, "Insects Affecting Park and Woodlands," published in 1905, more than 100 years ago.
With my newfound knowledge, this past week I discovered fall webworms in a tree easily reached and photographed on Maple Road in Ashville. When you come across a web nest in a tree, take a close look inside to appreciate these handsome, hairy and harmless caterpillars.