Silky webs are coating trees across Yellow Springs. Many black walnut trees are completely missing leaves.
Wright State University Professor of Biological Sciences, Don Cipollini said the city is experiencing an outbreak of Hyphantria Cunea also known as fall webworms.
“They are here every year. They’re a native insect,” Cipollini said. “This is just a heavy year for them.”
When the larvae hatch, they begin to spin the silk web. Cipollini said the web is a nest for the growing insects.
“This webbing is just the shelter in which they will stay together,” Cipollini said. “It does provide some thermoregulatory benefits you know it stays a little warmer overnight than if they were completely exposed. It provides some protection against predators like birds and things like that.”
He estimates each nest can hold around 100 caterpillars.
“Caterpillars are gregarious and when they’re young they have to be in a group in order to feed and grow normally,” Cipollini said. “If you were to separate them out into as individuals when they’re young they won’t feed, and they’ll die.”
Fall webworms will feast on the leaves of any tree. Cipollini said they have been documented on over 600 different species of trees and shrubs.
“The trees in this area most heavily invested by these insects are black walnuts to the point where they’ve defoliated the entire tree,” Cipollini said. “You can see their nests on a wide variety of other trees as well and you can see trees that they absolutely avoid, but clearly this year their preference for black walnuts is obvious.”
Many black walnuts already appear leafless.
“However, the effect on the tree is very minimal,” Cipollini said, “especially a large healthy tree and it’s partly because of the timing of the damage.”
Cipollini said the trees have already stored enough carbohydrates to survive the winter.
“Losing leaves this time of year is not that detrimental,” Cipollini said. “Within a month from now or so they’re going to start to drop leaves anyhow.”
Cipollini found a terminal bud on an ash tree impacted by the fall webworms. He said this is where the tree will begin to regrow next spring.
“They do not have a severe effect on trees. Smaller trees, smaller ornamental trees can be a different story,” Cipollini said. “These trees have fewer recourses to start out with and so when they get defoliated, they might suffer more detrimental effects to the point of possibly being killed, but that is a fairly uncommon occurrence.”
Fall webworms are native to Ohio. Cipollini said they come and go. A large outbreak isn’t expected in the same city every year.
“We’ve had two pretty heavy years of these, at what you might call, outbreak levels. Typically, what happens to tamp down outbreaks are the rise of predators, parasites pathogens that like to eat fall webworms.”
He expects the population to be smaller next year.
Right now, the caterpillars are large enough to start exploring.
“These are very hairy caterpillars, they are not toxic, they don’t sting, but the hairs can break free of the body, and they can become irritating to certain people that are sensitive to that sort of thing,” Cipollini said. “Your pets may try to eat them, same story they’re hairy, they can irritate their stomach and cause them to vomit.”
During the winter fall webworms will crawl into the bark of the trees to pupate.
“Next year they’ll emerge as adults (moths), mate, lay eggs, and start the process all over again,” Cipollini said.