SEPTEMBER 3, 2009 ISSUE
Fall Webworms Are Here
Webworms envelop branches on a tree in Watauga County. Photo by Ken Ketchie
The Watauga County Cooperative Extension has fielded “a lot of phone calls from people concerned [about webworms],” said Meghan Baker, commercial and consumer horticulture agent for Watauga County. “They are a very dramatic pest.”
Fall webworms’ white nests are easily spotted wrapped on numerous tree branches throughout the area. They do not usually, however, cause lasting damage to trees, Baker said.
“They are considered an aesthetic pest [because] they consume the foliage, but they only do it a month or so before the natural leaf drop, [and] it doesn’t affect the overall health of the tree,” Baker said.
“The one exception to that is if a young tree [is] new to the landscape,” she said. “[Webworms] can kill young trees in some situations. If people are really concerned, if they can reach the webs, ripping open the nest allows predators access.”
Natural webworm predators include birds, predatory wasps and parasitoids, which lay eggs in the caterpillars, which then become the parasitoids’ food source, Baker said.
Webworms favor persimmon, hickory and cherry trees, but their nests can be found in almost any type of tree. They are not very discriminatory, Baker said.
Although the webworms make an appearance in early fall of each year, they are considered to be worse in drought years.
“The overall population seems less to me than last year,” Baker said.
The nests may be “unsightly, but they’re not going to cause long-term harm, especially to mature trees,” she said.
The webworms are the larval stage of the moth and “their entire job is to eat as much as they can for a few weeks,” Baker said.
They then crawl down the tree, pupate and overwinter in the soil or debris such as leaf litter or mulch. Adults emerge typically in May and lay eggs that will become larvae.
The adults start one, and possibly two, new generations of webworms each spring, Baker said.
“If people really do want to do something early in the season, one [product] that we recommend [is] Bt,” she said.
Bt, an abbreviation for Bacillus thuringensis, is a natural organism approved for organic growers that only targets caterpillars. The product should be sprayed when webs are young and small, so that as the webworms expand their nest, they chew the leaves that were sprayed and the population’s growth is slowed down, Baker said.
“A lot of people call them bagworms [but] that’s incorrect,” Baker said, explaining that bagworms look like upside-down conifers and make much smaller nests.
“Bagworms can really defoliate a tree and be a bigger problem [than webworms],” she added.
Eastern tent caterpillars also build nests resembling those of the webworms. The Eastern tent caterpillars, however, build nests in the crotches of tree branches in the spring or early summer, whereas webworms build theirs toward the end of branches in the fall.
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