Webworms out earlier than usual

Webworms out earlier than usual

Gauzy, Halloween-like webbing from insects known as fall webworms may be decorating trees in the Rogue Valley a little early this year.

Usually found in forested areas, the creepy crawlers, known scientifically as Hyphantria cunea, have infested trees in the valley in unprecedented numbers the past two years, and some tree experts predict this year's infestation will continue that trend.

Although infestations are fairly easy to control and don't cause much damage to trees, the insects' decorations don't sit well with many locals, said Richard Hilton, entomologist at Oregon State University's Southern Oregon Research and Extension Center in Central Point.

Unlike tent caterpillars, which create similar webs during the spring, webworms usually infest trees in the fall. Because the insect is a late-season pest, it has less impact on trees that have already grown leaves or fruit, Hilton said. Although the pests can weaken branches, they usually don't kill trees. "It's more of a cosmetic nuisance," he said.

Webworms begin to lay eggs in the summer. When the eggs hatch, the hairy caterpillars — which are in the same insect group as woolly bear caterpillars — web up the leaves they feed on. The insects start at the tips of branches and expand inward on the tree, and can encompass an entire branch with webs.

Last year's webworms were especially noticeable on black walnut and madrone trees, but they can infest more than 80 species of trees and shrubs, including fruit trees, nut trees and rose bushes, Hilton said. They typically don't appear in oaks.

The fully grown, yellow and black caterpillars will exit the web and find a home in the crevices of tree bark or ground soil for the winter.

So, why the recent influx of the pest? Most entomologists aren't sure, Hilton said.

"We're really just guessing," he said. Infestations are usually cyclical, with booms and busts in webworm populations during a several-year period. Unusually cool and wet springs the past few years are one possible explanation for the reoccurrence, he said.

City of Medford arborist Bill Harrington said the number of predators that eat webworms, such as wasps and yellow jackets, has been down, which may have contributed to higher numbers of webworms. Although he hasn't seen any infestations yet, he expects the visual nightmares to start popping up soon.

"Phones are really starting to ring," said Willie Gingg, arborist at Southern Oregon Tree Care. "We're getting calls two weeks earlier than we did last year."

Most people call because the webs are unsightly or they're afraid of them.

"Some people call fearing the worms are going to crawl across the yard and in their house," he said.

Ed DeLong, an arborist at Advance Tree Services & Landscaping, said he saw serious infestations last year in Josephine County near Grants Pass and in pockets of Jackson County.

To tackle webworm infestations before they become serious, Harrington cautions against pruning and recommends leaving the nests alone or using the insecticide Bacillus thuringiensis, known as Bt. He also said residents can blast the nests using a high-pressure hose.

Although insecticides are an option, DeLong said the products are only 65 to 70 percent effective.

His usual course of action is to cut the infested part of a branch off, put it in a garbage sack, tie it up and leave it in the sun.

"One-hundred percent effective," he said. "It bakes the little buggers."

Reach Universtiy of Oregon reporting intern Josephine Woolington at 541-776-4368 or

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