Invasion of the web worm

'Creepy-looking' nests show up in trees throughout the Rogue Valley, ruining leaves and branches
A cooler-than-usual summer has led to an infestation of fall web worms in the Rogue Valley, with web-like nests showing up in area trees and even vineyards.Mail Tribune / Bob Pennell

Around the Rogue Valley, web clusters looking like Halloween decorations gone awry are filling trees and jamming the Grange Co-op phone lines with requests for advice and remedies.

The creepy-looking fall web worm nests, generally found in outlying areas and madrone and oak stands, have shown up en masse in urban trees, even vineyards, this summer. The worms feed off the host trees, destroying foliage and branches.

What you can do

If you've got fall web worms, Rick Hilton, an entomologist with the Oregon State University Extension Service, said there are a few options:

  • Cut out small nests, making sure to remove egg masses.
  • With larger infestations, applying Bt insecticide is more effective.
  • Hosing the nests down with water is an option, but may not wipe out the worms.

"The last 10 days to two weeks, we're getting at least 50 to 75 calls per day and that tells me there is an issue," said Steve Soderlund, Grange Co-op store manager on South Pacific Highway. "We've sold out of Thuricide, which has an active ingredient called Bacillus Thuringiensis — a 24-hour concentrate."

Dave Keiry was outside helping a neighbor Saturday morning when someone showed him the cluster in his ornamental Japanese plum.

"I went to the Grange and the clerk told me I was the 35th or 40th person to get the same stuff," Keiry said. "As I drove home, I saw the trees were covered along Biddle Road, past the airport and into Central Point."

The creepy crawlers have been called a variety of things, ranging from tent caterpillar to the ugly nest caterpillar, but Rick Hilton, an entomologist with the Oregon State University Extension Service, said there is no doubt about this pest.

"There is another moth where the caterpillar makes a similar tent," Hilton said. "But we're dealing with fall web worms."

When these caterpillars emerge from their cocoons, they are plain white.

"They are generally found in the mountains and hills," Hilton said.

The theory is that the invasion was spurred by two years of wet, cold springs and late summers.

"The wet conditions and late summer threw things off so the predators and parasites that control them were not active," Hilton said. "We saw them last year in the organic orchards. There was a big population last year and so there was a large population to start with this year,"

The arithmetic is simple for Soderlund.

"Thuricide is an item we commonly carry and it wouldn't be a problem," he said. "Except when every third body walks in with a tent caterpillar issue. You don't normally sell 20 bottles one year and 200 the next."

The active ingredient in Thuricide, Bacillus thuringiensis, often is called Bt. It's a biological pesticide that occurs naturally and is considered safe for people, pets and wildlife.

Hilton said the OSU Master Gardener's program has reported the highest volume of calls ever received as a result of the web worms.

Hilton said as soon as the eggs hatch, the webs show up.

"They feed on anything from 90 to 250 plants," he said. "At first it's just a leaf and then it grows. The webs usually have at least 100 eggs and the worms grow to an inch long. When you have 100 caterpillars and they start getting bigger, they need a lot of food. That's why a web can take up a whole branch. The sooner you get to it, the better."

Reach reporter Greg Stiles at 541-776-4463 or email

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