Pesky Western caterpillars are back and busily munching on Whatcom County trees


Western tent caterpillars, Bellingham

Western tent caterpillars in the Sunnyland neighborhood of Bellingham, Thursday, May 22, 2014.


They're back.

Hordes of white-orange-and-black caterpillars have emerged from their silken nests, or tents, to munch on tender tree leaves - signaling a third spring when Western tent caterpillars are out in big enough numbers to alarm gardeners and homeowners.

This year's numbers are part of a cyclical population boom that started in 2012, when their numbers exploded.

"We're on the downward slope," said Chris Benedict, agriculture agent at the Washington State University Extension office in Whatcom County.

"They're still going to show up," he said, but the population boom is on the other side of the bell curve.

The outbreaks often end on their own as trees activate their defenses and the parasites and predators that are the native caterpillars' natural enemies find them.

"They are native and they look like they're taking over the world," said Beth Chisholm, master gardener and coordinator of the Community First Gardens project through WSU Extension.

But there's the ick factor, with people posting their concerns on Facebook and calling the extension's office trying to figure out what to do.

"We don't want people to burn them," Chisholm said. "It's a hazard for yourself and you don't want other things to catch on fire."

The wiggling caterpillars are considered a nuisance but largely harmless. They are the larvae form of brown, stubby-looking moths that will emerge from cocoons later in June or July to mate and lay eggs.

They hatch from eggs over-wintering in "tents" on the tips of tree branches. Once they hatch, they start eating.

They prefer the foliage of most deciduous trees and shrubs such as alder, roses and fruit trees.

While the hungry caterpillars put a visible dent in the surrounding greenery, they usually don't kill the healthy trees they feast on and most trees can withstand the temporary munching, according to gardening experts.

Still, small trees might not be able to recover from such defoliation.

To get rid of the caterpillars, Chisholm recommended waiting until morning when it's cool and they are back in their nests. Then cut off the infested branch, put it in a bag, tie it up and put it in the garbage.

The WSU Extension has posted information on its website at for dealing with the caterpillars.

After an outbreak, they nearly disappear for several years.


-- Insecticides are discouraged. Unless trees already are weak, tent caterpillar damage isn't permanent; trees will grow new leaves after the caterpillars have left. Poisons also can kill beneficial insects, including those that attack the caterpillars.

-- In fall or winter, pick out and destroy the moth's eggs, which appear in foamy, gray, half-inch cases around twigs or in flattened masses on tree trunks.

-- Strip or prune out the tent-like nests found at the end of branches as soon as they appear. Remove the nest in the early morning or evening, when most of the caterpillars will be inside. If you can't do that, spray them off with hard bursts of water from a hose.

-- Bring questions and caterpillar-infested plant samples to the WSU Extension's Master Gardener diagnostic clinic during regular business hours at the extension office, 1000 N. Forest St. in Bellingham, or call 360-676-6736.

-- Learn more by going online to A section on tent caterpillars has been posted down on its homepage.

SOURCE: WSU Extension

Reach Kie Relyea at 360-715-2234 or .

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