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Washington plans aerial spraying in two counties to kill tree-destroying spongy moth

By Bill Lucia, Washington State Standard
Published: May 9, 2024, 5:49pm
2 Photos
A spongy moth caterpillar on a dead branch.
A spongy moth caterpillar on a dead branch. (Washington State Department of Agriculture) Photo Gallery

When the invasive spongy moth is in its caterpillar phase, it isn’t a picky eater.

That’s part of the reason the bugs are so destructive. “They eat between three and five hundred different types of plants,” said Karla Salp, a spokesperson for the Washington Department of Agriculture. “Basically, if it’s out there growing, they’ll eat it.”

The insect is permanently established in 20 states across the Northeast and Midwest, where it has stripped millions of acres of forest and urban trees of leaves, according to the state agriculture department. When severe, the damage the bugs cause can be deadly for trees.

While the pest prefers leafy hardwoods, like oaks, Salp said she’s seen them feed on conifers – the category that includes iconic Northwest species like the Douglas fir and western hemlock.

Fortunately, Washington has had success over the past five decades in detecting and eradicating the pests – formerly known as gypsy moths.

That work will continue in the days and weeks ahead, with aerial spraying in the Olympia region and around the town of Concrete in Skagit County. In these areas, there’s “imminent danger of an infestation,” according to an emergency proclamation Gov. Jay Inslee issued Wednesday.

Salp explained that, last year, the state captured about 77 of the moths in the vicinity of Steamboat Island Road, northwest of downtown Olympia, along with 11 near Concrete. The state’s total catch last year was 103.

“It’s one of the highest catches in the past decade or two,” Salp said. During the COVID years, she noted the total dipped as low as six.

 An adult spongy moth. The moths are a threat to trees when they’re in their caterpillar phase. (Washington State Department of Agriculture)So what’s the latest plan for dealing with the moths?

The state targets spraying in the spring based on where the moths are found the prior year.

The Department of Agriculture plans to treat about 1,400 acres in Thurston County, around Steamboat Island Road and U.S. Highway 101, and 900 acres in Skagit County with a naturally occurring soil bacteria – the kurstaki subspecies of Bacillus thuringiensis, or Btk for short.

Salp said it’s been widely used for caterpillar control for decades and is known to be safe when used around people, pets and wildlife. It doesn’t kill the caterpillars on contact. Instead, they ingest the bacteria and it upsets the chemistry in their stomachs.

The Btk formulation the state will use against the spongy moths is Foray 48B, which is approved for use in organic agriculture.

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Btk can leave a sticky residue, which the Department of Agriculture said can be removed with soapy water. People who want to minimize their exposure to the spraying are advised to remain indoors with doors and windows closed until about 30 minutes after it’s completed.

The department also said people should let the spray dry before allowing children to play outside.

A contractor is scheduled to begin spraying at the Thurston County site on Friday. It’s possible that people in the area could see a low-flying plane carrying out the work. The sites will each be treated three times, about three to 14 days apart.

Spraying in Skagit County is expected to begin in mid to late May.

The agriculture department has an interactive map on its website where you can enter your address to see if it’s in the zone where spraying is planned. You can find the map here. And you can find more information on spongy moth treatments here.


The Washington State Standard is a nonprofit, nonpartisan news outlet that provides original reporting, analysis and commentary on Washington state government and politics. We seek to keep you informed about Washington’s most pressing issues, the decisions elected leaders are making, how they are spending tax dollars and who is influencing public policy.

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