(The Center Square) – The Washington State Department of Agriculture plans to begin treatments on Friday to eradicate spongy moth caterpillars.

Officials don’t want the invasive species, which can destroy entire forests, to establish a foothold in the state.

Plans call for aerial spraying to treat about 1,400 acres in Thurston County and 900 acres in Skagit County with a biological product that contains Bacillus thuringiensis kurstaki, or Btk, a naturally occurring soil bacteria that interferes with the spongy ​​moth caterpillars’ digestive system, eventually killing the caterpillars.

Only the Thurston County site is anticipated for targeting on Friday, with additional sprays in the coming weeks.

“We don’t do treatments every year but we do monitor every year,” said Karla Salp, with WSDA communications. “In fact, this is our 50th year that we’re trapping.”

She explained the process.

“In the summer we trap for the adult months, and in winter we analyze results, and then if it looks like we have a reproducing population in an area, that’s when we propose a treatment,” she said.

The Washington State Department of Health recommends people who want to avoid contact with the material being sprayed remain indoors for 30 minutes after the treatment is complete.

The WSDA release says Btk is sticky, so residents in the treatment areas may choose to cover cars parked outside to protect them from the spray. However, Btk can be cleaned off vehicles with soap and water.

Eliminating the species is critical, according to Salp.

“If you take a look back east at what they’re dealing with, you’ll see why we try to keep them from establishing here,” she said.

The moths’ path of destruction is something to behold.

“I went back a couple of times when they were having outbreaks and basically what you see is a nice state park in the middle of July, and it looks like it’s winter because there are no leaves anywhere to be seen, because they’ve all been consumed by the caterpillars,” Salp said. “They’ve had entire forests completely defoliated by these moths and they’ve lost millions of trees in outbreaks.”

Spongy moth caterpillars, depending on the subspecies, can eat 300 to 500 different kinds of vegetation.

“Here in the Northwest where we have mostly conifers that don’t re-leaf out, that will kill the tree in one year,” Salp said.

Another nasty impact from an infestation is how much they eat and digest.

“They eat so much that they are constantly pooping and if you walk under an infested tree, it literally sounds like it’s raining from their feces dropping off the leaves,” Salp spelled out. “It’s basically raining poop.”

The area to be sprayed on Friday is along Steamboat Island Road Northwest. Area residents or other interested parties can enter an address on the agency website to see if it’s in the spray zone.

WSDA puts out at least 20,000 traps every year, according to Salp, “just to monitor for this pest which speaks to how damaging it could potentially be.”

Visit the agency’s spongy moth web page at agr.wa.gov/moths to learn more or contact the WSDA Pest Program via email at pest@agr.wa.gov or call 1-800-443-6684.