Gov. Inslee declared emergency for spongy moth infestation. How much of a threat are they?

On May 8, Governor Jay Inslee declared a statewide emergency over a pair of spongy moth infestations, one just outside Olympia and the other in Skagit County. The declaration claimed the infestation “endangers the agricultural and horticultural industries” and “threatens the economic well-being and quality of life” in Washington.

But how much of a threat do the moth infestations really pose? Here’s what you need to know.

Why are spongy moths dangerous?

The spongy moth is an invasive moth species that tends to feed on leaves. That makes them a threat to the trees in their surrounding areas, according to Washington State Department of Agriculture Public Engagement Specialist Karla Salp.

“Spongy moths are one of the worst invasive pests that have been introduced to the United States. Back east, there are like 20 states that are permanently infested with spongy moths… The caterpillar numbers kind of explode periodically. And when they do that, they can defoliate entire forests,” Salp said in a phone interview.

Salp said that infestations can kill deciduous trees – which have leaves that fall off and regrow annually – over the course of a few years. For conifers – trees that grow needles and cones – Salp said that spongy moths can kill a tree in as little as a year. Washington has a high proportion of conifers, making a potential infestation especially threatening.

Are spongy moths a threat?

WSDA monitors spongy moth populations across the state. Last summer, the state found 103 spongy moths after finding 30 in 2022. The biggest infestations occurred in Thurston County, near Steamboat Island Road, where 77 moths were found.

That was enough evidence of a growing spongy moth population for the Department of Agriculture to decide to use aerial treatments to get rid of the Thurston County infestation, along with an infestation in Concrete, WA, where 11 moths were found.

The emergency declaration was less a warning about a looming disaster than a routine step taken so WSDA could use aerial treatments, according to Salp.

“In order to do aerial treatments – which is by far the best way to do it, the most efficient and cost effective and effective way to make sure you get the best coverage – you have to have a declaration from the governor declaring a state of agricultural emergency,” Salp said.

That doesn’t mean this is an annual process, though. Prior to the emergency declaration, WSDA hadn’t used aerial treatments since 2021. They were a near-annual occurrence between 2015 and 2021, but had only been used twice in the decade before that.

If the results from those years are any indication, you’re safe from widespread spongy moth destruction for now. But Salp said her visits to states on the east coast with permanent infestations serve as a good reminder of why it’s so important to take these precautions.

“I went to state parks, this was in July, and it looked like it was winter because there was no leaves anywhere,” Salp said.