Beetles killing more trees in WA, likely due to drought

Washington had fewer dead or dying trees last year, but beetles that feed on dry trees caused greater damage across the state, a concerning trend for environmental officials.

The state’s annual forest health survey from the Washington Department of Natural Resources and the U.S. Department of Agriculture Forest Service found Washington’s forests are suffering from increased heat, drought and wildfires, making them more susceptible to deadly beetles that feed on bark. 

Glenn Kohler, a forest entomologist at DNR, said it’s hard to tell for sure if the drought is causing the beetle populations to spread, but it is likely. 

“I think they’re just breeding up in drought-stressed trees and moving to other drought-stressed trees,” Kohler said. “As long as these drought effects remain, I anticipate we’ll see more.” 

All forested areas experienced below-normal spring precipitation and some level of drought conditions in the fall, according to the 2023 survey. Kohler also noted that the 2021 heat dome likely had a lasting effect on trees, which once damaged, can’t defend as well against the insects. 

The outlook for this year isn’t better. Much of Washington is already considered abnormally dry or in a moderate drought, according to the U.S. Drought Monitor. The Department of Ecology last month declared a drought emergency across the state, except for the Seattle, Tacoma and Everett metro areas. 

The forest health survey is done aerially every year and looks at recently killed and currently damaged trees across the state. 

Last year’s efforts covered about 22 million forested acres in Washington, which accounts for the majority of the state’s forested land. Of those, about 517,000 acres were suffering from some level of tree mortality or disease. Aside from 2020, when no survey was completed because of the pandemic, that’s the fewest damaged trees since 2018, which saw 469,000 acres damaged. 

Despite fewer damaged trees, Kohler said the increase in tree deaths due to bark beetles is a concerning trend.

About 348,000 acres of trees last year showed signs of bark beetle damage, according to the survey. That’s slightly up from 2022’s 346,000 acres.

One worrisome data point is the increased impact of the Douglas fir engraver beetle. It caused 25,600 acres of tree damage last year — the highest number recorded since 1969. That’s also about 26% more than the 20,300 acres affected in 2019. Two other beetles, the Scolytus monticolae and the Douglas-fir pole, also caused significant damage to trees, the report found.  

Also alarming was the California Fivespined Ips beetle’s first occurrence in Western Washington. Before 2022, these critters only killed trees in the Columbia River Gorge. Now, they’ve moved to urban areas in Western Washington, as far north as Bothell. 

“We’re not talking about thousands of acres, just dozens of trees in urban areas, but it’s something we’re trying to keep an eye on,” Kohler said. 

As Washington continues to have hotter, drier summers, land managers can take steps to lessen the effects on trees and prevent bark beetles from spreading.

Thinning land to give more dominant trees room to grow or removing invasive weeds on the forest floor are two actions that can help lessen the need for a tree to fight for water or nutrients in the soil, Kohler said. Another way is to remove any root disease, which can also weaken or kill trees. 

But if a tree is affected by low precipitation and high temperatures once, it’s likely affected forever. 

“Even if we have a good year, the drought effects are cumulative on a tree,” Kohler said. “If the poor tree has dealt with three or four drought events, the damage is kind of done.”