The most destructive forest pest in North America is now in Oregon

The invasive emerald ash borer threatens the state’s salmon habitat, urban forests and agency budgets.


The emerald ash borer is a half-inch long wood-boring beetle. The species has already killed more than 100 million trees in the U.S.

Oregon foresters and entomologists have spent years dreading the inevitable arrival of the insect that experts call the most destructive forest pest in North America: the emerald ash borer. This slender green wood-boring beetle has been the subject of invasive species planning forums, quarantines, research papers, grant applications, detection surveys and mitigation plans across the nation since 2002, when it first appeared in Michigan. From there, it began eating its way across the landscape, leaving behind more than 100 million dead ash trees by 2010 — trees that have cost billions of dollars to treat, remove and replace.

The insect is difficult to detect, often going unnoticed until it’s well established. It spreads quickly and can cause widespread tree mortality in just a few short years. Last year, Oregon adopted a comprehensive management plan to prepare for its arrival; scientists estimated that it might reach in the state within five years.

By then, however, it was likely already here.

On the last Thursday of June, Dominic Maze, an invasive species biologist for the city of Portland, was sitting on his tailgate outside his kids’ summer camp in suburban Forest Grove, Oregon, scoping out the plants around the parking lot. He noticed that all the trees were in terrible shape, with very few green leaves and lots of shoots at the base to compensate. Then he noticed they were ash trees.

Immediately, “I had a real sinking feeling,” he said. Other parents in the parking lot watched as he shredded bark off the trees until he finally found the culprit: one tiny green menace, wedged in the bark. Overhead, shiny green beetles flew off into the distance.

Oregon Department of Agriculture Biocontrol Entomologist Max Ragozzino points to galleries dug by an emerald ash borer in Forest Grove, Oregon.
Jim Gersbach/Oregon Department of Forestry

The ecological and economic threats to Oregon are hard to overstate. “There is a high level of alarm,” said state entomologist Christine Buhl of the Oregon Department of Forestry. “(It’s) very difficult to slow its spread and it’s impossible to eradicate. That’s never been successful in any other state.”

The Oregon ash, native to the western Pacific Northwest, is uniquely suited to wetland habitats, where it provides vital shade and cooler waters for the region’s endangered and threatened salmon. Other ash species are widely used as street trees in cities where canopy shade is already shrinking.

The Oregon ash is uniquely suited to wetland habitats, where it provides vital shade and cooler waters for the region’s endangered and threatened salmon. 

As agencies statewide begin their efforts to destroy infected trees, detect other pockets of infestation, and protect areas where the beetle isn’t yet active, here’s what’s at stake:

99% of all ash trees in Oregon. Nearly every tree infected by the emerald ash borer dies within two to four years of infestation. After years of drought, experts say, trees in the state are especially vulnerable to pests.

10 species of ash in Oregon, including the Oregon ash, the only ash native to the Pacific Northwest. This tree thrives along the region’s many rivers and streams and is well suited to wetland conditions where researchers say no other native tree can grow or replace it.

11 threatened and endangered species living in those streamside, or riparian, habitats, including salmon and steelhead. They rely on the cooling shade of Oregon ash, which make up 95-99% of the tree canopy along some portions of the Willamette and Columbia river systems. The endangered peacock larkspur, the recently de-listed Oregon chub, freshwater mussels and endemic caddisflies are among the many other species likely to be harmed by widespread ash mortality.

Native ash thrive in wetland habitats across western Oregon, where tree mortality from an emerald ash borer invasion would harm 11 threatened and endangered species.

42,000 young ash trees planted at Smith and Bybee Wetlands Natural Area in Portland in 2015, and tens of thousands like them used in habitat restoration efforts throughout its range, which runs from California to northern Washington. (Oregon is the only West Coast state to have detected the emerald ash borer so far.) Oregon ash have been an important tool in erosion control and restoration projects in Oregon and California, though many municipalities recently began limiting its use due to the looming threat of the insect.

$50 million in urban tree removal and replacement costs in the city of Portland alone. Urban tree canopy plays an important role in climate change mitigation and equity strategies by reducing heat islands and improving air quality. Ash canopy loss was linked to more than 20,000 cardiovascular and respiratory disease deaths in the five years after the ash borer was detected in the U.S. Recent reports show that Portland’s canopy is already shrinking — and the city recently opted not to renew a popular tree-planting program — so although ash species make up less than 5% of the city’s trees, losing them could make a known problem worse.

600,000 ash tree seeds researchers had planned to collect in 2022 to preserve ash genetics and test for emerald ash borer resistance. Scientists have already gathered about 350,000 of their million-seed goal, but they are uncertain how the insects’ arrival will affect their genetic preservation efforts. The project, launched last year, is the first of its kind in the U.S. and could play a vital role in protecting — or replanting — ash trees nationwide.

Officials across Oregon urge residents and visitors not to transport firewood, which is the most common vector for accidentally spreading the emerald ash borer. They also recommend learning what ash trees and ash borers look like — the insect is frequently mistaken for several other common green beetles — and reporting any credible signs of infestation.

Sarah Trent is an editorial intern for High Country News based in southwest Washington. Email her at [email protected] or submit a letter to the editor. See our letters to the editor policy.

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