Emerald ash borer is in Oregon. First detected in the state in 2022, the invasive beetle has now been confirmed across five counties in the Willamette Valley: Washington, Yamhill, Marion, Clackamas, and Multnomah. The Oregon Department of Agriculture has established a permanent quarantine across all five counties to slow its spread, but the pest poses a serious long-term threat to Oregon’s native ash trees and the riparian ecosystems they anchor.
Where It Has Been Found
The initial discovery came in 2022, and since then, detections have spread across the northern Willamette Valley. The five quarantined counties form a broad corridor that includes the Portland metro area and surrounding communities. Given that emerald ash borer (EAB) typically goes undetected for years before populations grow large enough to notice, the beetle was likely present well before 2022.
All ash species are at risk, but the biggest concern in Oregon is the native Oregon ash. This is the dominant hardwood along rivers and streams throughout western Oregon, and it plays a critical role in shading waterways, stabilizing banks, and supporting wildlife. As EAB kills these trees, that riparian habitat will fundamentally change.
How It Damages Ash Trees
The destruction comes from the larvae, not the adult beetles. After eggs are laid on bark, larvae bore inward and feed on the tissue that moves water and nutrients through the tree. They carve winding, S-shaped tunnels beneath the bark, gradually severing the tree’s internal supply lines. The first visible sign is usually thinning in the upper canopy, where branches begin to die back. Heavy infestations can kill a vulnerable tree in as little as two years.
The most recognizable evidence of EAB is the D-shaped exit hole left behind when adult beetles emerge in June and July. These holes are tiny, roughly 1/8 inch wide, and distinctly flat on one side. If you peel back splitting bark on a declining ash tree, you may find the S-shaped larval tunnels packed with a fine sawdust-like material called frass. Vertical bark splits are another telltale sign, caused by the tree attempting to grow callus tissue around the galleries.
Oregon’s Quarantine Rules
The ODA’s permanent quarantine restricts the movement of ash tree materials out of Washington, Yamhill, Marion, Clackamas, and Multnomah counties. This covers a wide range of materials: logs, green lumber, nursery stock, chips, mulch, stumps, roots, branches, and firewood of any hardwood species. The quarantine also applies to white fringe and olive trees, which are in the same plant family and can harbor the beetle.
If you live in one of these counties, do not move ash wood or hardwood firewood outside the quarantine zone. This is the single most important thing residents can do, because EAB spreads long distances primarily through human transport of infested wood, not by flying. Exceptions for nursery stock or commercial wood products require a formal compliance agreement with ODA.
Treating Ash Trees on Your Property
If you have ash trees you want to protect, treatment is an option, but it works best as prevention or early intervention. The key threshold to know: trees that have already lost more than 50% of their canopy should not be treated. At that point, the tree can no longer pull insecticide up through its trunk and roots effectively, and removal is the better path.
For trees that are still healthy or only lightly affected, professional trunk injections offer the strongest protection. Products containing emamectin benzoate are particularly effective because a single treatment can protect a tree for two years, and university research suggests some formulations may last three or more years at the highest labeled rate. Other systemic insecticides need to be reapplied annually. Soil-applied treatments work well for smaller trees (under six inches in trunk diameter), though larger trees may need two consecutive years of treatment before they’re fully protected.
Spring applications have proven more effective than fall treatments in university trials. One thing to be aware of: trunk injections require drilling into the tree, which creates small wounds that can potentially allow disease-causing fungi to enter. For most healthy, mature ash trees the tradeoff is worthwhile, but it’s a consideration for smaller or already stressed trees. A certified arborist can assess whether your specific tree is a good candidate.
Biological Control Efforts
Oregon is also deploying a longer-term strategy: tiny parasitic wasps imported from Asia, where EAB evolved alongside natural predators that keep its populations in check. The USDA raises these wasps at a facility in Michigan and ships them to Oregon for release. Three species are currently being used, each attacking a different life stage of the beetle.
One species is an egg parasite. The female wasp lays her egg inside a freshly laid EAB egg, and the developing wasp consumes it from the inside. This species reproduces without mating, so every individual released can immediately begin destroying EAB eggs. The other two species target larvae already tunneling beneath bark. They use a drill-like organ to deposit eggs inside the larva, and the hatching wasps kill it before it can mature into an adult beetle. A single parasitized larva can produce anywhere from 6 to 45 new wasps depending on the species.
Biocontrol won’t eradicate EAB. The goal is to reduce beetle populations enough that some ash trees survive and the species isn’t wiped out entirely from Oregon’s forests. This approach has shown promise in midwestern and eastern states where EAB arrived years earlier, but it takes time to establish self-sustaining wasp populations.
What This Means for Oregon’s Landscape
EAB has killed hundreds of millions of ash trees across North America since it was first detected in Michigan in 2002. It is considered the most damaging forest pest in North American history. Oregon’s situation is still relatively early, which is both a reason for urgency and a reason for cautious optimism: the quarantine, biocontrol releases, and treatment of high-value trees are all more effective when started before populations explode.
The most significant ecological impact will be along western Oregon’s waterways, where Oregon ash is often the dominant tree. Losing that canopy means warmer stream temperatures, less shade for salmon and other aquatic species, and reduced bank stability. For homeowners and urban foresters, it means rethinking which trees to plant. Diversifying away from ash in new plantings is one of the most practical steps communities can take right now to build resilience against the losses ahead.