An invasive pest that kills oak trees continues to spread through the Willamette Valley between Portland and Salem.
Oregon officials completed a two-year study this summer that found the Mediterranean oak borer beetle across the valley in Clackamas, Marion, Multnomah, Polk, Washington, and Yamhill counties. Officials announced the findings this month.
“We’re not seeing a huge escalation in how many we are finding on the landscape, but it will continue to spread into other oak areas, and we want to know where those areas are,” said Christine Buhl, an entomologist with the Oregon Department of Forestry.
Mediterranean oak borers are reddish-brown beetles native to Europe, western Asia and northern Africa. They are what entomologists call ambrosia beetles, meaning they bore into trees and introduce fungi that ultimately kill trees.
The Mediterranean oak borer leaves small, round holes in the cut face of wood.
Photo courtesy Bob Rabaglia, U.S. Forest Service
These oak borers made their first North American appearance in California in 2017, then showed up in Multnomah County the following year. That’s bad news for the Willamette Valley, an ecoregion that relies heavily on native Oregon white oaks.
Oak savannas and woodlands once dominated the valley, supporting hundreds of plants and animals and providing a natural defense against wildfires. But these habitats have been devastated by urbanization, agriculture, invasive species and climate change.
Oak borers could shrink what little remains of the valley’s oak habitats. They could also kill oak species that shade residential areas.
“Oregon white oak is an extremely drought-tolerant tree,” Buhl said. “As climate change progresses, that’s one of our few trees that we can continue planting that can really endure these drought-stress times.”
Infested oak trees will experience widespread canopy die-back, which could look like large clumps of dead leaves or bare branches. People might find pale dust around the tree’s trunk, a sign that the beetle bored into the tree.
This isn’t the only pest endangering the valley’s native trees. Officials have found the emerald ash borer in five counties since 2022, and they expect it to kill 99% of native Oregon ash trees in time.
Ash trees line many waterways in the valley. Losing them to the emerald ash borer would likely increase water temperatures, in turn killing fish and degrading the region’s drinking water.
Landowners can inoculate their ash trees against emerald ash borers. But there isn’t a proven treatment for eradicating or preventing Mediterranean oak borers from infesting trees.
Buhl said some researchers are testing an insecticide on large-diameter oak trees, but the results so far haven’t been promising. Some arborists are recommending the treatment to landowners anyway.
“There’s some caution there,” Buhl said. “It can kill many other insects that are not harmful. So be aware of that. Also be aware that it may not work at all. It may be a waste of money.”
Still, Oregon forestry officials are interested in hearing from landowners who test the insecticide. More information can be found on the Oregon Department of Forestry’s website.
Oaks that are already stressed — like from drought or diseases — are most susceptible to the oak borer. At least, that’s what Oregon officials can tell so far.
“We are primarily seeing Mediterranean oak borer attacking some already stressed trees, but this is just observationally what we’ve seen thus far,” Buhl said. “We have a lot more to learn.”
Funding for this most recent round of traps has run dry, so Buhl and her colleagues are trying to figure out what’s next for assessing oak borers in the valley.
Mounting evidence suggests climate change will likely worsen infestations, because drought and wildfires can weaken trees, leaving them more vulnerable to infestations.
Tree-killing pests like the oak borer and the emerald ash borer often spread to new areas when people move firewood. Local officials warn people not to move firewood between counties.
