Summer Trapping Shows MOB Is Present Across Northern Willamette Valley (Photo)
- 11/06/25
SALEM, Ore. - Trapping for Mediterranean oak borer (MOB) this summer showed this new pest from Europe, North Africa, and western Asia is now present across the northern Willamette Valley in at least six counties – Clackamas, Marion, Multnomah, Polk, Washington, and Yamhill. MOB is a concern because it has infested Oregon white oaks, sometimes with fatal consequences. The tiny beetles carry fungi that can infect a tree with a vascular wilt, which can lead to tree death.
Before now, state and federal officials did not know how extensive Mediterranean oak borer’s presence in Oregon might be. Individual borers had shown up in traps here and there in the Willamette Valley even before the first infested Oregon white oaks (Quercus garryana) were found in Troutdale and Wilsonville. So this summer they set up traps all over the Willamette Valley and beyond to try and determine where MOB had reached.
What they found was that more than 500 of the tiny borers turned up in more than 30 traps on both sides of the Willamette River from the Portland area south to Salem. Catch rates varied from a single borer at a number of traps to 160 at a trap east of Oregon City. The borers were even caught in locations where no Oregon white oak trees had been reported declining.
“The results show us that, unfortunately, Mediterranean oak borer is firmly and widely established in the northern Willamette Valley,” said Cody Holthouse, current chair of the state’s Interagency MOB Task Force.
Of 944 oak trees inspected since MOB was first detected in Oregon, a total of 38 are currently infested with MOB. Another 22 thought to be infested have been removed.
Forest Entomologist Christine Buhl leads the Oregon Department of Forestry’s efforts to track and evaluate MOB’s potential impact on trees in the state. She said that Oregon scientists are still trying to learn all they can about the borers and the fungus they carry.
“We still have a lot of questions about MOB that we’re working to find answers to,” said Buhl. One basic question is what other tree species potentially could be affected by MOB.
“So far in Oregon, only Oregon white oaks have shown signs of infestation,” said Buhl. “In California, other native oak species have been attacked and killed, including valley oak (Quercus lobata) and blue oak (Q. douglasii). In Europe, MOB has been reported in a large number of oak species, and even on some non-oak trees.”
Buhl said researchers don’t know what percentage of Oregon white oak trees might eventually become infested, let alone if they could start infesting other trees here.
“So far we haven’t seen widespread decline of Oregon white oak. In many cases, even trees near those that have been heavily infested have not shown signs of infestation or decline. This raises a question in my mind about whether MOB will prefer to infest stressed or otherwise unhealthy trees,” said Buhl.
Buhl points out seeing recent construction in the root zone of many of the trees infested with MOB. “This can lower trees’ ability to cope with a variety of pests and diseases,” said Buhl. She advocates for protecting the root zones of Oregon white oaks from disturbance, ideally up to twice the width of the tree’s canopy.
“My hope is there might not be widespread, rapid die-off of Oregon white oaks, like there is with emerald ash borer, which is killing most ash trees across the country,” said Buhl. She stressed that more research is needed to find out what level of resistance oaks might have to the fungus carried by MOB.
Oregon white oak is an important native tree growing from British Columbia into California. Numerous species of birds, mammals, insects, and reptiles rely on the long-lived tree for food and shelter. A number of Native American people have traditionally leached out bitter tannins from the tree’s acorns, then processed their kernels into an acorn flour used in various dishes.
With funding from the USDA Forest Service, ODA hosted a multi-agency collaborative program to trap Mediterranean oak borer (MOB) in Oregon. That program, which ended in October 2025, aimed to improve the understanding of MOB distribution and pest status in the state. ODA has no more funding to continue coordinated trapping for MOB. The agency’s focus is shifting to supporting awareness, outreach, and detection through visual observations from professionals and the public.
An excellent article by Kayla Seaforth of the Bonneville Environmental Foundation summarizes what’s been learned about MOB in California and Oregon. Read it here.
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