Ash and Oak Trees at Risk
Destructive Insects
Emerald ash borer (EAB) is considered the most destructive forest pest in North America and can kill all species of ash trees. In the greater Portland area and throughout the Willamette Valley, native Oregon ash (Fraxinus latifolia) is of particular concern. This tree is critically important for forested wetlands and riparian areasRiparian areas The land alongside a stream, creek, river, or floodplain. For some ash-dominated wetlands, there is no native canopy-forming tree that grows in the same conditions. As of summer 2025, EAB has been found as close as six miles from Southwest Portland, leading us to step up our outreach and education to folks who might have ash trees on their properties.
The Mediterranean Oak Borer (MOB) is a tiny beetle that spreads fungus as it drills into oak trees, which eventually kills the trees. MOB infestations have already been found on over a dozen oak trees in the North Willamette Valley, and particularly concerning is their targeting of Oregon White Oak trees (Quercus garryana). Oregon white oaks are important to the human culture and ecology of this area, and are resilient to increasing high heat and droughtDrought A longer than normal time with not enough rain conditions. The Oregon Department of Forestry has been documenting the nature of MOB in Oregon and California, where is has been causing problems for the last decade. In Oregon, it hasn’t become an unstoppable infestation yet. Visit the MOB Infestation Dashboard to see the status on MOB in Oregon.
What You Can Do
Though these two beetles target different types of trees, what you can do to slow the spread and keep your trees safe is the same.
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Don't move firewood
If you use firewood, buy and use firewood locally (within 10 miles of harvest location) and transport only firewood that has been heat-treated and certified as pest-free. Transport of firewood and nursery stock are the major pathways for the spread of this destructive beetle.
Read more at DontMoveFirewood.org -
Learn to identify at-risk trees
Learn to recognize ash trees and Oregon white oak trees and map any ash trees you find on iNaturalist. In 2025, we hosted oak and ash workshops, to help people recognize particularly the native Oregon white oak and Oregon ash trees, to help track their health and recognize when they’re sick. Both insects themselves are extremely small and may look like other bugs, so we encourage people to learn to identify the trees and signs of sickness instead of the insects themselves. If you’re better learning in-person than reading and looking at photos (like lower on this page), we encourage you to register.
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If you spot the signs, report!
Look for the signs of poor tree health and symptoms of infestation outlined below. Report any signs of infestation to: oregoninvasiveshotline.org or call 1-866-INVADER. Remember, you’ll be reporting the individual infested plant, so multiple pictures and a specific location are crucial in each report.
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Take care of your trees.
- Protect the area immediately under, and up to 2 times the diameter of, the canopy, where the majority of the tree’s roots are.
- Avoid paving, parking vehicles or heavy equipment, construction, competing turf or weeds, or soil disturbance in this area.
- Don’t overwater your oak tree(s), except during periods of extreme heat and drought – perhaps two deep soaks during that period of summer.
- You can apply organic mulch to this area to conserve soil moisture underground but keep the trunk free to breathe.
- Consider hiring a reputable arborist savvy about oak and ash and their invasive pests if you have dead and dying tree limbs; they may recommend proactively and thoughtfully pruning those limbs before they break and leave an open wound that might invite EAB or MOB.
- Read more and find great resources on our page about oak habitat.
What to Look for in Ash Trees (Signs of EAB)
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Dead or dying ash trees or branches
Dead or dying ash trees or branches, particularly high up in the canopy, where the adult beetles typically start their feeding and laying of eggs. Tree die-back can be subtle at first, so it’s important to look closely
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Epicormic branching
Epicormic or sucker branching is when leaves or branches grow out of the trunk or straight up out of the base of larger branches. This is the tree’s last gasp at life. .
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Splits along the bark and S-shaped tunnels under the bark
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Patches of lighter wood
Patches of lighter wood are places where woodpeckers have pecked to get to insects. Use binoculars to look high in the canopy. Also look lower down on the trunk.
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Very small D-shaped holes in the bark of the tree
Very small D-shaped holes in the bark of the tree are a telltale sign of EAB. Adult beetle exit holes are 3 mm in diameter — about the size of a pencil tip. These are hard to see!
Read More -
See these signs? Make a report!
If you see a dying ash tree, S-shaped galleries, or D-shaped exit holes, or two of the other signs, take pictures and make a report of suspected EAB infestation. Give as much detail as possible so experts can follow up.
Make a report to Oregon Invasives Hotline
What to Look for in Oak Trees (Signs of MOB)
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Dead or dying branches
Dead or dying branches, particularly high up in the canopy.
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Dusty trunk
When the MOB drills into the tree, it leaves dust or “frass” (made of its waste plus sawdust of the tree) on the tree trunk and/or at the base of the tree.
The part of this picture that looks like sawdust (at the bottom of the tree and in its indentations) is actually frass, which is the part of the tree that the MOB chews through, mixed with its waste. Seeing it on the trunk of a tree is a sign of Mediterranean Oak Borer infestation.
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See both? Make a report!
If you see both of these signs on an oak tree, take pictures and make a report of suspected MOB infestation. Give as much detail as possible so experts can follow up.
Male a report to Oregon Invasives Hotline!
We provide guidance and resources for native tree care on your property.
Kammy Kern-Korot
Senior Conservationist
Contact me about:
Oregon oak, savanna, wetlands and riparianRiparian areas The land alongside a stream, creek, river, or floodplain habitats; Emerald Ash and Mediterranean Oak Borers; conservation planning and native plantings for pollinators and other wildlife on rural lands.