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You may have heard concerns about a destructive invasive pest from Asia, the infamous Emerald Ash borer, that has decimated native Ash tree populations in the eastern United States, and is now threatening our native Oregon Ash (Fraxinus latifolia). The Oregon Ash tree is vital to the health of Riparian Habitats in Oregon, and we have seen the amount of destruction it can have in very little time in states like Michigan, where the EAB was first detected in the U.S. If you haven’t heard of this pest, read our blog post How to Cope with Emerald Ash Borer, to learn more.
Now, there’s a newly discovered pest in Oregon (and in Marion County!) that may threaten the health and population of Oregon White Oak trees. It’s important to know about the signs and symptoms of this pest, what is at stake, and what can be done to prevent the spread, so that we can protect our white oak trees, their habitats, and the hundreds of species they support.
Mediterranean Oak Borer (Xyleborus monographus) is a tiny, woodboring ambrosia beetle native to Europe, Western Asia (Turkey, Iran, Israel, and Russia), and northern Africa (Algeria and Morocco) (1). Ambrosia beetles, also known as ‘fungus farmers’, live in a nutritionally symbiotic relationship with ambrosia fungus, which they cultivate within the trees they tunnel into. Rather than feeding on the wood of trees, these beetles feed on a fugus they bring into the tree, and cultivate in the galleries, which spreads into the branches and the trunk.
There are native ambrosia beetles, which only tunnel into dead wood. Most ambrosia beetles are generalists and it is rare to find a specialist like this one, which prefer red and white oak species. Most observed mortality due to MOB has been in Valley Oak, in California, but there are a lot of unknowns for our oaks in Oregon, as this is a relatively new pest in the area (2).
Ambrosia beetles are some of the first beetles to emerge in the spring. They overwinter at all life stages, and are only inactive for two months in California (November-January) they are able to fly long distances all other times of the year (2).
Female MOB beetles bore into bark cracks starting on the upper branches, broken branches, or freshly cut firewood of oak trees. They’ve been found to infest weak or vigorous trees in California, while attacking mainly dead and dying trees in Europe.
These beetles carry a fungi within their mouth parts, and inoculate the newly made tunnels with the fungi. They then lay their eggs, which hatch and eat the fungus growing in the galleries. Meanwhile, the fungi left here, such as Raffaelea montetyi, can cause wilt disease in susceptible trees (1). The fungus spreads inside from the sapwood and grows into the water conducting cells of the xylem, this depletes the tree of water sources. You will see a saw dust at the base of a tree that has been infected by a borer, indicating the beetle spends most of its time in the sap wood. The beetles continuously re-infest trees over the years, adding generations of fungi carrying beetles that tunnel deeper into the tree trunk, until the tree has been depleted of water and dies (3).
The first MOB in Oregon was detected in Multnomah county in 2018 (2). Since then, there has been detections each year in different counties, including Washington, Clackamas, and Marion counties in 2021 and 2022. As of September 2023, 6 white oaks have been found to be infested with MOB at a Homeowner’s Association in Wilsonville. Urban forestry and industrial forest land are thought to be more vulnerable to MOB infestations (2).
It is speculated that MOB has been brought to the West Coast in multiple different introduction sites, rather than one location from which it spreads. One possibility for bringing over MOB to the states is thought to be in oak wine barrels imported from Europe (2).
1) MOB is a tiny reddish-brown beetle that can be confused with many other beetles that reside in
oak. It’s very rare to see an adult.
2) Pale boring dust is kicked out by the beetle and found on the exterior trunk of the tree.
3) MOB galleries look like tiny black holes from the exterior of the wood
4) Small diameter branches (<10″ diameter) die first, branching black stained trellis galleries; ‘flagging’ is first sign.
5) MOB create tiny, perfectly round entrance holes
MOB attacks uppermost small limbs first, then proceed down branch. Eventually this girdles the tree and causes full wilt. This can take 3-5 years. If you can catch the signs very early, you can trim the infected limbs and hope to save the tree (5).
Preventative steps and Best Management Practices (BMPs)
To report a sighting, call 1-866-INVADER (1-866-468-2337) or file an online report at Oregon Invasive Hotline (link above).
This new invasive pest is raising concern in the Willamette Valley, because this is the historical home to our beloved Oregon white oak trees. This is the native keystone tree species of our home, and this ecoregion’s inhabitants have depended upon and lived in harmony with these trees for millennia.
Although these big beautiful and resilient trees have provided home, shelter, food, and cultural importance to the valley for thousands of years, here are a few examples of what is to lose if the Mediterranean Oak Borer is able to spread throughout these native habitats:
MSWCD related topics: