Mediterranean Oak Borer

Keeping our oaks healthy and all eyes out for the Mediterranean oak borer!

By Kristin Currin

A new threat to Oregon white oaks has arrived in Oregon. First detected in the state in 2018, new sightings in the last year at the Sandy River Delta and recently in heritage trees near Portland confirm that the Mediterranean oak borer (MOB) has the potential to spread. Many eyes are needed on this and together we can look out for our local trees by knowing the signs to watch for and how to report sick trees.

While the first detection of MOB in the United States occurred in California in 2017, this small beetle was not brought to Oregon from California. This beetle can be transported to new areas via infested firewood, packing materials, pallets, live plants, and other wood products, and although we are not yet completely sure, it is believed that MOB came to Oregon in untreated, imported oak staves used to make wine barrels. Local wine makers can help then by using only properly treated oak barrels (heat treated), as well as locally and sustainably produced barrels. And all of us can help deter the spread of MOB and other insects and diseases by adopting and supporting the "Don't Move Firewood" ethics and campaign.

The Mediterranean oak borer is small with a length of about 1/10th of an inch. It is a species of ambrosia beetle that farms fungus, which it carries in specialized pits near its mouth. As it tunnels into the outer wood (or sapwood) of an oak tree, it lays eggs and releases the spores of the fungus, which grows in the tunnels and later feeds new beetle larvae. The tunnels that any tree-boring beetles create are called galleries. Different species of beetles can have distinctive and unique galleries, and this can be a way to identify the species of beetle that created them. MOB galleries occur in the outer wood and smaller branches of trees and are darkly stained from the fungus that grows within them. Seeing darkly stained galleries/tunnels in the outer wood of an oak tree is one way to determine if MOB is present in that tree.

Photo of MOB tunnels/gallery in the outer wood of an Oregon white oak tree at Sandy River Delta. Photo provided by Brance Morefield (USFS)

An example of dark tunnelling in Oregon white oak created by a beetle, likely native, that is not of concern. Photo provided by Brance Morefield (USFS)

It is important to keep in mind that other native beetles that are less harmful and not of concern can also create dark tunnels in the wood of oak trees. Common native ambrosia beetles may even have similar looking galleries (with less branching), but do not attack live trees, as the MOB does, and instead prefer dead or dying trees.

Notice the fine, powdery, whitish wood dust (or frass) that has accumulated over the moss and bark on the side of this oak tree. Photo provided by Brance Morefield (USFS)

As the MOB beetle tunnels into an oak tree, it creates whitish boring dust from the wood that is pushed out and accumulates on the side of the tree, between the cracks of the bark, or around its base. Seeing this kind of whitish dust, also called frass, on the side or base of an oak tree is another way to tell if MOB is present in a particular tree.

An oak tree killed by the Mediterranean oak borer at Sandy River Delta. Photo provided by Brance Morefield (USFS)

An attack of MOB on a tree typically starts in the uppermost limbs and moves down the tree. This results in branch dieback and wilted or dead leaves. Eventually, within 3 to 5 years, the tree is entirely girdled and dies.

Oregon white oaks are one of the most important trees in the Pacific Northwest and Oregon white oak habitats are some of the region's most threatened habitats. These are trees that support hundreds of species of wildlife, yet 97% of Oregon white oaks and white oak habitats have been lost in some areas due to human development. It is important that we do what we can to protect what remains of these trees and the critical habitat they provide.

As many hands make light work, so can many eyes cast a wider net to catch the early signs of Mediterranean oak borer and stop the spread of this damaging insect. If you see:

1. White boring dust accumulating on the sides of trees

2. Darkly stained tunnels/galleries in the outer wood or small branches of an oak tree when cut

3. Branch dieback and red or dead leaves in conjunction with the two above signs

please report it to the Oregon Invasive Species Council at or by calling 1-866-INVADER. It is important to report any suspect trees and have an expert help to properly identify whether MOB is in fact present.

For more information about the Mediterranean oak borer and how to detect, prevent, and report it, download these Oregon Dept. of Forestry and Oregon Dept. of Agriculture information sheets.