Mediterranean Oak Borer Monitoring Project
Photos taken by CLERC Fire and Forestry Program Manager Tracy Cline.
About
The Mediterranean Oak Borer (MOB), Xyleborus monographus, is an invasive ambrosia beetle from the Mediterranean region. It was first detected in Napa County around 2017-2018 and has since spread to Lake, Sonoma, Sacramento, and several counties in Oregon. MOB primarily targets oak trees (Quercus spp.), including confirmed species like valley oaks and blue oaks, but it may also impact many other oak species that have not yet been identified. Early reporting and detection are crucial to protecting our heritage oaks.
MOB tunnels into tree trunks, disrupting water transport and creating pathways to farm the fungus Raffaelea montetyi, which it feeds on. This pathogen, to which oaks have no natural resistance, weakens the trees, making them more susceptible to further damage. Repeated attacks by MOB over several years can cause the tree to wilt and die within 3-5 years.
From afar: If the tree canopy is thinning and browning, starting at the ends of a limb traveling towards the trunk, there might be signs of MOB. You may see a section of the tree brown and dying while the rest is green and healthy.
What to Look For
To identify signs of the Mediterranean Oak Borer (MOB) infestation, start by examining the tree canopy. Look for sections where the leaves are browning and wilting, starting from the ends of limbs and moving toward the trunk, while other parts of the canopy remain green and healthy. This patchy canopy appearance is a key indicator.
Up close, check the trunk for small holes and fine, white or light brown dust, which is a sign of beetles boring into the wood. If you peel back the bark, you may see black lines in a trellis pattern, another sign of MOB activity. The beetles often start by attacking small branches and the uppermost limbs, so watch for weakened or broken branches in the canopy.
Though MOB is difficult to spot due to its small size, early detection is crucial. Thanks to the diligent work of scientists, there are resources available to help slow its spread. The first step is learning to identify the signs of infestation and report any findings to experts.
How do I know if my oak is Infected
Did the canopy recently show brown, shriveled, or wilted leaves in some sections, while others are healthy?
Is there fine, white sawdust at the base of the tree or in bark crevasses?
After taking a sample, do you see black staining and the characteristic branching gallery pattern in the wood?
What You can do
The most important thing you can do to stop the spread of MOB is to report any odd behavior from your oaks!
Protect your Trees
While the information about the Mediterranean Oak Borer (MOB) may seem alarming, there are effective steps you can take to protect your trees, especially since the beetles often target trees that are already stressed. Here are some practical measures:
Provide additional watering during summer to help trees stressed by drought.
Thin tree stands to reduce competition, allowing each tree better access to resources.
Prune trees carefully, cutting back to the node to prevent the beetle from spreading to the trunk.
Properly dispose of infested materials by chipping, burning, or masticating them on-site. Avoid moving these materials off your property to prevent spreading the infestation.
To further contain the spread, either burn infested materials or cover them with a tarp for six weeks in the winter or six months in the summer.