The Mediterranean oak borer (Xyleborus monographus), an invasive insect that primarily feeds on white oaks, was recently detected in Novato, prompting the Marin County Agricultural Commissioner and UCCE Marin to notify professional arborists and the public to keep an eye out for evidence of this invasive tree pest.
Joe Deviney, Marin’s Agricultural Commissioner, said the beetle – native to Europe, the Middle East, and North Africa – was identified in a valley oak on May 23, 2025. The insect was first detected in Napa County in 2019 and has since been found in nine counties.
“Education about the beetle is important, but unfortunately it can’t be eradicated and it’s becoming more widespread in our area,” Deviney said. “It was only a matter of time for the presence of this invasive pest to be confirmed in our county.”
Deviney’s staff is working closely with the Marin office of the University of California Cooperative Extension (UCCE), which has programs about agriculture and natural resources, forest health, horticulture, human-wildlife interactions, and others. Those County of Marin departments are collaborating with State of California pest management and wildfire agencies on efforts to detect, study, and educate about the borer.
The ambrosia beetle species is tiny – approximately 3 millimeters, or one-eighth of an inch – and bores galleries (tunnels) into stressed trees, specifically oaks. The beetle primarily feeds on valley oak but also feeds in other white oaks, including Oregon white and blue oak.
The beetles initially attack branches in the crown of a tree, killing limbs as the infestation grows. Eventually, the infestation spreads to the trunk and ultimately kills the tree.
The beetles carry and grow fungus in the galleries they bore; then they feed off the fungus, so they can live and reproduce within a tree for generations. Typically, the female beetles fly, but the males don’t.
Although the fungus may cause disease in living trees, the borers usually prey on trees that are already stressed and declining as a result of preexisting conditions. Lots of oak groves and forests throughout California fit that model.
Homeowners are encouraged to monitor tree crowns, and, if they see a decline in leaf growth in one or more branches or observe other symptoms of an infestation (learn more at mobpc.org), they are encouraged to seek expert evaluation from a certified arborist.
UCCE has created flyers about the insect in English and Spanish. Learn more about the Mediterranean oak borer on a recent UCCE webinar.