Agriculture officials in Marin County are alerting property owners and arborists to be on the lookout for a tiny invasive beetle that attacks oak trees, after the insect was detected last month in Novato.

The Mediterranean oak borer (Xyleborus monographus) is about one-eighth of an inch long and bores tunnels into stressed oaks, mostly valley oak but also other white oaks like Oregon white and blue oak, Marin agriculture officials said.

"Education about the beetle is important, but unfortunately, it can't be eradicated and it's becoming more widespread in our area," Marin Agricultural Commissioner Joe Deviney said. "It was only a matter of time for the presence of this invasive pest to be confirmed in our county."

The beetles carry and grow fungus in tunnels, then feed off the fungus. They can live and reproduce inside the tree for generations. Female beetles can fly, but the males don't.

Initially, the beetles attack the crown of a tree, killing limbs as the infestation grows, and eventually it spreads to the trunk and kills the tree.

The borers usually prey on trees that are already stressed. "Lots of oak groves and forests throughout California fit that model," the agriculture department said in a news release.

Native to Europe, the Middle East, and North Africa, the beetle was identified in a valley oak in Novato on May 23, 2025. The insect was first detected in Napa County in 2019 and has since been found in nine counties.

Research is still being done on the best control options, including whether any California trees have natural resistance.

In Europe, where the Mediterranean oak borer is native, it only attacks trees that are already dying, and it only lasts one generation per year. In California, the beetle infests both stressed and healthy trees and can have multiple generations per year, according to University of California Agriculture and Natural Resources Department's website.

The extent of the beetles' distribution in Napa, Sonoma and Lake counties hasn't been determined, according to the UC agriculture department. It's also spreading rapidly in Sacramento County, and has been detected in El Dorado, Yolo and Mendocino Counties.

Officials don't think the beetle has yet spread to other parts of the state.

"However, there is considerable potential for the beetle's range to expand as they can be moved in infested wood and the females can fly," the UC agriculture department said in a post on its website.

Agriculture officials are working with the Marin office of the University of California Cooperative Extension and Marin County is collaborating with state pest management and wildfire agencies on efforts to detect, study, and educate about the borer. 

Homeowners are urged 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, they are encouraged to seek expert evaluation from a certified arborist.

UCCE has created flyers about the insect in English and Spanish. More information is available at and .

• Marin County