The Mediterranean oak borer (Xyleborus monographus), an invasive insect, was detected in Novato, Calif., on May 23, 2025. The ambrosia beetle species is approximately one-eighth of an inch and bores 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. (Curtis Ewing/County of Marin via Bay City News)

ALTHOUGH HALF THE NATION would probably identify redwoods as the iconic California tree, many Californians might instead think of native deciduous and live oaks. The image of singular twisted oaks on golden rolling hills is as emblematic as they come. But oak trees are increasingly under threat by a quartet of bad actors, with contributing stressors of drought and climate change.

The newest culprit is the Mediterranean oak borer, or MOB, recently confirmed in Hopland and Potter Valley. The beetle has been around since at least 2019, said University of California Cooperative Extension forest advisor Michael Jones, speaking from his Ukiah office.

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