MENDOCINO CO., 8/20/25 — 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.
“It’s new in the sense that we’re now trying to understand it,” he explained. He suspects that the insect had been around for possibly a decade before arborists began taking note of it and its population started to increase.
“I wouldn’t put it at the top of the list of tree pests, but urban forests are where it will have the greatest impact,” Jones said. The MOB likes the white wood of oaks, so its favorite targets are large older valley oaks. Those oaks are often the beloved trees of downtown squares, urban parks, and leafy neighborhoods.
“When those oaks die,” he said, “the loss will be significant. It won’t happen overnight,” though he noted that heavily infested trees die quickly.
Jones said that he hoped that management practices can be developed to save the oaks or at least allow a new cohort of younger, more resistant trees to take root. The MOB is also attacking Oregon white oak and blue oak. They are not attracted to live oaks.
But other pests are. The quartet of MOB, goldspotted oak borer, invasive shothole borers, and sudden oak death yields a killer for every kind of native oak. The goldspotted oak borer is native to Arizona, and is now primarily around Los Angeles but spreading north. Likewise, the invasive shothole borer is found in the Santa Cruz mountains and in Santa Clara County but again is heading north. And MOB is in Mendocino, Lake, Sonoma, Napa, Marin, and other Northern California counties as well as Oregon.
“Pretty much every significant native oak we have has a serious pest,” concluded Jones.
The beetles attack trees that are stressed. Jones pointed out that while drought is a known stressor, the conditions of this past winter likely exacerbated MOB’s impact.
“Drought is not great,” he said, “but this year we had average precipitation across California. But that’s not a good metric. When rain comes in big atmospheric rivers, there’s a lot of runoff.”
Which means that water does not percolate into the soil, where oaks need it.
“We had that January dry spell and then a really early season heat wave in May. Trees were confused, and now they’re really struggling. That’s why we’re noticing so much mortality,” Jones said.
Jones said that scientists believe the Mediterranean oak borer came in wine barrel shipments from Europe. “Likely not in the barrels themselves but in packing materials,” he said. “We’re noticing it first in wine-growing areas.”
The hope is that in urban areas, with fewer deer, young valley oaks have a chance to grow past the browsing stage, and that they manage to survive climate- and human-caused stressors. And that our other native oaks are able to adapt to changing conditions and pests from other regions.
“The question is how do we establish a future for our native oaks?” Jones said.
These are in Ukiah as well, one flew in our house, thought it was one of the infamous red colored roaches that are in Ukiah now. Nope it was this beetle. I’ve swatted 2 off my wall so far, I’ll keep the next one for scientific research.
Put a photo on iNaturalist so others can see the range map of where this is being found.