The strength of Marin County’s two dominant native oak species, which dot golden hills and shade streets and parks, has been tested mightily over the past decade. Coast live oaks, which thrive under the damp marine layer, have been decimated by sudden oak death. Now, valley oaks, native to West Marin’s dry valleys and popular in urban spaces, face a new threat from a pest barely 3 millimeters long.

The Mediterranean oak borer—referred to as MOB—is a species of Ambrosia beetle native to Europe, North Africa and the Middle East. It was discovered last month in Novato. Ambrosia beetles are natural decomposers, boring tunnels into wood to lay their eggs. MOBs carry a unique species of fungus, infecting the tree and eating the resulting growth. The fungal infection can cause the premature failure of limbs and, eventually, of entire trees, as the beetles work their way down the trunk.

A large percentage of West Marin’s oaks may be at risk. So far, MOBs have attacked valley oak and blue oak, a less populous species. They typically target stressed, aging or otherwise compromised trees, making Marin’s oaks—which faced extended drought conditions in recent years—particularly vulnerable.

“It was inevitable that [MOBs] would get here,” said Tom Gaman, a forester who lives in Inverness. Because of drought and other stressors, he said, “We’ve lost most of our native valley oaks already.”

Oak borers are not uncommon insects in California, but MOBs are invasive and new to the region. They were first discovered in 2019 in Napa County and quickly spread through the hot and dry climate.

“It’s such a novel insect, we’ve only known about it being in North America for six years,” said Michael Jones, a forest advisor for the U.C. Cooperative Extension and one of the leading researchers of the insect in the North Bay. “We’re still trying to understand its life cycle and behavior, as well as its aggressiveness.”

The Mediterranean oak borer’s sudden emergence has left arborists with a short runway to develop treatment measures. The insect’s small size makes it hard to detect and exterminate, and while ecologists across the region are hard at work developing tools to combat it, “There’s not a ton of science to support potential treatment options,” Mr. Jones said.

Currently, the only form of prevention is to destroy infected material, either through chipping material into small pieces or burning or burying it. Officials emphasize that the best way to prevent the spread of MOBs in Marin is to catch infestations early.

David Lewis, director of the U.C. Cooperative Extension of Marin, believes the levels of infestation found in Novato show MOBs were present in Marin for some time before their discovery. He urges residents to “look up high into the canopy of a tree. If there is one branch that is not leafing out, or has lost leaves compared to other branches, seek out an arborist.”

Other symptoms of infestation include damage or tunneling in sapwood, leaking sap, and boring dust on tree bark. Marin’s native oaks are inextricable from its biodiversity, delicate ecosystems and urban greenery. The best way to prevent a MOB infestation is to keep Marin’s native oaks healthy.

“A tree in a good, healthy site with access to sunlight is going to be more resistant to any kind of bark beetle,” Mr. Gaman said.

Find a fact sheet about the pest at https://ucanr.edu/sites/default/files/2024-09/330538.pdf. If you suspect your tree may have a MOB infestation, contact an arborist.