11/13/2025
There is a new threat to our Oaks in Sonoma County. Please read and study up on how to minimize their spread or how to remove them when found. The article is from Sonoma Ecology Center.
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Serious New Threat to Sonoma’s Oak Trees
Nov 6, 2025
Photo by Alma Shaw
Sonoma’s native oak trees, a defining feature of our landscape and cultural heritage, are in critical danger due to the rapid spread of the Mediterranean oak borer, a small invasive beetle. Please read the following update from Sonoma Ecology Center’s Research and Restoration Teams, about this serious threat to some of our most treasured trees and the important measures you can take to help prevent further spread.
The Mediterranean oak borer (Xyleborus monographus, MOB) is a very small invasive beetle which originated in Europe and North Africa, in regions surrounding the Mediterranean Sea. In their natural habitat, the beetles serve an important ecological role by helping break down wood in trees that are already dying, aiding in the tree’s decomposition and the eventual return of its nutrients to the soil. In the United States, however, the beetle is a rapidly spreading invasive species, aggressively infesting both healthy and stressed trees alike.
Female beetles tunnel into the trunks and branches of trees to lay their eggs and in the process introduce a fungus which spreads through the tissues of the tree, blocking water and nutrient flow. Further damage from the beetles’ tunneling weakens and eventually kills the tree over time.
Infestations of oak trees in Sonoma County were first confirmed in early 2020, and as of 2025, MOB has already killed hundreds of thousands of oaks in Sonoma County. Local infestations are found primarily in valley oak and some blue oak, but Oregon white oaks are also susceptible.
What you can do:
MOB spreads by natural dispersal of the female beetles and through the movement of infested firewood, untreated wood products, and yard waste or mulch. The beetles are extremely difficult to detect because they are approximately 1/10th of an inch, about the size of a sesame seed.
Firewood in particular is one of the easiest ways invasive species like MOB spread. When it comes to procuring firewood, always try to follow the guideline “buy it where you burn it.” Ask your firewood seller where the wood is from and if it isn’t local, or the origin is unknown, consider obtaining your firewood somewhere else. Similarly, avoid bringing outside oak mulch or untreated oak products onto your property–ask what kind of wood your mulch is and where it was sourced.
Best practices for disposal of infested wood include burning, burying, and chipping. Reducing wood to chips smaller than one inch and composting wood at temperatures above 160°F are both effective ways to kill the beetle. If chipping and burning are not possible, cover the infested wood with a tarp for at least six weeks in winter or six months in summer to prevent the beetles from emerging.
One of the best ways to protect your trees is to ensure the overall health of your valley, blue, and Oregon white oaks this fall before the leaves drop, and be alert for changes next season. Deep watering during extended dry periods and mulching around trunks can help reduce stress on the trees. If possible, have a qualified arborist inspect your trees regularly, especially in late spring and early summer when the beetle’s activity is at its peak. Look out for defoliated and browning crowns, small gallery holes, and beetle frass (sawdust-like waste pushed out of galleries). Another sign of infestation are dark, wet looking areas on the bark of the tree where sap and moisture may be seeping.
Sonoma County’s oak trees and the ecosystems they support are not only beloved for their beauty, but also play a key role in maintaining biodiversity and storing carbon. Collaboration across our community is critical to prevent further infestation.
If you see signs of MOB, take pictures and contact an arborist or Sonoma Ecology Center’s Restoration Team at [email protected].
Additionally, you may contact the Sonoma County Agricultural Commissioner’s office at 707-565-2371 or the California Department of Food and Agriculture at their Pest Hotline at 1-800-491-1899.
Please report any suspected infestations to the University of California Agriculture and Natural Resources through their Mediterranean Oak Borer/Pathogen Complex Disease Reporting Survey: https://ucanr.co1.qualtrics.com/jfe/form/SV_9zwapzQGP0m1HBH
For more information, including images of symptoms to watch for, visit the University of California’s Agriculture and Natural Resource site on the Mediterranean oak borer: https://ucanr.edu/site/mediterranean-oak-borer.
The Mediterranean Oak Borer (Xyleborus monographus) is an ambrosia beetle that was found infesting several valley oak trees in Calistoga, (Napa County) California in 2019. The extent of its distribution within Napa County and neighboring Sonoma and Lake Counties is currently undetermined.
