Memorandum
Date: October 21, 2025
To: The Honorable Eddie Crandell, Chair, Lake County Board of Supervisors
From: Eddie Crandell, Chair, District 3 Supervisor
Jessica Pyska, District 5 Supervisor
Subject: 11:30 A.M. - Consideration of Presentation Giving an Update on the Invasive Mediterranean Oak Borer in Lake County
Executive Summary:
According to the University of California Agriculture and Natural Resources (UCANR) the Mediterranean Oak Borer (MOB), Xyleborus monographus, an invasive ambrosia beetle from the Mediterranean region was first confirmed in Napa County around 2017, has since spread throughout Lake County, as well as Amador, El Dorado, Marin, Mendocino, Napa, Sacramento, Sonoma, and Yolo Counties and more.
In California, UCANR has found that the MOB can attack at least 12 species of oaks, but primarily infests two species of white oak, the two most dominant oak species in Lake County, valley oak and blue oak. Recurring and ongoing drought can continue to stress trees, particularly older trees, years after a drought ends, making heritage oaks particularly susceptible to MOB.
As of 2024, aerial surveys conducted by the United States Forest Service have determined that there were over 69,000 acres of dead conifers in Lake County from drought and native bark beetle infestation. Wide-spread conifer mortality continues. According to the University of California Cooperative Extension Forest Advisor for Lake County, Dr. Michael Jones, who also is an entomologist, there are over 100,000 acres of oak and mixed oak woodlands in Lake County which are all now imperiled by MOB.
Before your Board today, Dr. Curtis Ewing, Senior Environmental Scientist, Forest Entomology and Pathology Program with the California Department of Forestry and Fire Protection (CAL FIRE), Dr. Michael Jones, Forest Advisor for Mendocino, Lake, and Sonoma Counties with UCANR, and Tracy Cline, Stewardship Director, Clear Lake Environmental Research Center (CLERC) who will give an update on the status of MOB in Lake County and California, and remain available to answer any questions your Board may have.
Thank you for your consideration of this important update.
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Recommended Action: Informational presentation. Consider Direction to Staff, as appropriate.