Mediterranean Oak Borer — New Invasive Insect

Gold Ridge RCD
4 min readOct 30, 2023
Photo Credit: Curtis Ewing, Cal Fire
Photo Credit: Curtis Ewing, Cal Fire

The Mediterranean oak borer (Xyleborus monographus), or MOB, is a type of invasive ambrosia beetle that was first identified in Northern California in 2019. It was collected from affected oak trees near Calistoga (Napa County), although specimens collected a year before were also found to belong to the same species (Campbell, 2022). The Mediterranean oak borer (MOB) is native to the Mediterranean regions of Europe, the Middle East, and North Africa. In California, we have seen MOB detected in valley oak (Q. lobata), blue oak (Q. douglasii) and Oregon white Oak (Q. garryanna) (Jones, 2020). There’s been evidence of the beetle likely for five to ten years in California. Tree Death from MOB has been detected in Lake, Sonoma, and Sacramento Counties. There is also evidence that the MOB has been introduced to Oregon. Unfortunately, since 2020, the infestation zone has doubled in size (Jones, 2023).

Photo Credit: Michael Jones — Map of area doubling in size since 2020.

The MOB first attacks the canopy of host trees, killing the branches and if it persists, the infestation will spread to the trunk and eventually kill the tree. In California it seems the MOB is responsible for the death of the tree, although we also have a different ambrosia beetle (Raffaelea montetyi) which bore tunnels and carry symbiotic ambrosia fungi for food and could be also causing damage. Usually, the fungi from these other ambrosia beetles are weak and only colonize near tissue in the bored tunnel. There is research currently happening to see if this other ambrosia beetle could cause similar disease to the MOB in oaks. The extensive network of MOB tunnels can weaken trees and make them more susceptible to failure (Jones, 2020).

Signs of Infested Trees:

  • Canopy loss, dead branches — this usually can be seen as one large branch in the upper canopy declining while the rest of the canopy appears healthy
  • Tiny exit holes and boring dust in cracks of the bark
  • Occasionally sap flux
  • Progression of declined canopy and in some cases heavily infested branches can produce extensive epicormic sprouting with leaves diminished in size and densely clustered (sometimes referred to as “popcorn foliage”).
  • Confirming architecture of tunnels, which are trellis like, intersecting, and fan out of a single plane (Fig.A)
  • The tunnels are distinguished from native Monarthrum spp. of ambrosia beetles, which are shown as a single point and do not intersect neighboring galleries (Fig.B)
  • The native beetles only attack trees that are already dead, dying, or diseased
Photo Credit: Curtis Ewing, Cal Fire
Photo Credit: University of California Agriculture and Natural Resources

What Can You Do To help?

  • Supplemental watering during the dry season — if you water the outside edges (5 feet from the trunk) it will encourage growth of the trees feeder roots making the tree stronger with new growth
  • A layer of mulch a few inches away from the trunk and root crown — this insulates and feeds the roots and slows evaporation
  • Thinning and pruning the branch node below the decline — sanitize tools after pruning

Proper Disposal of Infested Materials:

  • Keep materials on site. DO NOT MOVE. The likelihood of spreading the insect is much higher if you move the materials — (
  • Chip, burn, or masticate infested material
  • Solarize material (put large plastic tarp over material for 6–10 weeks, keep separate from other chippings)
  • If you want to burn materials, again solarize by putting a large plastic tarp over the materials for at least 6–10 weeks

If you see damage on your tree and are unsure if it is the Mediterranean Oak Borer please reach out to the follow:

  • Pest Hotline: 1–800–491–1899
  • Report a Pest:
  • Curtis Ewing from CALFIRE


Jones, M. (2020, December 17). New Invasive Insect: Mediterranean Oak Borer. ANR Blogs.

Cambell, F. (2022, October 12). Mediterranean oak borer — Don’t Move Firewood.

More info here:

Webinar of Mediterranean Oak Borer workshop by UCCE Mendocino County