Identifying Signs & Symptoms

Valley Oak in Calistoga. Photo by Dr. Akif Eskalen, UC Davis

Oak trees infested with MOB are most easily identified by damage caused by the beetle’s tunneling activity (galleries) in the xylem. MOB galleries are often:

  • Trellis-like
  • Very crowded and intersecting
  • Fanning out in a single plane
  • 1.2-1.5 mm in diameter


(Click thumbnail for larger image)


Other signs and symptoms of MOB tunneling activity include boring dust in cracks of the tree bark, and sometimes oozing sap (“sap flux”). Similar symptoms can be produced by tree diseases and other boring insects, so MOB galleries or specimens must be identified in addition to the presence of boring dust and/or sap flux.