Invasive Spotlight: Shot Hole Borers

Small beetles are causing big problems in Southern California. Two closely related species, the polyphagous shot hole borer and the Kuroshio shot hole borer (collectively referred to as invasive shot hole borers), have been attacking more than 60 species of trees. These invasive beetles create a series of tunnels, or galleries, where they lay eggs and cultivate a Fusarium fungus to use as a food source. The fungus causes branch dieback, general tree decline, and can result in tree death. The beetles have been found in Los Angeles, Orange, Riverside, San Bernardino, Ventura, Santa Barbara, and San Diego counties.

What should you look for?

Invasive shot hole borers are 1.8 to 2.5 mm in length and brown or black in color. Beetles typically spend most of their lives inside the tree, so it may be difficult to find them or identify them. However, signs of a beetle infestation can include:

  • Perfectly round entry holes about the size of the tip of a ballpoint pen
  • Wet staining, gumming, white powdery exudate, or frass associated with holes
  • Dead or wilting branches on trees

To learn more about these invasive shot hole borers, management options, and reporting, see this blog post and the UC ANR Invasive Shot Hole Borers website.