The Department of Agriculture/Weights and Measures has confirmed the presence of invasive shothole borer (ISHB) in the Redwood Shores community, marking the first detection of this pest in the County. These beetles, which are about the size of sesame seeds, can severely damage or kill a variety of trees. Previously limited to Southern California, ISHB infestations have also recently been confirmed in Santa Clara and Santa Cruz Counties.

The Department is conducting further surveys and trapping to assess the extent of the infestation. Residents and land managers are asked to inspect trees for signs of ISHB, such as small, round entry holes, wet staining, frass, boring dust, and sugary buildup around entry points.

Invasive shot hole borers (ISHB) are two species of non-native ambrosia beetles (Euwallacae fornicatus and Euwallacea kuroshio) that bore into trees and introduce fungi which causes a disease called fusarium dieback (FD). This insect-pathogen complex has been responsible for the death of tens of thousands of urban and riparian trees in Southern California (UC IPM Pest Notes, Publication 74179).

ISHB has been known to attack 66 different tree species in California, including but not limited to California natives like box elder, valley oak, arroyo willow, black cottonwood, and California sycamore. 

ISHB entry holes are perfectly round and about the size of the tip of a ball point pen (approximately .85 mm in diameter). In addition to entry holes, infested trees will typically exhibit symptoms of beetle attack:

  • Staining: may be wet and dark or dry and light-colored (e.g. sycamore trees, oak trees)
  • Gumming: thick resin that sometimes pushes the beetle out of the gallery (e.g. silk trees, Koelreuteria species like Goldenrain tree)
  • White powdery exudate: may form "sugary volcanoes" that appear to be crystalline foam (e.g. avocado)
  • Frass: produced by the beetle's boring activity, may be present on any host tree especially when infestation level is high. It can appear as long "matchsticks" of frass (e.g. box elder, coral tree, and willow species)

Photos and descriptions of these symptoms can be found at the UC ANR Website here.

 ISHB Staining Staining Gumming Gumming White Powdery Exudate White Powdery Exudate Frass Frass

For more information, please visit www.ISHB.org. If you are a resident of San Mateo County and are concerned that your tree(s) may be infested with ISHB, please contact the San Mateo County Department of Agriculture/Weights and Measures office at (650) 363-4700.

While eradication is challenging once the pest is established, managing the pest is possible through regular monitoring, removal of severely infested trees, and proper handling of green waste. Chipping and composting or solarizing infested material on-site helps prevent the beetle from spreading.

Residents who suspect ISHB damage should complete the ISHB Detection Assessment or contact the San Mateo County Department of Agriculture/Weights and Measures at (650) 363-4700 for assistance.