The goldspotted oak borer, Agrilus auroguttatus, (GSOB) is an invasive pest found currently contributing to oak mortality in San Diego County, CA. This flatheaded borer is newly introduced to California yet poses a significant risk to its oak tree population. The pest is actually naturally occurring in southeastern Arizona, and a related species can be found in Mexico and Guatemala. Although GSOB was identified in California in 2004, it wasn’t until 2008 that it was linked with extensive damage to oak trees. As of 2010 over 21,000 trees in San Diego County have died due to goldspotted oak borer infestation. The damage extends over more than 1,800 square miles of forest and park land as well as on residential landscapes.
The larvae of this pest damage tree tissue by feeding beneath the bark near the interface of xylem and phloem, the tissue which conducts nutrients and water throughout the tree. Additional damage is incurred in the cambium, a layer of single cells responsible for the radial growth of the tree. Multiple generations of borers prey on trees for years causing damage, stress and eventual death. Oak death is causing economic, environmental, aesthetic and cultural disturbances across San Diego County. GSOB continues to pose a major threat in the state of California and oaks in Oregon are also at risk.
Goldspotted oak borers seem to prefer 3 species of oak trees; coastal live oak, Quercus agrifolia, canyon live oak, Quercus chrysolepis and California black oak, Quercus kelloggii. The pest has also been identified on Engelmann oak, Quercus engelmannii, but damage to the specimen(s) was not reported. They usually target mature and older trees, and do not appear to attack trees with diameters at breast height (DBH) of 12 cm or less. Unfortunately, finding these borers currently attacking only 4 species does not fully exclude other oak species from risk of infestation and populations require continued monitoring.
Evidence of an affected tree can include: red or black staining on bark ranging from the size of a dime up to about 6 inches, blistering and oozing on bark’s surface, thinning crown, branch and twig die-back and/or premature leaf drop. External, visible damage is caused by larval galleries created during feeding. Exit holes; small, D-shaped holes in bark, left after larvae pupate and attempt to leave, may be present on trunk. Excessive woodpecker activity may also indicate the presence of GSOB. These signs and symptoms will vary among the 3 known affected species, so if suspicious damage appears on an oak tree contact your local agency, agricultural extension or arborist for more information and advice on steps to take.