What Kills Native Oaks in Southern California? - Orange County Registry

The Golden Spotted Oak Moth, or GSOB, is an invasive beetle that kills native oaks in several areas of southern California.

Sensitive oaks include live oak from the coast, live oak from the canyon and black oak from California. In many cases, GSOB damaged or killed mature oak trees valued for their beauty, wildlife habitat and shade. Areas with a large number of native oaks are particularly threatened. Unfortunately, oaks injured over several years by several generations of GSOB often die.


Although the gold-spotted oak borer was first identified in San Diego County in 2004, it was not until 2008 that oak deaths were directly linked to them. In 2010, they killed over 20,000 oak trees growing in the forests, parks and urban areas of San Diego County. Subsequent infestations occurred in Idlyllwild in 2012, in Orange County in 2014 and in Los Angeles County in 2015.

The three most recent outbreaks have all occurred in San Bernardino County. The first occurred at Oak Glen in 2018, followed by infestations of California black oaks in the Sugarloaf area of ​​Big Bear in August 2019 and at Wrightwood in early November 2019.

GSOB is native to southeastern Arizona, where it is not destructive to otherwise healthy native oaks. This may be due to natural enemies and / or resistant oak species that have co-evolved with them.

What are the damages: The larvae feed on the vascular system (water and nutrient-conducting tissues) inside the trunks and branches. Infested trees have bark stained with black and may ooze sap under the red bark blisters. Adult beetles leave a distinctive D-shaped exit hole. Damage caused by leaf-feeding GSOB adults is not a major concern.

How to identify them: GSOB larvae are about 0.8 inches long, white, and without legs with two pinch-like spines at the tip of their abdomen. Adult GSOBs are smaller (about 0.4 inch long) and are mostly black with six golden spots on their fore wings. Soft-bodied pupae resemble adults in size and shape and are found in the outer bark from late spring to early summer.

How to control them: Prevention is important because there are no known control methods once trees are infested with GSOB. Keeping infected firewood in place is the most effective way to stop its spread. Wood should never be moved off site as this is the primary method of spreading GSOB. No known natural enemies have been identified and insecticides are generally not effective.

It is important to monitor sensitive tree species and to quickly identify and report new infestations. If you suspect there is an infested oak on your property, please submit photos of the entire tree, a close-up of a leaf (to confirm the species) and a close-up of the bark surface from the main trunk to the ucan site .edu. If possible, include a photo of an unsharpened No. 2 pencil tip next to all of the visible exit holes, as they are both about 0.15 inches wide.

A team of scientists from UC, the California Department of Forests and Fire Protection, CALFIRE and the US Forest Service and other agencies are working together to reduce the devastation caused by this insect and identify control agents effective. More detailed information and additional photos can be found here: https://ucanr.edu/sites/gsobinfo/.