A tiny beetle responsible for killing hundreds of thousands of oak trees in Southern California has reached Ventura County, marking a troubling expansion.
This is the farthest north the goldspotted oak borer has been found in the state. Given the less-than-one-half-inch insect’s track record of devastating oaks since being first detected in San Diego County in 2008, scientists and land stewards are alarmed — and working to contain the outbreak.
“We keep seeing these oak groves getting infested and declining, and a lot of oak mortality,” said Beatriz Nobua-Behrmann, an ecologist with UC Agriculture and Natural Resources, or UCANR. “And as we go north, we have tons of oak woodlands that are very important ecosystems over there. It can even get into the Sierras if we don’t stop it. So it is scary.”
A goldspotted oak borer emerges from a tree.
(Shane Brown)
Although officials are only now reporting the arrival, they first found the beetle in Ventura County in the summer of 2024. Julie Clark, a community education specialist for UCANR, recalled getting a call from a local forester who spotted an unhealthy-looking coast live oak while driving in Simi Hills’ Box Canyon.
“He saw die-back. He saw all the leaves on the crown were brown, which is one of the characteristic signs of a GSOB infestation,” Clark said in a blog post published this week, using the acronym for the invasive insect.
The forester examined the tree and found D-shaped holes — the calling card of the goldspotted oak borer — where the beetles had chewed through the tree to emerge from the bark.
Foresters debarked and chipped the highly infested tree to kill the beetles inside. Surrounding trees, however, were not afflicted.
Still, the beetle continued its march in the county. Last April, another dead, beetle-infested oak was found in Santa Susana, according to the California Department of Forestry and Fire Protection, or Cal Fire. A month later, several more dead and injured trees were discovered.
The beetle, named for six gold spots that adorn its back, doesn’t fly far. It reaches faraway areas by hitching a ride on firewood. Nobua-Behrmann, an urban forestry and natural resources advisor, is among a contingent calling for regulations limiting the movement of firewood.
The goal, they say, is to prevent the slaughter of the state’s iconic oaks.
The beetles lay their eggs on oaks. When the larvae hatch, they bore in to reach the cambium. The cambium is like a tree’s blood vessels, carrying water and nutrients up and down. The insect chews through the layer, and eventually the damage is akin to putting a permanent tourniquet on the tree.
An infested tree will often display a thinning canopy and red or black stains on the trunk, injured areas where the tree is attempting to force out insects. The “confirming sign” is the roughly eighth-inch exit hole.
In the Golden State, the beetles are attacking the coast live oak, canyon live oak and the California black oak.
The goldspotted oak borer is native to Arizona, where the ecosystem is adapted to it and it doesn’t kill many trees. It’s believed that it traveled to San Diego County via firewood. It has since been found in L.A., Orange, Riverside and San Bernardino counties, and, according to research by UC Riverside, has killed an estimated 200,000 oak trees.
In 2024, the beetle was discovered in several canyons in Santa Clarita, putting it just 14 miles from the roughly 600,000 coast live oaks in the Santa Monica Mountains. Reaching the scenic coastal mountain range was described as “the worst case scenario” for L.A. County in a 2018 report.
Researchers, fire officials and land managers, among others, are working to control or slow the beetles’ death march. They acknowledge they’re unlikely to be eradicated in the areas where they’ve settled in.
Experts advise removing and properly disposing of heavily infested trees, which entails chipping them. (To kill the minute beetle, chips must be three inches in diameter or smaller.)
If trees are lightly or not-yet infested, they can be sprayed or injected with insecticides.
However, there are drawbacks to the current options. Pesticides may harm nontarget species, like butterflies and moths. And the treatment can be expensive and laborious, making it impractical for vast swaths of forest.
There’s another nontoxic tactic in play: educating the public to report possible infestations and burn firewood where they buy it.
People can also volunteer to survey trees for signs of the dreaded beetle, allowing them to “do something instead of just worrying about it,” Nobua-Behrmann said.
UCANR, along with the Cal Fire, is hosting a “GSOB Blitz” surveying event next month in Simi Valley.
