A small beetle that has killed hundreds of thousands of oak trees in Southern California has reached Ventura County, marking a troubling expansion.
This is the furthest north the goldspotted oak borer has been found in the state. Given the insect’s record of less than half an inch of oak destruction since it was first discovered in San Diego County in 2008, scientists and rangers are alarmed — and working to contain the outbreak.
“We’re seeing more and more of these oak forests filling up and dying, with 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 a lot of oak forest that is a very important natural area out there. It could even go into the Sierra regions if we don’t stop it. So it’s scary.”
The golden-spotted oak leaves the tree.
(Shane Brown)
Although officials are only reporting arrivals, they first found the beetle in Ventura County in the summer of 2024. Julie Clark, a public education specialist for UCANR, recalled getting a call from a local forest ranger who saw an unhealthy-looking shoreline while driving through Simi Hills’ Box Canyon.
“He saw die-back. He saw all the leaves on the crown were brown, which is one of the symptoms of a GSOB infestation,” Clark said in a blog post published this week, using an acronym for the invasive insect.
A forester examined the tree and found D-shaped holes – the calling card of the goldenspotted oak borer – where the beetles had chewed through the tree to get out of the bark.
The forest workers cut down the heavily infested tree to kill the beetles inside. However, the surrounding trees did not suffer.
Nevertheless, the beetle continued its march in the region. Last April, another dead, beetle-infested oak tree was found in Santa Susana, according to the California Department of Forestry and Fire Protection, or Cal Fire. A month later, several dead and damaged trees were found.
The beetle, named for the six golden spots that adorn its back, is not far behind. It reaches distant places by climbing wood. Nobua-Behrmann, a consultant in forestry and urban natural resources, is among those calling for laws restricting the movement of wood.
They say the goal is to stop the killing of the state’s oak trees.
Beetles lay eggs on oak. When the larvae hatched, they penetrated to reach the cambium. The cambium is like the blood vessels of a tree, carrying water and nutrients up and down. This insect chews through the layer, and in the end the damage is like putting a permanent tourniquet on the tree.
A heavily infested tree will show a thin canopy and red or black spots on the trunk, areas where the tree is trying to expel the insects. The “confirmation mark” is an exit hole about eight inches.
In the Golden State, the beetles attack coast live oak, canyon live oak and California black oak.
The golden-spotted oak borer is native to Arizona, where the ecosystem adapts and does not kill many trees. It is believed to have gone to San Diego County for wood. It has since been found in LA, Orange, Riverside and San Bernardino counties, and, according to a study by UC Riverside, has killed an estimated 200,000 oak trees.
In 2024, the beetle was found in several canyons in Santa Clarita, which sits just 14 miles from the nearly 600,000-year-old oak trees in the Santa Monica Mountains. Access to the beautiful coastal mountain range was described as “the worst” for LA County in a 2018 report.
Researchers, fire officials and landscape managers, among others, are working to control or slow the death march of the beetles. They agree that it is unlikely that they will be eliminated from the places where they are based.
Experts advise removing and properly disposing of overgrown trees, including cutting them down. (To kill a small beetle, the chips should be three inches in diameter or smaller.)
If the trees are young or not infected yet, they can be sprayed or injected with insecticides.
However, there are drawbacks to the current options. Pesticides may harm non-target species, such as butterflies and moths. And treatment can be expensive and difficult, making it impractical for many forests.
There is another non-toxic tactic in play: educating the public to report possible attacks and burn wood where you buy it.
People can volunteer to check trees for signs of the dreaded beetle, allowing them to “do something instead of worry about it,” Nobua-Behrmann said.
UCANR, along with Cal Fire, is hosting a “GSOB Blitz” pilot event next month in Simi Valley.
