Invasive Beetle Kills At Least 90,000 SoCal Trees: Can Indigenous Cultural Burns Help?

Tree limbs and tops began falling off century-old black oak trees on the La Jolla reservation in northern San Diego County around 2014 and nobody knew why. The die off was puzzling and tribal fire chief Wesley Ruise Jr. couldn’t remove the dead trees fast enough.

The oak trees’ acorns are a vital food source for the birds, deer and other animals on the reservation. But at one point they were also a vital food source for the Indigenous peoples across the state.

“We'd go and gather them and run them through a process to kind of cure them,” Ruise said. “We'd make what we call wiiwish, and it was like a kind of thick pudding that we ate.”

Around that time entomologists from the U.S. Forest Service identified the culprit — the goldspotted oak borer, an invasive beetle that likely hitched a ride on some firewood from Arizona to Southern California. Researchers suspect that could have happened around the early '90s.

The goldspotted oak borer is tiny, but the yellow spotted beetle can cause big damage to California oak trees. The pest has already forced officials to remove at least 90,000 oak trees throughout Southern California.

The beetle has appeared in Green Valley, a small community in the Sierra Pelona Mountains northeast of Santa Clarita in recent years. The beetle has also popped up in L.A., Orange, Riverside and San Bernardino counties and killed off oak trees in city, county and state parks as well as federal and tribal lands.

Could cultural fire be the answer?

The La Jolla Band of Luiseño Indians began an insecticide program on the reservation in 2014, but nobody was available to volunteer until 2019. That’s when Joelene Tamm, a graduate student at the University of California Riverside’s entomology department, volunteered to manage the program.

Tamm, who’s also a Squaxin Island tribal member, is the La Jolla Band of Luiseño Indians natural resource manager. She also does outreach with the California firewood task force, and the University of California Agriculture And Natural Resources.

Her research found burning piles of infested wood stops the beetles from continuing their life cycles by 98%.

The tribe plans to use cultural burns once again in May, targeting the lower part of the tree, where goldspotted oak borer reproduction happens.

“GSOB pupates in the outer bark,” Tamm said. “So we're thinking that the fire, if it's timed in the spring, will have the most heat impact on the outer bark, and it might even burn open their pupil chamber, allowing additional predators to come in like ants or other kinds of flies and insects that can prey on the GSOB.”

Ruise is confident that "good fire" will work. He pointed to the 2007 Poomacha fire that burned about 95% of the reservation and scorched thousands of oak trees. The 5% that did not burn was the campground area, which is where the goldspotted oak borer is currently incubating.

“History shows that in the past, our ancestors used fire to maintain the forest, to maintain the areas around where we lived for the plants and the trees,” Ruise said. “So what we're trying to do now is bring back what we call cultural fire or good fire, back to our tribal land as it was in the past.”

A UC Berkeley study published in Proceedings of the National Academy of Sciences in 2022 found that Karuk and Yurok tribes carried out cultural burns in the Klamath Mountains for thousands of years, which played a big role in the area's biodiversity and forest structure.

How the beetles survive

Female goldspotted oak borers lay their eggs in the cracks and crevices in tree bark from spring until August. The eggs can take a few weeks to a month to hatch and when it does, the larvae feed on the tissue just below the tree’s bark, the cambium — eventually killing the tree through suffocation.

Female goldspotted oak borers lay their eggs in the cracks and crevices in tree bark from spring until August. When the eggs hatch, the larvae feed on the tissue just below the tree's bark, the cambium — eventually killing the tree through suffocation.

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Mike Lewis

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University of California, Riverside

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By the fall, the larvae will tunnel further into the tree where they continue growing. Come May, the beetles will metamorphose and emerge hungry from oak trees. They’ll eat oak foliage and mate to start the whole process over again.

“The scariest thing about this insect, I think, is its potential,” said U.S. Forest Service entomologist Beth Kyre. “To see the trees that aren’t yet infested and understand that there’s a strong possibility that at some point they could get there.”

What’s being done to manage them?

The U. S. Forest Service starts with removing heavily infested trees. After that the agency uses a topical chemicals that targets the beetles as they land and exit trees. There are also systemic insecticides that move throughout the tree and kill off any living larvae hiding inside.

“It's really important when you start to consider this chemical management that you think of it in terms of an integrated pest management approach,” Kyre said. “So you're thinking of prevention, chemical application, you're also thinking of: ‘Do we need to remove this tree?’”

Kyre also recommended burning firewood in the areas where it was purchased and not transporting the logs elsewhere.

But the agency is also looking into the centuries-old Indigenous practices that the La Jolla Band of Luiseño Indians plan to use to manage the beetle.

“We can learn from them and apply that to broader scale management,” she said.

Green Valley

Goldspotted oak borers have caused fire officials in Los Angeles County to remove close to 10,000 trees in Green Valley since 2015. L.A. County Fire deputy forester Melissa Valente said many people move to the area for the oak trees and rely on them for firewood in the winter and shade during the summer.

“So for them it's extremely disappointing to watch their trees die,” Valente said.

While it’s hard to say if progress is being made, Valente said the number of dead trees annually has fallen from around 200 three years ago to about 70 this year.

Valente credited basal injections, which the county implemented in Green Valley last summer. Trees are injected with syringes toward the bottom of the trunk where the insecticide moves into the tree and kills the larvae.

Still, Valente’s said she’s worried the beetle’s next stop could be the rich oak forests in the Santa Monica Mountains

“It's a huge concern for the fire department and all of the other agencies in the Santa Monica Mountains,” she said.