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Invasive Beetle Killing Coast Live Oaks

Picnic area in Cuyamaca Rancho State Park where the Gold Spotted Oak Borer has killed dozens of trees.
Ed Joyce
Picnic area in Cuyamaca Rancho State Park where the Gold Spotted Oak Borer has killed dozens of trees.

Oak Borer Could Wipe Out Trees From San Diego To Oregon

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Invasive Beetle Killing Coast Live Oaks
There's a killer in San Diego County. Its prey: oak trees. Hundreds of Coast Live Oaks have already been decimated in Cuyamaca Rancho State Park.

Hundreds of Coast Live Oaks have already been decimated in Cuyamaca Rancho State Park in San Diego County.

The trees have been killed by the Gold Spotted Oak Borer.

We walked through a section of the Park with UC Riverside Natural Resources Specialist Tom Scott and U.S. Forest Service Entomologist Tom Coleman.


In just one section of the park, the picnic area of the Green Valley Falls Campground, the pernicious beetle has killed dozens of trees, including a 200-year-old oak tree.

"If you lived in San Diego, if you grew up here, this is the kind of place you'd come in the summer where you'd have a closed canopy forest that you could sit under with the the dappled light from the oak trees," said Scott. "And if you look at this now, what you have is an area that's basically exposed."

These trees will not be the only or last trees to fall victim to the Gold Spotted Oak Borer.

At risk are hundreds of thousands of iconic Coast Live Oak trees from San Diego County to Oregon.

Scott said the beetle was already killing trees before it was even noticed.


"All the large trees in this picnic area have died, most of them in the last five years," said Scott. "Most of them before we even knew there was a problem. These trees already had the beetle and had begun their progression toward death."

Scott said the Gold Spotted Oak Borer is not native to San Diego County.

He said the pest likely arrived here between 1996 and 2000.

How the invasive species got to the county is not clear.

Scott said the best guess is the beetle hitched a ride in a load of firewood brought here from Arizona.

Scott and others are now warning people in San Diego and other nearby counties about the potential dangers lurking in firewood.

"If you buy firewood that's green or you transport firewood that's green to the area where you live and you have oak trees, chances are in a year or two you're going to start to see your oak trees thin," Scott explained. "And then you're going to start to see the kind of decline that we have in the scene behind me right now."

U.S. Forest Service Entomologist Tom Coleman has been tracking the beetles path of devastation.

He said the pests choke the tree, keeping nutrients from getting to its roots.

"What really killed this tree, it's the larval feeding, and you can see this on the wood surface here," said Coleman, pointing out the pattern on a dead oak stump. "All this kind of scribbly lines, this meandering feeding pattern. What the beetle does is almost girdle the tree and you would get the same effect if you were to come up to the tree with an ax or a hatchet and just take all the bark off."

Coleman said the beetles have a voracious appetite for the oldest and tallest oaks.

"The beetle prefers these really large diameter trees," said Coleman. "When the beetle initially will move into some of these stands, this is what it attacks first. And it'll slowly move from this larger size class to the smaller size class."

He said the mature Gold Spotted Oak Borer beetles lie dormant in the winter waiting for warm weather. In spring, the adults feed on the trees and lay their eggs.

Coleman said it is these tiny offspring which cause the big problems.

"The larvae are really the damaging agent that are killing the trees, it's not the adults, it's the damage from the larval feeding that girdle the tree and cause the mortality," said Coleman.

Damage to the base or main stem of the coast live oaks is what tipped Coleman and others off to the beetle's presence.

Coleman said another sign is sparse or bare tree tops. It takes several years for the trees to die once the larvae start to feed.

The Coast Live Oak is a hearty tree.

But Tom Scott with UC Riverside said it doesn't have a natural defense for the Gold Spotted Oak Borer.

"And it's a striking thing to see a tree that can go through catastrophic fires, droughts and have survived for 200 years in this unpredictable climate to be waylaid by a beetle that's less than a centimeter long," said Scott.

Scott and Coleman said researchers are studying ways to control the beetle. But for now, there's no way to eradicate the pest.

"Seventeen percent of the trees are dying every year," Scott said. "Do the math, you don't have to go many years until you've got every tree in decline or dead."

Meantime, the oaks face the potential for widespread devastation.

"The line in the sand right now is urban Los Angeles. If we don't stop this beetle and it makes that jump across Los Angeles, it won't stop until it gets to Oregon," said Scott.