San Diego County

Invasive beetles are killing San Diego County's oak trees — and the bugs on the move

"It’s a very quiet problem."

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A drive to the mountains in San Diego County means you're surrounded by nature, trees especially, which have been facing a major threat from something so small you might not even notice it.

You will notice, however, how it's affecting oak trees.

Anyone walking around the San Luis Rey Picnic Area off state Route 76 can see the contrast, with a mix of healthy oaks, dying trees and those that appear dead. Experts say it's caused by an invasive beetle.

"Today we’re going to talk about the gold-spotted oak borer, or GSOB, as you’ll hear us refer to it," said Beth Kyre, an entomologist with the U.S. Forest Service.

It's easy to understand how the GSOB got its name.

"It attacks oak trees and it has gold spots," Kyre said.

According to Kyre and Joelene Tamm — a grad student studying these invasive insects at the University of California, Riverside — the beetles may have gotten to San Diego in the 1990s. They were first scientifically detected in 2004, however.

”They move primarily because of humans, so we move them around from place to place, oftentimes through firewood and wood product," Kyre said.

Laying their eggs in tree bark, the GSOBs nestle into crevices at the base of the tree. That's just the beginning, though.

"Then those eggs will hatch, the larvae will tunnel into the living tissue inside the tree and that tissue is responsible for moving around water and nutrients up and down the tree to make its leaves, to keep it growing," Kyre said.

"They will eat their way throughout the tissue, the cambial tissue within the tree, and then pupate — so, slow down and stop — and that is when turn into adult beetles," Kyre saiod. "The adult beetles will emerge in the spring, crawl up the tree foliage, mate, and the cycle starts again.”

When they exit, the beetles leave behind "D" shaped holes in the bark. Another sign that they've made their way into the tree is what's behind the bark. Their peak emergence is typically around late June through early July, according to Tamm.

”You will see gallery wall under the bark where the larvae were developing and feeding as the journeyed their way to beetles," Kyre said.

As the bugs invade the tree, the oaks start to show symptoms too. The crown of the tree thins, leaving behind dead branches, woodpeckers start to search for larvae in the bark, and there's bark staining.

"It’s a very quiet problem," Tamm said.

The dying trees present a major risk when wildfire season peaks in California.

”It’s such a nuisance to oak trees because we are really concerned about the wildfire hazards that we’re experiencing in California," Tamm said. "GSOB is just compounding that threat with increased fuel loading due to excessive tree mortality."

Meantime, Kyre and Tamm said, there are a couple of treatment options being utilized. If the tree is badly infested, it could be cut down and removed. Another option is injecting trees with insecticide. Tamm has been surveying the trees, protecting the ones that have a chance at being saved.

"I pick the healthiest tree with the best chance of fighting the beetle off with the help of the insecticide, pick that tree, and then ensure I can treat a certain number of trees every 50-100 feet to ensure that shade coverage when other trees start to die around it," Tamm said.

Kyre said that, when it comes to stopping the spread of the bugs, she encourages those who purchase firewood where they burn it.

"Trying to keep people from moving firewood, trying to protect our campsites," Kyre said.

The impacts the beetles have on oak trees could be long-lasting, saidTamm. While they are centered in eastern San Diego County, the beetles are moving northward.

"Eventually, it's going to move up through the Sierra Nevada and along the coast, and impact the whole state," Tamm said.

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