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Invasive Beetle Devouring San Diego’s Oak Trees

Dead trees in the Cleveland National Forest in San Diego County are pictured ...

Credit: Cleveland National Forest

Above: Dead trees in the Cleveland National Forest in San Diego County are pictured in this undated photo. They are a casualty of the goldspotted oak borer beetle.

More than 80,000 oak trees in San Diego County have been killed by the invasive goldspotted oak borer beetle over the last decade, according to the Center for Invasive Species Research. The Cleveland National Forest has taken the brunt of the massive oak die-off, but the beetle has also devoured trees in Descanso, La Jolla and most recently, Idyllwild.

The hardest hit in the oak family have been the coast live, canyon live and black oaks. The trees are ultimately killed after the half-inch-long beetles burrow their eggs in the underside of the bark, and the hatched larvae feed on the water- and food-conducting tissue of the tree.

Photo credit: Center For Invasive Species Research

The goldspotted oak borer, pictured, was found in San Diego County in the early 2000s. But it wasn't until recently that officials discovered that the beetle, combined with the drought, has killed tens of thousands of oak trees.

Photo credit: Center For Invasive Species Research

Mature larvae leggless and about 18 mm long and have a bulbous head used for boring into bark and phloem.

“No one worried about these trees 20 years ago because they were perfectly capable of taking care of themselves, said Tom Scott, a natural resource specialist with the University of California. "The fact that we have to actively manage them now to maintain our oak woodlands is a stunning change in events.”

Scott said warmer temperatures and less rainfall from climate change could continue to weaken the region’s remaining oaks and make them more susceptible to beetle attacks.

“So what we’re worried about is when we go back into a drought and we have a hot summer -- that could basically cause a lot of trees which are attacked by the beetles and hanging on, to suddenly die in large numbers together again like they did in 2002.”

Goldspotted Oak Borer

The coast live is a keystone tree species in southern California and their widespread loss is causing immense impacts, including the loss of habitat and food resources used by native animals such as acorn woodpeckers, mule deer, ground squirrels, and the arroyo toad.

The region could face an ecosystem collapse if the beetle can't be controlled, warned Scott, because there are currently no effective treatments to eradicate the beetle once it becomes established.

"The only thing that’s saved us so far is the fact that you don’t have multiple life cycles of these beetles," said Scott. "They only have one life cycle per year."

Scott said the beetle isn't a pest in its native home in Arizona and Mexico, likely because it has effective natural enemies keeping the population in check. But in California, the bug has no known predators.

Scott said the beetle likely moved to the county in a load of infested firewood. California Forestry Department officials are urging people to avoid transporting firewood outside the area where it is obtained.


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