New Oak COWncern

May 12, 2009

Oak trees in California are facing a new threat. A U.S. Forest Service study reported in Science Daily suggests that there is reason for COWncern. U.S. Forest Service scientists have completed a study on a beetle that was first detected in California in 2004, but has now attacked 67 percent of the oak trees in an area 30 miles east of San Diego. Scientific surveys have not yet found the oak-boring beetles in Northern California, though scientists fear they could spread to coast live oaks and California black oaks in many parts of the state. Forest officials are not sure how the infestation started and they do not know how to stop its spread. They do know that the formerly-rare beetle, “Agrilus coxalis” has now become a serious threat to oaks in the San Diego area and seems poised to expand its range. Oak trees have a nearly continuous distribution in the state, reaching from the infestation area north to the Oregon border. Drought and climate change may weaken oaks and make them more oaks susceptible to this insect and help the beetle extend its range northward. While the peril to oak trees is so far restricted to a limited region, the study suggests that all of us who have coast live oak, canyon live oak, and California black oak trees should stay on top of the situation.

The Agrilus coxalis is a metallic-green, bullet-shaped wood borer. It was previously so rare that it doesn’t yet have an accepted common name, although the name “the gold-spotted oak borers” has been proposed. Control and eradication efforts for the Agrilus coxalis may include spraying oaks with insecticides and banning the removal of oak fire wood from infested areas.

These bugs are apparently not a concern for evergreen trees. It appears to be killing trees in the backcountry when the larvae spreads over large patches and kills the cambium, a tree layer that produces new cells.

With all of California, and our area particularly, already reeling from the ravages of Sudden Oak Death (SOD), the last thing we need is the specter of another threat to our beautiful and ancient oaks. Sudden Oak Death is a very serious oak disease that has been a focus of local attention ( See our article here) and was recently the subject of a second annual ‘SOD Blitz’ held in Portola Valley in April. As experts say, SOD is “everybody’s problem. There are no barriers to infection between properties.”

Death of oak trees, whether from SOD or this new beetle, will increase fire danger and destroy wildlife habitat. This new threat just reinforces the need for good management of our trees including vigilance, strengthening the trees by use of appropriate fertilizers annually, and careful pruning.

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