By Sharon Cohoon, Sunset senior garden writer

California live oaks, California black oaks, and canyon live oaks in eastern San Diego County have been noticeably declining since at least 2002.  The problem was initially blamed on the prolonged drought, but U.S. Forest Service entomologist Tom Coleman has determined that the gold-spotted oak borer (GSOB) Agrilus coxalis is actually the chief villain.  GSOB is not a native insect.  It comes from southern Mexico and northern Guatemala.  It is believed to have been introduced via oak firewood brought in from Mexico.

GSOB is a small, bullet-shaped beetle about 0.4 inches long.  It has six golden yellow spots on its dark green forewings.  Here is what the pest looks like:

Because it is so small, the beetle is hard to observe.  It lays its eggs in bark crevices, and its larvae feed under the bark.  This kills patches of strips of cambium tissue which causes dark staining and sap flow on the bark surface.  (See the photo below.)  Prolonged infestations cause crown-thinning, limb die-back, and eventual tree death. 

What can you do to keep GSOB from spreading?

The most important thing is not to transport any oak firewood from infested areas.  This is not a problem we want to spread any further.

If you have dead or dying trees infested by GSOB on your property, remove the trees and cover the wood with a layers of thick, clear plastic sheeting, sealing the edges with soil.  Keep the sheeting in place until the wood is thoroughly dried out to kill any living larvae remaining in the wood.  Chipping wood into one-inch pieces, which can be used as mulch, has the same effect.

For more information about the gold-spotted oak borer, go to this U.S. Forest Service link.  Or call Vicent Lazaneo, urban horticulture advisor at the San Diego UC Cooperative Extension. 858/694-2845.

Photos supplied by Tom Coleman.