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Native Oaks Threatened by New Pest

By Vincent Lazaneo

Oaks have been an integral part of our ecosystem for centuries, but the survival of some native species in Southern California is now threatened by a small beetle, the gold spotted oak borer.  GSOB, which is not native to California, was detected in 2008, infesting oaks in the mountains east of San Diego.  Coast Live Oaks, Canyon Live Oaks, and Black Oaks were attacked by the borer and over 17,000 trees have been killed in a 28,000 acre area in and around the communities of Descanso, Guatay, and Pine Valley.  The Engelmann Oak seems to be resistant to the pests.

Countless numbers of additional oaks could die if the borer spreads to other areas of the county and state.  Officials are concerned that the public may, unknowingly, spread the borer by transporting oak firewood infested with the pest.  If you use firewood, please do your part to combat GSOB by not buying or transporting oak firewood that may be infested.  A map of infested areas is available at http://groups.ucanr.org/gsob/ along with photos of the pest and other information.  You can also report possible infestations on the site.

If you visit a campground or park, there are three additional things you can do to help protect native oaks. 

  1. Do not transport oak firewood into or out of campgrounds or parks.
  2. Leave oak firewood at home and use local sources of firewood when you camp or have a picnic.
  3. When you use local firewood, leave any unburned wood on-site when you return home.

It's important NOT to transport any oak firewood because even sound-looking wood can contain borers.  The female GSOB lays eggs in bark crevices. The larvae feed on live tissue (cambium) beneath the bark on the trunk and larger branches.  Mature larvae are white, legless, slender, and about three-fourths inch long.  They pupate in the outer bark and leave "D" shaped exit holes about one-eighth inch wide when they emerge.  The adult GSOB is a small bullet-shaped beetle about three-eighths inch long with 6 golden yellow spots on its dark green fore-wings.  Adult beetles are active during warm months of the year and may feed on foliage, but are rarely seen.

Signs that a tree may be infested include crown-thinning, the beginning of significant leaf loss, and die-back of twigs. Larval feeding on the trunk and large limbs kills patches and strips of cambium which causes dark staining and sap flow on the bark surface.  Prolonged infestation causes limb die-back and eventual tree death. 

Researchers are studying GSOB and methods that could be used to combat the pest.  No chemical treatment is recommended at this time. Tests are being conducted to evaluate the effectiveness of insecticides against the borer but researchers do not yet know if any insecticide product will save infested oaks or help protect healthy trees.  People who have oaks on their property may be tempted to use an insecticide to protect their trees from GSOB but this should not be done until it has been shown that a specific product will be effective and safe for people and the environment.

Protecting oaks from GSOB with chemical treatment may not be a good option even if an effective product is identified.   At least one and probably more insecticide applications could be required every year to protect susceptible trees after GSOB is established in an area.  This would be expensive and could result in unwanted consequences.  Replacing susceptible oaks with Engelmann Oaks (Quercus engelmannii) would be a safer and less expensive option in the long run.  Young oaks grow quickly and can reach a height of 20' within a decade.  

Dead or dying oaks can harbor GSOB until the wood is thoroughly dry.  Chipping infested wood into 1" pieces is the most effective way to rapidly reduce the population of GSOB.  A 4" thick layer of chips can be spread on the ground to control weeds and conserve moisture.  You can contact a tree care company for information on chipping services.  Please check for information on free chipping programs that may be offered at http://groups.ucanr.org/gsob/ .