FROM THE ARCHIVES: [New] Beetle Killing Oaks

By Vincent Lazaneo, originally published in Let's Talk Plants! January 2009, No. 172.

Editor's note: In an online article, last updated by Faith Campbell, October 2015, as found on https://www.dontmovefirewood.org/pest_pathogen/goldspotted-oak-borer-html/ it specifies the scientific name for the goldspotted oak borer as Agrilus auroguttatus (formerly coxalis).

https://commons.wikimedia.org/wiki/File:Agrilus_auroguttatus.jpg
The Goldspotted Oak Borer (Agrilus auroguttatus). Photo credit: Mike Lewis, as found on Wikimedia Commons.

https://commons.wikimedia.org/wiki/File:Agrilus_auroguttatus.jpg

Custom Don’t Move Firewood poster built for the La Jolla Band of Luiseño Indians. Designed and published in 2021. Features beetle imagery, mountain style background, and customized wording including mention of the goldspotted oak borer by The Nature Conservancy, Don’t Move Firewood.

http://www.dontmovefirewood.org/wp-content/uploads/2021/09/DMF_quickposterLaJolla_2.pdf


 

If you want to burn oak in your fireplace this winter, be careful where it came from. Many oaks in the mountains east of San Diego have been attacked by a new insect pest, the goldspotted oak borer (GSOB) Agrilus coxalis. Transporting oak firewood out of the infested area could spread GSOB to new areas and endanger other oaks.


GSOB is not native to California and may have been introduced with firewood from Mexico. For the past seven years, Oak decline has been evident in eastern San Diego County near the communities of Descanso and Pine Valley. Over the past few months, US Forest Service entomologist Tom Coleman discovered that damage was due to the feeding activity of GSOB. Prior to the beetle’s discovery, drought was thought to be the main cause of oak deaths.


Photos from original article.

The beetle has killed California live oak, Canyon live oak and California black oak from the community of Alpine, east to Mt. Laguna and north to Julian. For now, Englemann oak seems to resist the pest.


As many as 70% of the oak trees in some areas are infested. Some campgrounds in the national forest have been closed and no permits are being issued to remove oak firewood.

The adult GSOB is a small, bullet-shaped beetle about 10mm (0.4 in. long) and has six golden yellow spots on its dark green forewings. It is active from summer to fall and may feed on foliage but is rarely seen. Eggs are probably laid in bark crevices and the larvae feed under the bark on the trunk and larger branches. Mature larvae are white, legless, slender and about 18mm (0.75 in. long). Larval feeding kills patches and strips of cambium tissue beneath the bark which causes dark staining and sap flow on the bark surface. Prolonged infestations causes limb die-back and eventual tree death. The larvae pupate in the outer bark and leave D-shaped exit holes about 1/8” wide when they emerge.


It is difficult to protect susceptible oaks from attack by GSOB. At this time it is not known if insecticides will help protect healthy oaks or save infested trees. Research is planned to evaluate the effectiveness of chemical sprays and systemic treatments. Before an insecticide product is used, always read and follow all label directions. For more information on GSOB visit www.fs.fed.us (in the search box enter CA under state and Cleveland under name).


How to keep GSOB from spreading?

  • Do not transport oak firewood or logs to uninfested areas

  • Remove heavily infested dying or dead oaks

  • Place cut wood and logs in a sunny location, then cover with two layers of clear plastic and secure tarp edges with soil.

  • Chip wood into 1” pieces and use as mulch

Read the whole original article here: https://sdhort.org/Resources/newsletters/2009/SDHS_Jan_09.pdf

 

Vincent Lazaneo, the San Diego Horticultural Society's 2004 Horticulturist of the Year, is UC Urban Horticulture Advisor Emeritus. He has a master’s degree in horticulture and a teaching credential in vocational agriculture from UC Davis. In 1983, Vince began the Master Gardener program in San Diego. Vince frequently contributes, or has contributed to, the San Diego Union-Tribune, Mira Mesa Living, and other publications, and he enjoys growing specialty plants in his home garden, reading, hiking, and fishing.