Three species of oaks native to southern California – coast live oak (Quercus agrifolia), California black oak (Q. kelloggii), and canyon live oak (Q. chrysolepis) – appear to be threatened by a beetle native to southeastern Arizona – the goldspotted oak borer (GSOB) (Agrilus auroguttatus). [A similar beetle, A. coxalis, is found in southern Mexico and Guatemala (Coleman et al. 2012.]. Oak decline has been evident in eastern San Diego County since 2002 (USFS PSW S&P R5-RP-022 Oct 28, 2008). An investigation of this decline by Forest Service entomologist Tom Coleman in May 2008 led to determination of the beetle as the causal agent. California Department of Food and Agriculture survey records indicate that the insect was first detected in California in 2002 (Coleman et al. 2012), but its impact was not then understood.
The outbreak began in the Cleveland National Forest and Cuyamaca Rancho State Park in eastern San Diego County. It is likely that GSOB was introduced to the areas on firewood brought into the area from Arizona since these are heavily visited recreational areas which include campgrounds (Coleman & Seybold. 2008. Pan-Pacific Entomologist 84: 288-300) and the beetle is genetically very similar to the Arizona populations (Coleman et al. 2012).
GSOB has contributed to death of more than 80,000 oak trees in three counties of southern California (San Diego, Riverside, and Orange) (http://cisr.ucr.edu/goldspotted_oak_borer.html). The outbreak at the town of Idylwild in Riverside County has now been confirmed to include trees within the San Bernardino National Forest (pers. comm. from Kim Corrella, CALFIRE to Faith Campbell, September 2015). GSOB has the potential to spread further north in California and cause similar mortality to these species in other parts of their ranges (Coleman & Seybold. 2009. SOD symposium abstract).
In December 2014, scientists discovered that GSOB was established at Weir Canyon in northern Orange County. The Weir Canyon outbreak is 60 or more miles away from previously known outbreaks in San Diego and Riverside counties – so it is almost certain that the beetles were taken there by people moving wood.
Weir Canyon Wilderness Park is a popular hiking area located just 37 miles southeast of downtown Los Angeles. According to Dr. Thomas Scott of the University of California Riverside, built-up sections of Los Angeles have between 250,000 and 300,000 coast live oak trees – a species very susceptible to GSOB infestation. Some 92,000 acres of single-family homesites are shaded by such oaks. All are now vulnerable to this nearby outbreak of the pest.
In August 2015, an outbreak was found in a fourth county, Los Angeles (County of Los Angeles Fire Department, Press Release 25 September, 2015). This outbreak is in the town of Green Valley, which is north of the main urban area, inside the boundaries of the Los Padres National Forest. First surveys identified 27 coast live oaks that had symptoms of GSOB attack (pers. comm. from Kim Corrella, CALFIRE to FTC, September 2015). The insect was probably taken to Los Angeles County by movement of firewood (County of Los Angeles Fire Department, Press Release 25 September, 2015).
Yosemite and Kings Canyon National parks and other campgrounds in the Sierra Nevada receive large numbers of campers from the Los Angeles area (Koch 2012). The California black oaks in these parks are now at heightened risk of GSOB attack, since the beetles could be transported to the parks in firewood.
Neither the state of California nor the National parks have adopted restrictions on firewood transport. Nor has California implemented a quarantine to prohibit movement of GSOB or wood that might be transporting this tree-killing pest.
GSOB is particularly damaging in California because there are very few other insects that attack the phloem of the main stem of oaks, so these oaks lack any evolved host resistance to such attacks. The only natural enemies of the GSOB are woodpeckers.
Evidence of insect attacks on oak trees includes the presence of the beetle larvae under the bark, D-shaped exit holes through the bark surface, woodpecker foraging, and bark staining on the trunk of the tree and larger branches. Unlike its relative, the emerald ash borer, GSOB larvae attack the bole from the base of the tree as well as upward to approximately 9 m along the main stem and larger branches (Coleman & Seybold. 2008. Pan-Pacific Entomologist 84: 288-300).
Little is known about GSOB in its native range of southern Arizona. There, its native hosts are Emory oak, Quercus emoryi Torrey, and silver-leaf oak, Q. hypoleucoides A. Camus; oak mortality from GSOB in this area is negligible (Coleman and Seybold 2011). Tree symptoms include D-shaped exit holes, and staining, but less reliably crown dieback (Coleman et al. 2012).
Widespread oak mortality can impact wildlife through loss of a food source and habitat. Dead oaks can create potential hazards, especially near dwellings, along roadways, and in recreational areas. Oak mortality also represents a significant change in fuel load structure and composition across the landscape, which can increase the probability and severity of wildfire (USFS PSW S&P R5-RP-022 Oct 28, 2008). Urban trees provide important ecological services, including shade which reduces energy use and expense associated with air conditioning; and storm water runoff reduction ( Zeleznik 2008).
Scientists are studying possible biocontrol agents. In Arizona, two parasitic wasps were found attacking GSOB larvae – Calosota elongata Gibson and Atanycolus simplex Cresson. Both are already present in California, although parasitism rates are much lower for C. elongata in California than in Arizona. The scientists recommend continuing the search for additional natural enemies of GSOB (Coleman et al. 2012).
To slow spread of GSOB to vulnerable oaks farther North, a consortium has been formed (http://groups.ucanr.org/GSOB) which aims at informing citizens. Part of the message urges people not to transport oak firewood or logs and to remove dead or recently dying oaks with heavy infestations to limit localized growth of the beetle population. Other management efforts include an expanded survey program in California, search for potential attractants for GSOB to increase survey efficacy, and investigation of the areawide impact of the beetle and the effect of drought on the success of GSOB in southern California.
As of November 2010, no one has studied whether oaks in other parts of North America are vulnerable to the goldspotted oak borer.
For more information on this pest, please visit:
- UC Cooperative Extension Goldspotted Oak Borer information a.k.a GSOB.org
- UC Riverside, Center for Invasive Species GSOB Species Profile
- UC Integrated Pest Management guidelines for GSOB
Coleman, T.W., V. Lopez , P. Rugman-Jones, R. Stouthamer, S. J. Seybold, R. Reardon, M.S. Hoddle. 2012. Can the destruction of California’s oak woodlands be prevented? Potential for biological control of the goldspotted oak borer, Agrilus auroguttatus. BioControl (2012) 57:211–225 DOI 10.1007/s10526-011-9404-4
Coleman, Thomas W. and Steven J. Seybold. 2008. Previously unrecorded damage to oak, Quercus, in southern California by the goldspotted oak borer, Agrilus coxalis Waterhouse (Coleoptera: Buprestidae). Pan-Pacific Entomologist 84: 288-300
Coleman, Thomas W. and Steven J. Seybold. 2009. Tree Mortality from the Goldspotted Oak Borer in Oak Woodlands of Southern California. In Frankel et al. Fourth Sudden Oak Death Science Symposium June 09 Meeting Abstracts
Koch, F.H., D. Yemshanov, R.D. Magarey, and W.D. Smith. 2012. Dispersal of Invasive Forest Insects via Recreational Firewood: A Quantitative Analysis J. Econ. Entomol. 105(2): 438-450 (2012). Online at http://admin.forestthreats.org/products/publications/Dispersal_of_invasive_forest_insects.pdf. Accessed August 5, 2013.
USDA Forest Service Pacific Southwest State & Private Forestry R5-RP-022 Oct 28, 2008
Zeleznik, Joe. 2008. Economic impact of emerald ash borer on North Dakota communities. CityScan 2008