Title: GSOB ≠ SOD. Tree mortality from the goldspotted oak borer in oak woodlands of southern California.
Author: Coleman, Tom W.; Seybold, Steven J.;
Source: In: Frankel, Susan J.; Kliejunas, John T.; Palmieri, Katharine M. 2010. Proceedings of the Sudden Oak Death Fourth Science Symposium. Gen. Tech. Rep. PSW-GTR-229. Albany, CA: U.S. Department of Agriculture, Forest Service, Pacific Southwest Research Station. pp. 58-66
Publication Series: General Technical Report (GTR)
Description: A new threat to oaks (Quercus spp.) in California was identified in June 2008 following years of misdiagnosis. The goldspotted oak borer (GSOB), Agrilus coxalis auroguttatus Schaeffer (Coleoptera: Buprestidae), is aggressively attacking and killing three species of oaks in oak woodlands in San Diego County. About 20,000 coast live oaks (Quercus agrifolia), California black oaks (Q. kelloggii), and canyon live oaks (Q. chrysolepis) have died in a 4903 km² area centered on the Descanso Ranger District, Cleveland National Forest and Cuyamaca Rancho State Park. Oak mortality has been continuous for the past 8 years and occurs on all land ownerships.
The goldspotted oak borer was first collected in California in 2004. Although the collection history for this species is very limited (68 pinned specimens or records from 26 museum and private collections surveyed), early records date back to 1889 and 1905 in southern Mexico and southeastern Arizona, respectively, where there have been no reports of damage or mortality to oaks. In fact, prior to 2008, the biology and hosts of GSOB were unknown. Previous collection history for GSOB, the pattern of oak mortality in California since 2002, an expanding level of infestation, and geographical separation of oak stands in southeastern Arizona and southern California strongly suggest that GSOB was introduced into San Diego County. Although molecular genetic analyses of the populations of GSOB are pending, the proximity of Arizona and California and the morphological similarity of specimens from the Arizona and California populations both imply that GSOB was introduced into San Diego County from Arizona. Firewood movement represents the most likely pathway into California, however, in support of an alternative hypothesis of origin, there are anecdotal reports of oak firewood brought into this area of San Diego County from Mexico for 20 years.
Larval GSOB kill native oaks by feeding primarily on the wood surface at the interface of the xylem and phloem. Larvae feed in a meandering pattern on the wood surface and galleries typically have a dark appearance when bark is first removed. Larval feeding can reach high densities and cause areas of the cambium to die, eventually leading to tree mortality. Oaks infested with GSOB can be identified by thinning crowns, D-shaped adult exit holes, woodpecker foraging, and dark black or red staining on the bole or larger branches. Evidence of colonization by other insects is absent in GSOB-infested trees until trees have declined severely (for example, when bark has cracked around areas of dead cambium). Studies are currently underway to assess GSOB biology, enhance survey and monitoring techniques, determine the distribution of GSOB in southern California, record its impact to native oak stands, manage its populations through insecticide and firewood treatments, and determine
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Coleman, Tom W.; Seybold, Steven J. 2010. GSOB ≠ SOD. Tree Mortality from the Goldspotted Oak Borer in Oak Woodlands of Southern California. In: Frankel, Susan J.; Kliejunas, John T.; Palmieri, Katharine M. 2010. Proceedings of the Sudden Oak Death Fourth Science Symposium. Gen. Tech. Rep. PSW-GTR-229. Albany, CA: U.S. Department of Agriculture, Forest Service, Pacific Southwest Research Station. pp. 58-66
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