Assessing the risks posed by goldspotted oak borer to California and beyond
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Gen. Tech. Rep. PSW-GTR-251. Berkeley, CA: U.S. Department of Agriculture, Forest Service, Pacific Southwest Research Station: 317-329
Goldspotted oak borer, Agrilus auroguttatus, has killed approximately 27,000 mature oaks in southern California. Consequently, the future spread of this insect is a significant concern to many oak woodland managers in California and across the United States. "Risk" reflects the likelihood that A. auroguttatus will continue to spread in North America and the magnitude of ecological, economic, or social impacts that this insect may cause. This research project measured several critical biological parameters to refine spatial risk assessments for this insect. Cold tolerance testing of prepupae, the primary overwintering stage, indicates that this insect should be unable to survive in U.S. Department of Agriculture (USDA) Plant Hardiness Zones 2b – 5b. Some survivorship might occur in Zone 6a, but this outcome depends on the degree of cold acclimation that larvae may achieve. Host range testing with cut logs confirmed expectations that California black oak, Quercus kelloggii, and coast live oak, Q. agrifolia, are hosts and that Engelmann oak, Q. engelmannii, is not a host for A. auroguttatus. Our assays suggest that interior live oak, Q. wislizeni, and valley oak, Q. lobata, could be hosts, whereas Oregon white oak, Q. garryana, is unlikely to be a host. More study is needed to determine conclusively the host status of blue oak, Q. douglasii, canyon live oak, Q. chrysolepis, and cork oak, Q. suber, though field observations suggest canyon live oak can be colonized and killed by this insect. Flight mill studies indicate that adult females might fly up to 4 to 5 km/day and 9.3 km in their lifetime. Collectively, these results suggest that A. auroguttatus poses the greatest risk nationally to California and southern Oregon. If dispersal only occurs through flight, the effects from this insect will likely remain concentrated in southern California for the next 5 to 10 years. Potential movement of A. auroguttatus via infested firewood or other human-mediated pathways and the unknown host status of oak species from eastern North America introduce considerable uncertainty into these models. This refined risk assessment supports the value of efforts to slow the spread of A. auroguttatus.
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Venette, Robert C.; Coleman, Tom W.; Seybold, Steven J. 2015. Assessing the risks posed by goldspotted oak borer to California and beyond. In: Standiford, Richard B.; Purcell, Kathryn L., tech. cords. Proceedings of the seventh California oak symposium: managing oak woodlands in a dynamic world. Gen. Tech. Rep. PSW-GTR-251. Berkeley, CA: U.S. Department of Agriculture, Forest Service, Pacific Southwest Research Station: 317-329.