Forest stand composition and impacts associated with Agrilus auroguttatus Schaeffer (Coleoptera: Buprestidae) and Agrilus coxalis Waterhouse in oak woodlands

From 2009–2011, we assessed the impact of the goldspotted oak borer, Agrilus auroguttatus Schaeffer, or its sibling species, Agrilus coxalis Waterhouse, at locations in southern California (denoted infested: ICA and uninfested: UCA), southeastern Arizona (AZ), and southern Mexico (MX). Our surveys examined forest composition of oak woodlands; the degree of injury and proportion of oaks infested with either A. auroguttatus (ICA and AZ) or A. coxalis (MX); and the progression of aerially mapped oak mortality in San Diego Co. (ICA). By most measures of impact that we evaluated, the effect on oaks by the two Agrilus spp. was relatively low in their native regions, but significantly higher by A. auroguttatus at ICA sites. Larger diameter red oak species have been the preferred hosts of A. auroguttatus in AZ and ICA sites, and red oaks greater than approx. 13 cm in DBH throughout California are likely at risk of injury from this invasive pest. At sites in AZ there was no evidence of infestation by A. auroguttatus on living or dead white oak species, whereas at ICA sites we recorded a minor amount of infestation by A. auroguttatus on living individuals of a white oak species (Quercus engelmannii Greene), but no mortality. In contrast, a white oak from MX sites (Quercus peduncularis) was more frequently infested by A. coxalis than were indigenous red oaks. Across all ICA sites, A. auroguttatus has infested 61% of the live larger diameter oaks and killed 13% of the oak component of the forest (vs. 4% infested and 2% dead in AZ, respectively). At survey plots near the predicted origin of the outbreak in CA, over 90% of the larger diameter red oaks have been infested. Nearly 90% of the dead oaks surveyed across all ICA sites showed evidence of previous injury symptoms from A. auroguttatus. Aerial oak mortality polygons associated with A. auroguttatus have expanded ∼50 km in nine years, but our analysis confirms that the outbreak appears to still be confined to San Diego Co. The distance of oak mortality polygons from the predicted origin of the outbreak explained the most variance in a principal component analysis. The invasive population of A. auroguttatus is a significant conservation and ecological threat to the oak woodlands of California and should be managed accordingly, especially by restricting firewood movement.

► A. auroguttatus has infested 61% and killed 13% of the oaks in California plots. ► The impact of the two Agrilus spp. was relatively low in their native regions. ► Larger diameter red oak species are the preferred hosts of A. auroguttatus. ► The levels of injury caused by A. auroguttatus were more severe in California. ► Aerial oak mortality polygons associated with A. auroguttatus have expanded ∼50 km.