Collection History and Comparison of the Interactions of the Goldspotted Oak Borer,Agrilus auroguttatusSchaeffer (Coleoptera: Buprestidae), with Host Oaks in Southern California and Southeastern Arizona, U.S.A.

Abstract

An invasive population of the goldspotted oak borer, Agrilus auroguttatus Schaeffer (Coleoptera: Buprestidae), is colonizing and killing three species of oaks in San Diego Co., California. However, the interactions of A. auroguttatus with oaks in its native range in southeastern Arizona have not been recorded. We present a complete inventory of the North and Central American collection records of A. auroguttatus and Agrilus coxalis Waterhouse from the literature and from a survey of the holdings of 27 museum and personal collections. We also discuss the relationship between this collection history and the behavior of A. auroguttatus as an intracontinental invasive species. Surveys of native populations of A. auroguttatus in oak forest stands from four mountain ranges in southeastern Arizona revealed injury patterns on Emory oak, Quercus emoryi Torrey, and silverleaf oak, Quercus hypoleucoides A. Camus, similar to those observed on other “red” oaks in California. No damage was observed on “white” oaks in Arizona, and observed only rarely on a white oak, Quercus engelmannii Greene, in California. In Arizona, adult emergence was confirmed from bark removed from Q. emoryi, representing the first developmental record of A. auroguttatus from a native host. Late instars of Agrilus sp. were also recovered from Q. hypoleucoides, but they were not reared to the adult stage for species identification. Nonetheless, our observations of damage and the presence of larvae in the same configuration and location in the outer bark as we would expect for A. auroguttatus suggest that Q. hypoleucoides is also likely a host. Two hymenopteran parasitoids, Calosota elongata Gibson (Eupelmidae) and Atanycolus simplex Cresson (Braconidae), and two likely coleopteran predators (Trogossitidae and Elateridae) emerged from, or were collected in southeastern Arizona from, Q. emoryi bark infested with A. auroguttatus. Based on the museum survey results, the morphological similarity of individuals from the California and Arizona populations, the spatial dynamics of the pattern of infestation in California, the geographic isolation of hosts in California from native populations of the beetle, and the proximity of San Diego Co. to southeastern Arizona, we hypothesize that A. auroguttatus was introduced to California from Arizona or less likely from the Mexican states of Baja California, Chihuahua, or Sonora, and that the introduction most likely occurred on oak firewood. Further, we hypothesize that the oak mortality in southern California is occurring from this intracontinental invasive species because the beetle is filling a vacant niche by colonizing and developing in non-coevolved trees with low host resistance in the absence of a diverse and coevolved insect natural enemy complex.