Densities of Agrilus auroguttatus and Other Borers in California and Arizona Oaks

Abstract

We investigated within-tree population density of a new invasive species in southern California, the goldspotted oak borer, Agrilus auroguttatus Schaeffer (Coleoptera: Buprestidae), with respect to host species and the community of other borers present. We measured emergence hole densities of A. auroguttatus and other borers on the lower stem (bole) of naïve oaks at 18 sites in southern California and on co-evolved oaks at seven sites in southeastern Arizona. We sampled recently dead oaks in an effort to quantify the community of primary and secondary borers associated with mortality-species that were likely to interact with A. auroguttatus. Red oaks (Section Lobatae) produced greater densities of A. auroguttatus than white oaks (Section Quercus). On red oaks, A. auroguttatus significantly outnumbered native borers in California (mean ± SE of 9.6 ± 0.7 versus 4.5 ± 0.6 emergence holes per 0.09 m² of bark surface), yet this was not the case in Arizona (0.9 ± 0.2 versus 1.1 ± 0.2 emergence holes per 0.09 m²). In California, a species that is taxonomically intermediate between red and white oaks, Quercus chrysolepis (Section Protobalanus), exhibited similar A. auroguttatus emergence densities compared with a co-occurring red oak, Q. kelloggii. As an invasive species in California, A. auroguttatus may affect the community of native borers (mainly Buprestidae and Cerambycidae) that feed on the lower boles of oaks, although it remains unclear whether its impact will be positive or negative.

Keywords: host suitability; invasive species; oak ecosystems; phloem and wood borer; population dynamics.

Figure 1

Mean densities of A. auroguttatus emergence holes on lower boles of dead oaks at CA sites containing host species pairs (A,B) and at AZ sites with more than one co-occurring host species (C,D). Significant differences between species are denoted by *. Sample sizes are in parentheses following species names. Oaks in Section Lobatae are red symbols; Section Protobalanus are gray symbols; and Section Quercus are white symbols.

Figure 2

Mean densities of emergence holes on the lower boles of dead oaks by borer group, tested separately by host species at CA sites containing host species pairs (A–D) and at Warner Springs, a site outside the current zone of A. auroguttatus infestation in CA (E). Letters above bars indicate significant differences according to Tukey’s HSD; sample sizes are the same as corresponding species and sites for Figure 1; and n = 10 trees (E). Oaks in Section Lobatae are red circles; Section Protobalanus are gray circles; and Section Quercus are white circles. X-axis categories are as follows: gsob = A. auroguttatus; bup + cer = other Buprestidae combined with Cerambycidae; bos = Bostrichidae; cur = Curculionidae (Scolytinae); sess = Sessiidae; and uk = unknown.

Figure 3

Mean densities of emergence holes from A. auroguttatus compared to that of other borer species combined, tested separately for each host species in CA (A) and AZ (B). * indicates significantly greater value according to restricted maximum likelihood estimation for the fixed effect borer group in mixed models. Sample sizes are: 90 trees across nine sites (Q. agrifolia); 91 trees across nine sites (Q. kelloggii); 40 trees across four sites (Q. emoryi); and 33 trees across three sites (Q. hypoleucoides).