The novel interaction between Phytophthora ramorum and wildfire elicits elevated ambrosia beetle landing rates on tanoak, Notholithocarpus densiflorus

The 2008 wildfires in the Big Sur region of California's central coast-the first to occur in forests impacted by Phytophthora ramorum, the non-native, invasive pathogen that causes sudden oak death-provided the rare opportunity to study the response of scolytid and other subcortical beetles to this novel disturbance interaction. We used sticky card traps attached to the main stem of tanoak, Notholithocarpus densiflorus, the tree species most susceptible to P. ramorum, to determine which subcortical beetle species may be using tanoak as a host and to compare insect landing rates on these trees in forest plots impacted by neither disturbance, either wildfire or P. ramorum disturbance alone, or both disturbances combined. Xyleborinus saxesenii and Gnathotrichus pilosus, two species of ambrosia beetles (Coleoptera: Scolytidae), composed the majority (48% and 40%, respectively) of subcortical beetles landing on tanoaks during both years of the study. Adults of two species of a small, branch-feeding flatheaded borer (Anthaxia sp.; Coleoptera: Buprestidae) were also captured in relative abundance landing on tanoaks in the combined disturbance plots during the second year of the study. All but two of the 2779 scolytid beetles collected in this study were trapped on tanoaks in forest plots disturbed by P. ramorum and/or fire, and 75% of these scolytids were trapped during the fall 2009 season. The majority of scolytids were trapped on tanoaks in plots containing both disturbances (81% in 2009 and 79% in 2010), and, of the two disturbances, more scolytids were trapped on tanoaks in burned plots than in P. ramorum-infested plots (92% more in 2009 and 476% more in 2010). Semiochemicals emanating from the tanoaks upon which the sticky cards were attached-either in the form of host volatile compounds or scolytid aggregation pheromones-presumably affected ambrosia beetle landing rates, and greater quantities of moribund and recently-killed trees in the plots disturbed both by P. ramorum and fire may have led to greater population densities of ambrosia beetles in these areas. Our findings of elevated ambrosia beetle landing rates in Big Sur forests with mixed disturbances suggest a heightened threat to tanoak in these areas, but additional research is needed to determine the actual frequency of ambrosia beetle gallery initiation in living tanoaks and whether colonization hastens or leads to tree mortality.