Attraction of ambrosia and bark beetles (Coleoptera: Scolytidae) to coast live oaks (Quercus agrifolia) infected by Phytophthora ramorum

An epidemic caused by the introduced oomycete pathogen, Phytophthora ramorum, has infected and killed large numbers of oaks and tanoaks in coastal California since the mid 1990s (“sudden oak death”). Infected coast live oaks, Quercus agrifolia, develop stem cankers that are frequently attacked by bark and ambrosia beetles. In this study, designed to evaluate the roles of these beetles in tree mortality, asymptomatic coast live oaks were either mechanically inoculated or wounded without inoculation, then half of each group was treated with insecticide to prevent beetle colonization. Sticky traps were attached to the insecticide-treated trees to monitor beetle responses for one year after symptoms developed. Inoculated trees accounted for more than 95% of the seven beetle species trapped. Insecticide treatment delayed initial beetle attacks and lowered total attacks for the year. The delay in first beetle attacks led to decreased mortality after four years in the insecticide-treated trees. Canker surface area, numbers of beetles trapped, and the number of beetle attacks all were positively correlated with tree mortality. The involvement of bark and ambrosia beetles in accelerating both disease progression and mortality of these trees has implications for putative host resistance. Once beetles breach the bark and tunnel deep into the sapwood of mature coast live oaks, the introduction of their associated fungi and the subsequent arrival of other saprotrophic insects and microorganisms may overwhelm any constitutive resistance to the pathogen.