Ambrosia beetles accelerate mortality of coast live oaks (Quercus agrifolia) infected by the exotic pathogen Phytophthora ramorum, the cause of sudden oak death in California

The epidemic known as Sudden Oak Death (SOD), caused by Phytophthora ramorum, continues to kill large numbers of coast live oaks, black oaks, and tanoaks in coastal California forests. Saprotrophic ambrosia and bark beetles selectively attack the bleeding cankers of P. ramorum infections on these tree species. In disease progression plots placed in naturally-infected forests, survival analyses conducted on each species showed that, for infected trees, beetle attacks reduced median survival to less than three years, compared with five to eight years for unattacked trees. In another study, coast live oaks were 1) inoculated with P. ramorum, and 2) wounded without inoculation, and half of each group was treated with insecticide to limit beetle attacks. Sticky traps were placed on the insecticide-treated inoculated and wounded trees to monitor beetle responses to the resulting treatments. In the 18 months following inoculation, greater than 95% of total beetle catch was from inoculated trees. The numbers of beetles trapped and the numbers of entrance tunnels produced by attacking beetles during the first beetle flights of the season predicted mortality five years later. After five years, the survival of insecticide-treated trees was greater than the untreated trees. Structural failure of living P. ramorum-infected coast live oaks was consistently associated with extensive tunneling by Monarthrum scutellare, the most abundant ambrosia beetle trapped on bleeding trees. The accumulated evidence suggests that attacks by both native and introduced ambrosia beetles may irreversibly alter the course of the disease by rendering ineffective the possible defensive responses of P. ramorum-infected trees.