An epidemic caused by an invasive pathogen, Phytophthora ramorum , is affecting forests from Big Sur, California, to southern Oregon. Species in eight plant families are known hosts, but oaks (Quercus spp.) and tanoaks (Lithocarpus densiflorus ) are most severely impacted. Infected coast live oaks, Q. agrifolia , consistently produce a viscous, red to black exudate (bleeding), that is associated with subcortical cankers on the main stem. In plots monitored for two years, up to 52% of living symptomatic (bleeding) trees were colonized by ambrosia (Monarthrum spp.) and oak bark (Pseudopityophthorus pubipennis ) beetles. During this period, every symptomatic tree that died (n=21) had been colonized by beetles while the foliage was green. This behavior is not characteristic of these saprotrophic insects, which normally infest trees in later stages of decline. In an effort to understand why beetles colonize P. ramorum -infected trees, we monitored the association of beetles with Q. agrifolia in natural forests. During beetle flight periods for two years, sticky traps were placed on trees classified as non-symptomatic, symptomatic (bleeding only), and symptomatic/beetle-infested. The mean number of beetles trapped on symptomatic/beetle-infested trees was significantly greater than on non-symptomatic and symptomatic trees. This implies pheromone-mediated colonization of diseased portions of trees, which may contribute to tree mortality. There was no significant difference in catch between symptomatic and non-symptomatic trees. Since infected mature trees have not been observed during the course of the disease in the absence of ambrosia and bark beetles, their ability to recover from P. ramorum infection is uncertain.
Species 1: Coleoptera Scolytidae Monarthrum scutellare (Oak ambrosia beetle)
Species 2: Coleoptera Scolytidae Monarthrum dentiger
Species 3: Coleoptera Scolytidae Pseudopityophthorus pubipennis (Western oak bark beetle)
Keywords: sudden oak death, Phytophthora ramorum
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