Insecticide treatment increases survival of coast live oaks (Quercus agrifolia) infected with Phytophthora ramorum, cause of sudden oak death, in coastal California

The introduced pathogen Phytophthora ramorum causes leaf, twig, and stem infections in a large proportion of the woody plants in coastal northern California forests. In most species, the resulting disease is not fatal. Coast live oak (Quercus agrifolia) is a dominant mast-producing tree in much of the coastal forest habitat from northern California to northern Mexico. Infected coast live oaks develop bleeding main stem phloem infections (cankers) within 2 m of the soil, a disease known as Sudden Oak Death. Saproxylic ambrosia beetles colonize a high percentage of infected trees, restricted to the bark overlying cankered areas. In July 2002, we inoculated 80 mature coast live oaks with P. ramorum and sprayed half of these trees with the insecticide permethrin twice per year through 2007. Beetles were attracted to and tunneled into bleeding trees within 3 months following inoculation. Although insecticide treatment prevented attacks only until May 2003, two months after beetle flight started, significantly fewer insecticide-treated than untreated trees were killed by 2010. Canker lengths measured in 2009 revealed considerable between-tree variation in disease expression. Evidence for resistance in coast live oak populations suggests that minimizing or preventing beetle attacks within the first year following infection may increase the likelihood of maintaining sustainable populations in the presence of the pathogen.