Oak croak: Mortality of oaks (Quercus spp.) in southern California and undocumented damage by Agrilus coxalis (Coleoptera: Buprestidae)

Extensive oak mortality has occurred on the Cleveland National Forest in southern California since 2002. Mortality of coast live oak (Quercus agrifolia Nee) and California black oak (Q. kelloggii Newb.) has impacted 10,264 ha. Oaks holds dominate canopy positions in southern oak woodlands, and increase the aesthetics of southern California’s rolling hills. Acorns are also an abundant food source for wildlife. For six years, drought was considered the sole mortality factor. However, newly discovered damage from Agrilus coxalis Waterhouse may have a primary or secondary role in observed tree mortality. This is the first record of damage and mortality associated with A. coxalis. The presence of A. coxalis in southern California may represent an introduction or range expansion from Mexico or southeastern Arizona. Larval galleries are abundant on the sapwood surface of both oak species. Larval feeding can strip kill areas of the sapwood and result in external staining along the main stem. Thinning crowns and woodpecker foraging are frequently observed on declining oaks. Trapping studies were initiated to assess flight periodicity, distribution and landing rates of A. coxalis. Long-term plots were established to assess oak susceptibility, forest stand mortality, regeneration, and gather additional insect life history information. Prior to this discovery, no information was known about the larval habitats and hosts of A. coxalis.