Mapping spread of the goldspotted oak borer (Agrilus auroguttatus)

The earliest signs of goldspotted oak borer (Agrilus auroguttatus, GSOB)-associated oak declines can be found in 1996 aerial photo images from the Descanso area of San Diego County. By 2014, GSOB had spread over a 4000 km2 area, with a patchy distribution similar to the early spread of the emerald ash borer (Agrilus planipennis). The GSOB occurs over about 26,250 ha (60 percent) of the oak woodlands in its primary range of infestation; with heaviest damage near rural housing and communities. Three GSOB diaspora have occurred 30, 40, and 55 km from closest known infestation areas, implicating human transport rather than adult flight as the agent of dispersal. Sequential aerial photo plots suggest that individual oaks (7916) took a median of 3 years to die after first sign of canopy decline, and 95 percent died within 8 years. Plots with >9 years of GSOB attack lost up to 76 percent of their oak canopies, with up to 25 percent of canopy loss/year in drought years. Oak canopy losses averaged about 3 percent of total area/year, or the equivalent of about four large, mature oak trees/ha/year. Years of high precipitation slowed GSOB mortality, followed by rapid increases in mortality when these years were followed by a drought year.