White ash survival in forested sites in the core of the EAB invasion

Since its discovery in 2002 in the Detroit metro area in southeast Michigan, emerald ash borer (EAB) (Agrilus planipennis Fairmaire) (Coleoptera: Buprestidae) has become the most destructive forest pest to invade North America. Millions of ash (Fraxinus spp.) trees have been killed and nearly 8 billion trees in U.S. forests are threatened by EAB. Previous studies in sites in the original core of the EAB invasion have reported 99% mortality rates for white ash (Fraxinus americana L.), green ash (F. pennsylvanica Marsh.) and black ash (F. nigra Marsh.). We have observed, however, that at least in some forested sites, an unexpectedly high proportion of overstory white ash trees remain alive, despite the presence of EAB in these core areas for at least 6 years. We used a combination of transects and variable and fixed radius plots to document the condition of white ash and other overstory trees in 30 forested sites in core areas of the EAB invasion. Number of EAB captured on two double-decker traps in each site was recorded as a relative index of EAB population levels. Each trap consisted of two purple or green prisms attached to a 3 m tall pvc pipe and baited with cis-3-hexanol or Manuka oil. Potential influence of overstory composition, density and size, surrounding vegetation and land use classes on white ash survival rates was assessed using field data and GIS analyses. Identification of traits consistently associated with high or low white ash survival can enable managers to project future EAB impacts.