Emerald ash borer (Agrilus planipennis; EAB) is an invasive wood-boring beetle in North America which has caused extensive ash (Fraxinus spp.) mortality. Closely related white fringetree (Chionanthus virginicus) also supports EAB populations, although it is a lower quality host than highly susceptible black ash (F. nigra). Mechanisms mediating white fringetree susceptibility and the influence of environmental stress on host defenses have not been assessed. Further, observations suggest that host trees grown in full sun are more resistant to EAB than those grown in shaded conditions. Previous studies have indicated that the activities of defense-associated enzymes and profiles of phenolic secondary metabolites within phloem tissue mediate levels of susceptibility in ash species. Here, we quantified constitutive and induced defenses and other chemical characteristics in the phloem tissue of white fringetree and black ash under differential light conditions. After sampling, all trees were inoculated with EAB eggs and larval development was assayed.
Results/Conclusions
We observed significantly lower activities of peroxidase, polyphenol oxidase, ß-glucosidase, chitinase and lignin concentration in white fringetrees compared to black ash at both the constitutive and induced levels. However, total phenolic, non-structural carbohydrate, and oleuropein concentrations were higher in white fringetree. Further, light limitation did not impact individual’s ability to mount induced defenses via enzymes or other chemical characteristics measured, although it did have impacts on growth and photosynthetic efficiency, indicating a response to the treatment. As suspected, EAB larval performance was worse on white fringetree, but light limitation had no effect. This study is the first to examine potential defense mechanisms responsible for variations in the suitability of white fringetree as a host for EAB. Our results suggest that phenolic profiles and abundance may be more important than the activity of tested defense associated enzymes. In addition, many studies have assessed the relationship between light and defense mechanisms, but usually in the context of light stress caused by high levels of irradiance. Our study directly investigates the potential relationship between light limitation and defense in deciduous trees but finds no observable impacts. As EAB continues to spread into the native range of white fringetree, and potentially to other novel hosts, our results may have implications for preemptive host detection and or chemical treatments to combat this invasive pest.