Mechanisms of Antixenosis and Antibiosis of Ash Against Emerald Ash Borer

Emerald ash borer (EAB), Agrilus planipennis (Coleoptera: Buprestidae), is an invasive forest pest causing widespread mortality of ash (Fraxinus spp.) in North America. Host resistance research and the development of resistant hosts offers a promising strategy for the long-term conservation of ash and management of EAB. Manchurian ash (F. mandshurica) shares an evolutionary history with EAB in Asia, resulting in its greater resistance relative to naive North American ashes. In the following studies I investigate antixenosis and antibiosis mechanisms of resistant and susceptible ashes. Antixenosis in Manchurian ash was demonstrated by quantifying substantially lower oviposition on this species relative to North American ashes. The potential underlying mechanisms of antixenosis were addressed by profiling the bark and canopy volatile organic compounds (VOCs) emitted by susceptible black (F. nigra) and resistant Manchurian ashes and major species differences in VOC profiles were demonstrated. To address antibiosis, the physiological responses of EAB larvae that had fed on Manchurian, white (F. americana), and green (F. pennsylvanica) ash were quantified. It was found that antioxidant and quinone-protective enzyme activities of larvae feeding on Manchurian ash were substantially higher, suggesting that larvae feeding on Manchurian ash experience relatively high levels of reactive oxygen species and quinone stress. Manchurian ash demonstrated substantially higher activities of defense-associated enzymes and reactions than black ash, especially phenolic-oxidizing enzymes. These results support the conclusions of the larval physiology study that Manchurian ash appears to be able to generates greater amounts of quinone- and ROS-stress in vivo than North American ashes. Lastly, larval performance and bark phenolic chemistry and physiology were compared for Manchurian ash that were experimentally girdled or not. Girdling reduced larval performance by half but bark defenses did not differ by treatment indicating that decreases in larval performance are associated with factors other than a reduction in levels of host defenses. It was concluded that Manchurian ash expresses antixenosis, which may be driven by the emission of certain volatiles. Also, that antibiosis appears to be related to the ability of Manchurian ash to generate an oxidatively stressful diet for larvae and larval success in compromised trees does not stem from a reduction in defense levels