Methyl jasmonate as a tool to investigate induced responses of ash to the emerald ash borer

Emerald ash borer (EAB) (Agrilus planipennis; Coleoptera: Buprestidae), an exotic wood-boring insect, has killed tens of millions of ash trees since its accidental introduction from Asia and has shown potential to functionally extirpate ash (Fraxinus spp.) from North American forests. The Asian species, Manchurian ash, which shares a co-evolutionary history with EAB, is resistant while North American species, including white, green, and black ash are highly susceptible. In some model systems, induction of plant defenses (e.g. phenolics and defensive proteins) to chewing insects is mediated by the phytohormone methyl jasmonate (MeJA). Thus, we hypothesized that resistant Manchurian ash differs from susceptible ash species in its ability to respond to MeJA and that these responses can be characterized biochemically. We have found that exogenous application of MeJA to the outer bark of Manchurian and white ash increased the concentration of total phenolic compounds in phloem tissues of both species to the same degree, and differentially increased concentrations of individual compounds that are shared and specific to both species of ash. Furthermore, the exogenous application of MeJA to the outer bark of ash species in the field provided the same protection from EAB as did the application of a permethrin insecticide formulation labeled for use against wood-borers. We are also employing a proteomic approach to identify genes that are differentially upregulated in phloem of resistant and susceptible ash species in response to MeJA treatment. Our goal is to find alternative measures for control of EAB using the endogenous defense machinery of susceptible North American ash.