The traits used by phytophagous insects to find and utilize their ancestral hosts can lead to host range expansions, generally to closely related hosts that share visual and chemical features with ancestral hosts. Host range expansions often result from ecological fitting, which is the process whereby organisms colonize and persist in novel environments, use novel resources, or form novel associations with other species because of the suites of traits that they carry at the time they encounter the novel environment. We examined the phytochemical underpinnings of host-switching in emerald ash borer, Agrilus planipennis (EAB) an ecologically and economically important invasive wood boring beetle. Once thought of as an ash (Fraxinus spp.) tree specialist, recent studies have revealed a broader potential host range than was expected for this insect, including white fringetree (Chionanthus virginicus), a North American native tree. We examined the susceptibility of a pair of closely-related ash (Fraxinus) and fringetree (Chionanthus) species from both the native and introduced range of this beetle and examined the extent to which phytochemical equivalence in the profile of phenolic metabolites in the phloem of these hosts predicted their degree of susceptibility to this beetle.
Results/Conclusions
The resistant Manchurian and susceptible black ash exhibited a striking similarity in profiles of phenolic metabolites in the phloem, but quantitative variation in some metabolites was apparent. Likewise, the resistant Chinese fringetree and the susceptible white fringetree exhibited similarities in their profiles of phenolic metabolites, but they were significantly different both qualitatively and quantitatively from each other and from the ash species. The resistant ash and fringetree species also exhibited higher activities of some phenolic oxidizing enzymes that may be involved in metabolic activation of phenolics. Alternate hosts displayed significant similarities in phloem chemistry with ancestral hosts, indicating that EAB is generally using ancestral traits to find and utilize these hosts, but there is significant qualitative and quantitative variation in some of these chemical profiles as well. This indicates that EAB likely requires a certain degree of chemical similarity in hosts to foster novel interactions but exhibits sufficient plasticity to be able to utilize somewhat divergent hosts. However, hosts that exhibit a high degree of phytochemical equivalence can still vary in resistance to this beetle if other traits that they possess modify the activity of their chemical defenses.