Plant secondary metabolites and the performance of an exotic, generalist leafroller in California

Abstract

As an exotic, generalist leafroller the light brown apple moth (Epiphyas postvittana) has become established in coastal California where populations have been shown to experience strong negative density dependence from an unknown cause. This study was conducted to determine whether plant secondary metabolites could be a potential source of the density dependence. Experimental observations were focused on two of its commonly used host plants (ribwort plantain and Japanese cheesewood) to test for detrimental effects of secondary metabolites on larval performance under laboratory conditions. A plant defense induction assay was used to test for density-dependent responses of the two host plants to prior larval feeding damage, and a leaf disk assay was used to quantify the impact of increased concentrations of selected groups of chemical defenses (iridoid glycosides and polyphenols). Contrary to expectations, the results from these assays provided no evidence that E. postvittana either induced a sufficient plant response to impact larval performance, or that experimentally increased chemical defense concentrations affected larval performance. Consequently, the observed results suggest that the larvae of E. postvittana may have adaptations that inhibit induced responses in their host plants and/or that allow them to tolerate secondary defenses through detoxification and excretion. In conclusion, there is no evidence that induced plant defenses can account for the strong negative density dependence observed in the dynamics of E. postvittana populations in California and further research will be needed to identify the true cause of this density dependence.