Analysis of plant leaf metabolites reveals no common response to insect herbivory by Pieris rapae in three related host-plant species

Studying the biochemical responses of different plant species to insect herbivory may help improve our understanding of the evolution of defensive metabolites found in host plants and their role in plant–herbivore interactions. Untargeted metabolic fingerprints measured as individual mass features were used to compare metabolite reactions in three Brassicales host-plant species (Cleome spinosa, Brassica oleracea, and Lunaria annua) to larval herbivore attack (Pieris rapae; Lepidoptera). Principal component analyses of metabolic fingerprints were able to distinguish among the three plant species and between uneaten control plants and plants that had been eaten. A large number of mass features (1186, 13% of mass features measured in control plants) were common to the three plant species. However, there were few similarities in the mass features that were induced (i.e. changed in abundance) following herbivory. Of the 87 and 68 induced mass features in B. oleracea and C. spinosa, respectively, there were only three that were induced in both plant species. By contrast, L. annua only had one mass feature induced by herbivory, and this was not induced in the other two plant species. The growth of the P. rapae larvae was poorer on the host plant L. annua than on B. oleracea and C. spinosa. The absence of common metabolites among the plants meant these induced responses could not be related to the performance of the herbivore. Thus, the response to herbivory by the same herbivore in these three host plants has evolved to be idiosyncratic in terms of the specific metabolites induced.