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Abstract

Plants are part of biodiverse communities and frequently suffer from attack by multi- ple herbivorous insects. Plant responses to these herbivores are specific for insect feed- ing guilds: aphids and caterpillars induce different plant phenotypes. Moreover, plants respond differentially to single or dual herbivory, which may cascade into a chain of interactions in terms of resistance to other community members. Whether differential responses to single or dual herbivory have consequences for plant resistance to yet a third herbivore is unknown. • We assessed the effects of single or dual herbivory by Brevicoryne brassicae aphids and/ or Plutella xylostella caterpillars on resistance of plants from three natural populations of wild cabbage to feeding by caterpillars of Mamestra brassicae . We measured plant gene expression and phytohormone concentrations to illustrate mechanisms involved in induced responses. • Performance of both B. brassicae and P. xylostella was reduced when feeding simulta- neously with the other herbivore, compared to feeding alone. Gene expression and phytohormone concentrations in plants exposed to dual herbivory were different from those found in plants exposed to herbivory by either insect alone. Plants previously induced by both P. xylostella and B. brassicae negatively affected growth of the subse- quently arriving M. brassicae. Furthermore, induced responses varied between wild cabbage populations. • Feeding by multiple herbivores differentially activates plant defences, which has plant- mediated negative consequences for a subsequently arriving herbivore. Plant popula- tion-specific responses suggest that plant populations adapt to the specific communi- ties of insect herbivores. Our study contributes to the understanding of plant defence plasticity in response to multiple insect attacks.