The ‘Evolution of Increased Competitive Ability’ (EICA) hypothesis predicts that invasive plant populations in the introduced range will have lower concentrations of defensive secondary chemicals than populations of the same species in the native range. An extension of this hypothesis to include inducible chemical defenses predicts that the relative inducibility of secondary compounds should be less variable in the native range than in the introduced range. This is because selection for inducibility should be relaxed in the introduced range in which insect herbivory is reduced. We tested this extension of the EICA hypothesis by comparing the induction of pyrrolizidine alkaloids in response to artificial mechanical injury in populations of houndstongue (Cynoglossum officinale) collected from its native range in Europe and from its introduced range in western North America. Pyrrolizidine alkaloids are implicated as defenses against insect herbivory. The hypothesis was supported in that within-population variation in inducibility was greater for the North American populations than for the European populations. The pattern suggests that selection by herbivore pressure constrains induction patterns of houndstongue in its native range.