Weevils of the genus Cionus (Curculionidae, Mecininae) sequester the iridoid glycosides (IGs) aucubin and catalpol from their host plants Scrophularia or Verbascum (Scrophulariaceae). Cionus hortulanus is the only member of the genus that feeds on both plant genera. We previously showed that sequestration patterns in C. hortulanus depend on the local host. To investigate whether IG patterns are driven by their availability in the hosts or genetic differences between populations, we collected C. hortulanus from S. nodosa in the field and reared them either on S. nodosa or on V. nigrum. The differences in IG concentrations were specific for the host plant upon which the weevils developed. Similar to monophagous species of the Cionini, individuals from S. nodosa had more aucubin than catalpol and mirrored the concentrations of their host plants. Specimens from V. nigrum, on the other hand, had higher concentrations of aucubin and of catalpol than their host. On V. nigrum, the ratio of catalpol to aucubin differed significantly between plant and beetle samples due to much higher catalpol concentrations in the weevils. Our data thus contradict genetically fixed differences between populations living on either plant but rather document the host plants' influence on the beetles' metabolism.