An understanding of how defensive metabolites from plants are processed by insects may help us develop more effective pesticides. Oligophagous insects can consume a wide range of different host plant species, but how these host plants vary in their metabolite compositions and the extent to which this variation affects the biochemistry of the insect herbivores is largely unknown. We studied the interactions between the oligophagous insect herbivore Pieris rapae (Lepidoptera: Pieridae) and five species of its larval host plants (family Brassicaceae and Cleomaceae) by examining untargeted metabolic fingerprints of the plants and the larval herbivores feeding on them. Visualisation of the metabolic fingerprints of the different host plant species showed highly distinctive clusters. Larvae could also be distinguished based on the species of host plant they fed on but clusters overlapped to a greater extent. The fingerprints of larvae feeding on Cleome spinosa plants were most distinctive due to a large group of abundant metabolites also found in high abundance in C. spinosa, but not in the other host plants examined. We conclude that host plants influence the biochemistry of their larval herbivores, and that some metabolites are conserved from one trophic level to the next.