The Glanville fritillary butterfly Melitaea cinxia feeds upon two host plant species in Aland, Finland, Plantago lanceolata and Veronica spicata, both of which produce iridoid glycosides. Iridoids are known to deter feeding or decrease the growth rate of many generalist insect herbivores, but they often act as oviposition cues to specialist butterflies and are feeding stimulants to their larvae. In this study, two iridoid glycosides (aucubin and catalpol) were analyzed by micellar electrokinetic capillary chromatography. We measured the spatial and temporal variation of iridoid glycosides in natural populations of the host plants of M. cinxia. We also analyzed the aucubin and catalpol content in plants in relation to their use by ovipositing females, and in relation to the incidence of parasitism of M. cinxia larvae in natural populations. The mean concentrations of aucubin and catalpol were higher in P. lanceolata than in V. spicata, and catalpol concentrations were higher than aucubin concentrations in both host species. Plantago lanceolata individuals that were used for oviposition by M. cinxia had higher aucubin concentrations than random plants and neighboring plants. Additionally, oviposition and random plants had higher catalpol concentrations than neighboring plants, indicating that ovipositing females select for high iridoid glycoside plants or that oviposition induces iridoid glycoside production in P. lanceolata. Parasitism by the specialist parasitoid wasp Cotesia melitaearum occurred most frequently in larval groups that were feeding on plants with low concentrations of catalpol, irrespective of year, population, and host plant species. Therefore, parasitoids appear to avoid or perform poorly in host larvae with high catapol content. Nutrition; Diet; Reproduction; Reproductive behaviour; Parasites diseases and disorders; Parasites; Insect parasites; Hosts; Insect hosts; Land and freshwater zones; Palaearctic region; Europe Melitaea cinxia (Nymphalidae); Food plants; Plantago lanceolata & Veronica spicata; Plant iridoid glycoside content, effects on oviposition choice & hymenopteran parasite prevalence; Egg laying; Hymenopteran parasites; Cotesia melitaearum; Finland; Aland Islands; Food plant chemistry effects on oviposition choice & hymenopteran parasite prevalence Cotesia melitaearum (Braconidae); Lepidopteran hosts; Melitaea cinxia; Prevalence in host larvae; Finland; Aland Islands; Prevalence in lepidopteran host larvae, food plant chemistry effects none