Melitaea cinxia; AGE-; Larvae, defensive responses to hymenopteran parasite relationships; DEFENSIVE-BEHAVIOUR; Larval responses to hymenopteran parasite attack, age relationships; HYMENOPTERAN-PARASITES; Cotesia melitaearum; Parasite behaviour & movement within & among host patches; Defensive responses & metapopulation level dynamics of interactions; POPULATION-DYNAMICS; Metapopulation dynamics of interaction with hymenopteran parasites Cotesia melitaearum; FORAGING-; Behaviour & movement within & among lepidopteran host patches; Individual & metapopulation aspects; LEPIDOPTERAN-HOSTS; Melitaea cinxia; Behaviour & movement within & among host patches; Host defensive responses & metapopulation level dynamics of interactions; POPULATION-DYNAMICS; Metapopulation dynamics of interaction with lepidopteran hosts Foraging behaviour and movement within and among host patches of the specialist parasitoid wasp Cotesia melitaearum (Braconidae) attacking the larvae of Melitaea cinxia (Nymphalidae) were studied in the field and in the laboratory. In the field, female wasps aggregated in large host groups in the autumn and caused positive spatial density-dependent parasitism in the field. Wasps stayed longer with groups of pre-diapause caterpillars than with post-diapause caterpillars, but attacked them less frequently. In the laboratory, wasps attacked larger larvae more readily than smaller larvae. Also in the laboratory, wasps exposed to larvae outside their protective webs showed differences in the rates at which they attacked larvae fed different diets, implicating host plant-derived chemicals as proximate cues for foraging wasps. Mark-recapture studies indicated that there was a low rate of successful movement of wasps among groups of young larvae within a habitat patch in the autumn and no successful movement of wasps across non-habitat. In contrast, wasps moved frequently among groups of late-instar caterpillars in the spring. Host caterpillars of different ages responded very differently to wasp attacks. Pre-diapause larvae remained in groups and used collective head-jerking behaviour to defend themselves, whereas post-diapause larvae dispersed away from the group immediately after being attacked. Population and metapopulation level dynamics of the host-parasitoid interaction are discussed in light of these observations of the behaviour of individual wasps.