1. The strength of interaction between the specialist parasitoid Cotesia melitaearum and the host butterfly Melitaea cinxia is influenced by the coincidence of the adult stage of the parasitoid with the larval stage of the host. 2. We show that there is great variation in this developmental synchrony among local populations and among years, ranging from complete synchrony to complete asynchrony. 3. The causal mechanism is early spring temperature, which affects parasitoid development differently than the development of the host. 4. At cool air temperatures the dark-coloured and mobile host larvae benefit from basking in the sun, while the white and immobile parasitoid cocoons develop slowly in shaded microclimates, becoming adults after hosts have pupated and are no longer available for parasitism. At warm temperatures many adult wasps emerge in time to parasitize host larvae. 5. We show that the host-parasitoid synchrony influences subsequent parasitoid population size and the rate of colonization of previously uninhabited host populations, contributing to parasitoid metapopulation dynamics. 6. We detected no direct effect of the phenological synchrony on local host population size, but the synchrony is likely to be important for overall host metapopulation dynamics via variation in the rate of colonization by the parasitoid Melitaea cinxia (Nymphalidae); Life cycle and development; Hymenopteran parasites; Cotesia melitaearum; Climate and weather; Temperature; Early spring temperature; Finland; Aland Islands, phenological synchrony, relations with early spring temperature Cotesia melitaearum (Braconidae); Life cycle and development; Lepidopteran hosts; Melitaea cinxia; Climate and weather; Temperature; Early spring temperature; Finland; Aland Islands, phenological synchrony, relations with early spring temperature none