A new internal marking technique using the rare stable isotope 44Ca was developed for the gregarious endoparasitoid Cotesia glomerata, which parasitizes Pieris brassicae larvae. Released wasps marked their host caterpillars reliably through oviposition. The effect of selected flowering nectar plants on parasitoid flight and parasitism was quantified in the field in mark-release studies over a period of 3 days, as well as in laboratory experiments. They proved that female wasps successfully convert floral nectar from dill (Anethum graveolens) into flight energy.