The rosy apple aphid Dysaphis plantaginea is one of the most important leaf sucking pests in pome fruit. As damage, caused by an infestation of a relatively small number of fundatrices in spring, easily exceeds the economic threshold level, pest management is crucial. Besides the use of IPM-compatible pesticides, natural enemies (ladybird beetles, parasitic wasps, saw flies...) can play an additional role in controlling aphids. In Europe, the solitary endoparasitoid Ephedrus persicae (Hymenoptera: Braconidae, Aphidiinae) is the dominant parasitic wasp attacking rosy apple aphid. As this parasitoid develops later than its host, control is determined by the population density and the parasitising efficiency of the wasp. The population increase within a season is determined by generation turnover and parasitizing capacity, a factor poorly understood in E. persicae. To be able to estimate the number of wasps required for successful control the parasitic behaviour was studied in semi-field circumstances. Artificially infested colonies of rosy apple aphid on apple trees grown in a greenhouse, were covered with cages of gauze in which young, mated female parasitic wasps were released. The number of aphids (alatae, apterae and mummies) as well as the number of adult parasitic wasps were recorded weekly until the end of infestation or parasitation. This test method allowed a comparison of the parasitizing efficacy of Ephedrus with that of the well-studied parasitoid Aphelinus mali (Haldeman), that efficiently controls the woolly aphid Eriosoma lanigerum (Hausmann).