Local adaptation is promoted when habitat or mating preferences reduce gene flow between populations. However, gene flow is not only a function of dispersal but also of the success of migrants in their new habitat. In this study I investigated mating preference in conjunction with phenotypic plasticity using Aphidius parasitoids adapted to different host species. Males actively attempted to assortatively mate, but actual mating outcomes were strongly influenced by the relative size of the adult males. Results are discussed in the context of assortative mating in combination with the success of migrant males in mitigating gene flow between host-associated parasitoid populations. Acyrthosiphon pisum (Aphidoidea): Host; Aulacorthum solani (Aphidoidea): Host; Aphidius ervi (Aphidiidae): Parasite. Acyrthosiphon pisum, Aulacorthum solani: Hymenopteran parasites, Aphidius ervi; Aphidius ervi: Mating, Assortative mating, Pair formation, Population genetics, Genetic divergence, Evolutionary adaptation, Host-associated local adaptation, Hemipteran hosts, Acyrthosiphon pisum, Aulacorthum solani. Arthropods; Hymenopterans; Insects; Invertebrates; True Bugs. none