Lipaphis erysimi (Kaltenbach) is a specialist crucifer feeding aphid and Myzus persicae (Sulzer) is a generalist feeding aphid. The foraging behavior of Diaeretiella rapae (McIntosh), a parasitoid with the ability to parasitize both of these species, was assessed using a series of attack rate and success bioassays, with turnip, Brassica rapa var rapifera, as the host plant. The attack rate of D. rapae was significantly greater on L. erysimi than on M. persicae when aphids were feeding on turnip leaf discs in Petri dishes, irrespective of the aphid species upon which the parasitoids were originally reared. Attack rate bioassays with leaf discs absent, using both satiated and starved aphids, revealed that background chemistry and internal aphid chemistry may have small effects on attack rate. Excision of D. rapae pupae from mummy cases and subsequent use of the fully developed adults in attack rate bioassays showed that cues received by D. rapae at the time of adult emergence provide cues that prime D. rapae to attack L. erysimi at a greater rate than M. persicae. However, the relative success of D. rapae on these two aphid species, in terms of the percentage of attacks resulting in a successful adult parasitoid, was not significantly different. Parasites diseases and disorders; Parasites; Insect parasites; Hosts; Insect hosts Diaeretiella rapae (Aphidiidae); Hemipteran hosts; Lipaphis erysimi & Myzus persicae; Parasite attack & success rate on specialist & generalist hosts Lipaphis erysimi; Myzus persicae (Aphidoidea ); Hymenopteran parasites; Diaeretiella rapae; Parasite attack & success rate on specialist & generalist hosts; Plant hosts; Brassica rapa var. rapifera none