none Aphidius rhopalosiphi; HEMIPTERAN-HOSTS; Myzus persicae, Rhopalosiphum padi & Sitobion avenae; Host selection by parasitoid, importance of host plant cues Myzus persicae; Rhopalosiphum padi; Sitobion avenae; HYMENOPTERAN-PARASITES; Aphidius rhopalosiphi; Host selection by parasitoid, importance of host plant cues; PLANT-HOSTS; Wheat; Plant importance in selection by hymenopteran parasitoid Female Aphidius rhopalosiphi De Stefani Perez reared on Rhopalosiphum padi (Linnaeus) made equal numbers of encounters with another suitable host aphid (Sitobion avenae (Fabricius)) (both Homoptera: Aphididae) in arenas where the aphids were on either pieces of wheat leaf or moist filter paper. However, the number of oviposition stabs made in arenas with wheat was far higher than with filter paper, demonstrating the importance of host-plant cues in the acceptance of the aphids by the parasitoid. Diet reared aphids (even if subsequently fed on wheat) appeared to be accepted more readily than totally wheat reared individuals. In another arena experiment, there were more encounters with and more stabs made into both S. avenae and a non-host aphid (Myzus persicae (Sulzer)) on wheat than on Brussels sprouts (a host plant of M. persicae), again showing the over-riding importance of host-plant cues.