Volatiles produced by the interaction of Rhopalosiphum padi and wheat, and semiochemicals which elicit the spacing behaviour of R. padi on wheat and oat, were evaluated in an olfactometer against the cereal aphid parasitoid Aphidius rhopalosiphi. The parasitoid was attracted by volatiles produced by the R. padi-wheat interaction at high density of aphids, but not by the R. padi-wheat interaction at low density. Compounds produced by the interaction of wheat or oat that was infested with a high density of aphids (approximately 9 aphids/cm2), 6-methyl-5-hepten-2-one (MHO), 6-methyl-5-hepten-2-ol (MHOH), 2-tridecanone (2-T), and methyl salicylate (MS), neither attracted nor repelled A. rhopalosiphi as pure compounds or in the naturally occurring mixture. The results are discussed in terms of the use of semiochemicals and parasitoids in the integrated management of aphid pests.