Previous field experiments indicated that the presence of the bird cherry-oat aphid, Rhopalosiphum padi (L.), on perennial grasses can decrease the effectiveness of predatory lacewings, Chrysoperla plorabunda (Fitch), in reducing populations of the Russian wheat aphid, Diuraphis noxia (Mordvilko). We tested the hypothesis that R. padi deflects predation away from D. noxia because it feeds in sites that are more accessible to predators. We quantified the behavior of lacewing larvae on crested wheatgrass plants bearing either D. noxia alone or an equal mixture of D. noxia and R. padi. On non-flowering plants, R. padi typically occurred on leaf sheaths or open blades, and was encountered and captured more often than D. noxia, which usually fed within immature, rolled leaves. Overall time-budgets of lacewings did not differ between the pure-D. noxia and mixed-species treatments, but >75% of the time spent consuming aphids in the mixed-species treatment was devoted to R. padi. On flowering plants, D. noxia usually aggregated on the flag leaf below the inflorescence, whereas R. padi occurred mostly on leaf sheaths. Predators again captured R. padi more often than D. noxia, and spent more time consuming aphids in the mixed-species treatment than in the pure-D. noxia treatment. These behavioral observations support the hypothesis that non-target prey can hamper the short-term effectiveness of biological-control agents against D. noxia.