Resource- vs. Enemy-Mediated Interactions Between Cereal Aphids (Homoptera: Aphididae) on a Common Host Plant

We examined the impact of the bird cherry-oat aphid, Rhopalosiphum padi (L.), on the per capita rate of increase of the Russian wheat aphid, Diuraphis noxia (Mordvilko), both in the presence and absence of a shared predator. In greenhouse and field experiments, we manipulated the initial density and composition of aphids on crested wheatgrass, Agropyron desertorum (Fischer ex Link) Schultes, and later introduced the common green lacewing, Chrysoperla plorabunda (Fitch), to half of the plants per treatment. A direct, competitive effect of R. padi on D. noxia was observed only at extraordinarily high aphid densities (\u3e2,000 individuals per greenhouse seedling). Lacewing predators significantly reduced the rate of increase of D. noxia in each experiment, but they were sometimes less effective on plants that received a mixture of both aphid species than they were on plants that received an equivalent number of D. noxia alone. The presence of R. padi may have reduced predation on D. noxia because the former species is more accessible to lacewing predators or because its faster growth rate caused it to outnumber D. noxia on plants that initially received an equal ratio of both species. Our results suggest that lacewing predators can suppress or even eliminate D. noxia populations on range grasses, but their effectiveness may be hampered by the presence of other cereal aphids, at least over short time scales