In the process of feeding, aphids modify the physiology of their host plants in species-specific ways, and plant responses to these aggressions are often genotype- or cultivar-specific. Thus, different aphid species sharing a host can influence each other's fitness via plant-mediated interactions, usually with negative and asymmetric impacts. Here, we show that feeding by the sugarcane aphid, Melanaphis sacchari Zehntner, can improve the suitability of sorghum as a host plant for the bird cherry-oat aphid, Rhopalosiphum padi L. Whereas the reciprocal effects were generally negative for sugarcane aphid, the specific life history impacts varied with sorghum cultivar. Line ‘PI 550610′ was a more suitable host plant for both aphid species than the hybrid ‘P8500′, contrary to expectations based on the former representing a source of resistance to Schizaphis graminum (Rondani). Whereas coinfestation with sugarcane aphid had positive effects on bird cherry-oat aphid survival, development, and reproduction on both cultivars, the presence of bird cherry-oat aphid reduced the sugarcane aphid fecundity on both cultivars, although other negative effects (lower survival and delayed development) were evident only on the less suitable P8500. A second experiment using previously infested plants yielded similar results, although a gradual decay of aphid-induced effects postinfestation was apparent. Bird cherry-oat aphid is the second aphid species, after S. graminum, shown to benefit from sugarcane aphid facilitation on sorghum. We conclude that facilitation by sugarcane aphid at least partly accounts for the various aphid species forming mixed infestations with this invasive species in fields of susceptible sorghum cultivars.
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Vol. 46 • No. 2