Competition between phytophagous insects could be mediated by differential impact of natural enemies or by induced changes in host plant quality. The interaction between the aphids Sipha flava and Rhopalosiphum maidis on the shared host plant, Sorghum halepense, was evaluated during the fall and spring seasons in the presence or absence of natural enemies in a garden experiment. During the fall, S. flava was negatively affected by R. maidis in all treatments. However, during the spring season, S. flava was not affected by R. maidis when natural enemies were excluded, and positively affected by R. maidis with natural enemies present. Rhopalosiphum maidis was negatively affected by S. flava during the fall and spring seasons under all treatments. Laboratory experiments demonstrated that changes in host plant quality induced by S. flava attack and the presence of the parasitoid Lysiphlebus testaceipes negatively affected R. maidis, but not S. flava. Under abiotic conditions simulating fall and spring (lower temperature and daylength in the fall than in the spring), S. flava showed lower performance than R. maidis under "fall"Conditions, and both aphid species showed similar life history traits under "spring "Conditions. For S. flava, the time to the first reproduction and longevity were longer, and the nymphal production was lower, under "fall" conditions than under "spring"Conditions. Our results showed that S. flava is benefited when R. maidis and the parasitoid L. testaceipes are present but the abiotic conditions are not adverse.