Ant-Aphid Interactions: Are Ants Friends, Enemies, or Both?

Interactions between ants and aphids range from mutualistic to antagonistic. Understanding the ecological basis for such interactions requires understanding the costs and benefits to the aphids of ant-tending. Such an analysis is not simple, because ants can simultaneously have positive and negative effects upon aphids. The aphids Pleotrichophorus utensis Pack & Knowlton and Uroleucon escalantii Knowlton (both Hemiptera: Aphididae) are occasionally tended by Formica obscuripes Forel (Hymenoptera: Formicidae) at field sites in central Colorado. To compare the relative effects of protection and predation by ants on aphid abundance, we experimentally crossed the presence of the ants and other predators on host plants on which one or both aphids occur. Within a week of the start of the experiment, ants had a strong negative impact on aphid numbers that lasted the course of the experiment. Nonant predators initially had a weak negative effect on aphids, but by the end of the experiment, the negative effect of nonant predators was similar in magnitude to the effect of the ants. The negative effect of ants and other enemies on aphids was nonadditive; simultaneous predation by ants and other enemies was not as strong as expected from estimates of predation rates by only ants or only other enemies. This study suggests that ants simultaneously protect and prey upon aphids. We suggest selection to appease ants and to gain protection from ants can both be important forces generating ant-aphid mutualisms.