Ant-aphid interactions are mediated by host plant sex and ant colony nutritional status

Background/Question/Methods   Interactions between ants and aphids can span a continuum from mutualism to parasitism, where ants protect aphids in exchange for carbohydrate-rich honeydew or, alternatively, ignore or prey upon aphids as a source of protein. We investigated how ant colony nutritional status and host plant quality influence the strength and direction of ant effects on the aphid Aphis valerianae feeding on the dioecious host plant Valeriana edulis in a high elevation (3000 m) meadow in Colorado, USA. Using a combination of field observations and manipulative experiments, we tested whether ant-aphid interactions differed between male and female host plants and between treatments where boxed colony fragments of the ant Formica podzolica were fed carbohydrate or protein diets.

Results/Conclusions   There were four key results for the effects of plant sex: aphids were more abundant on female than male plants; ants tended aphids more on female than male plants; aphid performance was not was not influenced by plant sex; and predators reduced aphid abundance to a similar extent regardless of plant sex. These results document sex-biased herbivory due not to variation in host plant resistance, but instead to plant mediation of ant-aphid interactions. There were two key results for the effects of ant diet on ant-aphid interactions: protein- but not carbohydrate-fed ants tended aphids and aphid abundance was more than four-fold higher on plants associated with non-tending (carbohydrate-fed) ants. These results document that carbohydrate availability determined whether or not ants tended, but also that ant tending itself negatively affected aphid abundance. In summary, by showing how aphid feeding upon female host plants and carbohydrate deprivation favors ant attendance of aphids, our results illustrate that multiple factors shape the pairwise ant-aphid interaction.