The effect size of aphid‐tending ants in an agricultural tri‐trophic system

Abstract

Most studies regarding ant–aphid interactions focus only on the direct effects of ants on tended aphids and aphidophagous predators, or the indirect effects on the host plant. Studies evaluating the effects of aphid‐tending ants on more than one trophic level are rare and evaluate only the presence or absence of such effects. Here we assessed the effect sizes of ants in a tri‐trophic system (common bean plants, aphids and lacewing larvae). We tested if the presence of aphid‐tending ants has positive effects on aphid abundance and host‐plant production and negative effects on aphid predator abundance. We also hypothesized that aphid‐tending ants affect more intensely trophic levels that are more directly related to them (i.e., first aphids, then aphid predators and then host plants). We tested these hypotheses in field mesocosms experiments using the presence and absence of ants. We found that aphid‐tending ants have great positive effects on final aphid abundance. Ants also positively affected the number of seeds; however, it was not possible to measure the effect size for this trophic level. Furthermore, ants had negative effects on lacewing larvae only at first release. The effect size of ants was greater for aphids, followed by lacewing larvae, and with no effects on the number of seeds produced. Ants positively affect aphids and host‐plant production, probably by way of honeydew collection preventing the development of entomophagous/saprophytic fungi. On the other hand, ants negatively affect lacewing larvae by excluding them from the host plant. In natural systems, several ant species may attend aphids, differently affecting the organisms of the various trophic levels within the ant–aphid interaction, thereby obscuring the real effect size of ants. Assessing the effect size of aphid‐tending ants on the organisms involved in ant–aphid interactions provides more realistic information about the effects of this interaction on natural systems.