Most aphid species produce some sort of sweet honeydew as waste while feeding on their host plants; ant-attended aphid species use this honeydew to attract ants. In many cases, the ants "milk" the aphids by stroking them to prompt release of the honeydew. While exploiting a colony of aphids, ants defend it as a food resource, protecting the aphids from predators. Aphid species that commonly rely on ant protection often lack defensive adaptations [$-a] found on species that don't interact with ants.
Ants tend aphids on a milkweed plant. Photo by dmills727.
Niesen et al. report the results of experiments performed ants attending colonies of milkweed aphids, Aphis asclepiadis, which are susceptible to a fungal pathogen that can wipe out aphid colonies in a matter of days. In two experiments, they introduced aphids into the ant-attended colonies, and tracked what the ants did to them. They found that
- Ants were more likely to remove the corpses of fungus-killed aphids than either the corpses of aphids killed by freezing or introduced live aphids; and
- Ants were more likely to remove live aphids contaminated with fungal spores (conidia) than live aphids without spores.
Nielsen, C., Agrawal, A., & Hajek, A. (2009). Ants defend aphids against lethal disease Biology Letters DOI: 10.1098/rsbl.2009.0743
Way, M. (1963). Mutualism between ants and honeydew-producing Homoptera. Ann. Rev. Entomology, 8 (1), 307-44 DOI: 10.1146/annurev.en.08.010163.001515