Some species of ants farm aphids, protecting them on the plants where they are feeding, and consuming the honeydew the aphids release from the terminations of their alimentary canals. This is a mutualistic relationship, with these dairying ants milking the aphids by stroking them with their antennae. Dairying ants also milk mealybugs and other insects. Although mutualistic, the feeding behaviour of aphids is altered by ant attendance. Aphids attended by ants tend to increase the production of honeydew in smaller drops with a greater concentration of amino acids.

• Hooper-Bui, Linda M. (2008). “Ant”. World Book Encyclopedia. ISBN 978-0-7166-0108-1.
• Stadler, Bernhard; Dixon, Anthony F. G. (2005). “Ecology and Evolution of Aphid-Ant Interactions”. Annual Review of Ecology, Evolution, and Systematics. 36 (1): 345–372. doi:10.1146/annurev.ecolsys.36.091704.175531.

Some farming ant species gather and store the aphid eggs in their nests over the winter. In the spring, the ants carry the newly hatched aphids back to the plants. Some species of dairying ants (such as the European yellow meadow ant, Lasius flavus) manage large herds of aphids that feed on roots of plants in the ant colony. Queens leaving to start a new colony take an aphid egg to found a new herd of underground aphids in the new colony. These farming ants protect the aphids by fighting off aphid predators. 

• Hooper-Bui, Linda M. (2008). “Ant”. World Book Encyclopedia. ISBN 978-0-7166-0108-1.
• Wootton, Anthony (1998). Insects of the World. Blandford. ISBN 978-0-7137-2366-3.
• Stroyan, Henry G. (1997). “Aphid”. McGraw-Hill Encyclopedia of Science and Technology (8th ed.). ISBN 978-0-07-911504-1.

An interesting variation in ant–aphid relationships involves lycaenid butterflies and Myrmica ants. For example, Niphanda fusca butterflies lay eggs on plants where ants tend herds of aphids. The eggs hatch as caterpillars which feed on the aphids. The ants do not defend the aphids from the caterpillars, since the caterpillars produce a pheromone which deceives the ants into treating them like ants, and carrying the caterpillars into their nest. Once there, the ants feed the caterpillars, which in return produce honeydew for the ants. When the caterpillars reach full size, they crawl to the colony entrance and form cocoons. After two weeks, the adult butterflies emerge and take flight. At this point, the ants attack the butterflies, but the butterflies have a sticky wool-like substance on their wings that disables the ants’ jaws, allowing the butterflies to fly away without being harmed.

• Neary, John (1977). Insects and Spiders. Time-Life Books. pp. 78–79. ISBN 978-0-8094-9687-7.

Another ant-mimicking gall aphid, Paracletus cimiciformis (Eriosomatinae), has evolved a complex double strategy involving two morphs of the same clone and Tetramorium ants. Aphids of the round morph cause the ants to farm them, as with many other aphids. The flat morph aphids are aggressive mimics with a “wolf in sheep’s clothing” strategy: they have hydrocarbons in their cuticle that mimic those of the ants, and the ants carry them into the brood chamber of the ants’ nest and raise them like ant larvae. Once there, the flat morph aphids behave like predators, drinking the body fluids of ant larvae.

• Salazar, Adrián; Fürstenau, Benjamin; Quero, Carmen; Pérez-Hidalgo, Nicolás; Carazo, Pau; Font, Enrique; Martínez-Torres, David (2015). “Aggressive mimicry coexists with mutualism in an aphid”. Proceedings of the National Academy of Sciences. 112 (4): 1101–1106. Bibcode:2015PNAS..112.1101S. doi:10.1073/pnas.1414061112. PMC 4313836. PMID 25583474.