Ants and aphids have a symbiotic (or mutually beneficial) relationship. The aphids provide the ants with a food-source – the sugar-rich honeydew they excrete when eating plants – and, in return, the ants protect the aphids from ladybirds and other insects that prey on them.
To ensure a constant supply of honeydew, some ant species cultivate large numbers of aphids, and prevent them from straying too far from the colony by biting and damaging, or even completely removing, their wings. The ants also secrete a chemical from their mandibles which inhibits wing development in juvenile aphids.
Ants communicate with each other using a large repertoire of chemical signals, which are actively secreted onto surfaces from exocrine glands on the legs. These signals can recruit nest-mates to food sources, and are also used to mark a colony’s territory. Ants secrete chemicals passively too – as an ant moves, hydrocarbons are shed from the cuticle (the waterproof outer lining of the exoskeleton), leaving a chemical trail.
Ants use behavioral signals called semiochemicals to manipulate aphids’ nervous systems. (Ant’s own behaviour can be manipulated too, by parasitic fungi.) Earlier work had shown that the presence of ants can somehow tranquilize aphids and limit their motor functions, but whether or not this required direct contact between the ants and aphids was unclear.
Using digital video cameras to measure their walking speeds, Tom Oliver of Imperial College London, and colleagues from Royal Holloway and the University of Reading have now shown that aphids move much more slowly on paper that had previously been walked on by ants than on plain paper. They believe that the chemicals laid down in the ants’ footprints are used to maintain an aphid “farm” near the ant colony.
Maintaining a populous aphid farm in a small area is obviously beneficial to the ants, as it would provide them with large quantities of honeydew. However, the relationship between the two species is complex, and it seems that the ants’ manipulation of the aphids’ behaviour is exploitative.
Normally, aphids wander off to new locations when conditions become crowded, to establish new populations nearby. And although ant-attended aphid populations are bigger and live longer than those not attended by ants, the ants prevent the aphid dispersal that is necessary to maintain a stable meta-population, and makes the aphids more vulnerable to parasites.
Oliver and his colleagues suggest that the aphids might use the chemical footprints to stay inside the area within which they are protected. They also note that their findings migth be useful in developing methods for controlling the dispersal of blackfly, which are considered as pests and sometimes carry diseases.
Oliver, T. H., et al. (2007). Ant semiochemicals limit apterous aphid dispersal. Proc. R. Soc. B. doi: 10.1098/rspb.2007.1251. [Abstract]