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Feb 22

Animal Relationships: Ants and Aphids

One of the most vital turning points in the history of humanity was when we started to domesticate animals. Now, instead of going out to hunt animals for their meat in the wilderness, we could keep large groups of them close to our villages, enabling us to permanently settle down in one area. Not only that, but in addition to eating their flesh we could also take from them milk, eggs and wool. And so farming was born.

Well, human farming, that is. See, farming has been going on elsewhere in the animal kingdom for a long time before Homo sapiens even arrived on the scene. This is one such animal that is being routinely farmed across the world right now. It’s an aphid.

600px-Acyrthosiphon_pisum_(pea_aphid)-PLoS

Photo: Shipher Wu

Aphids are tiny insects, commonly called greenflies, which can often be seen clinging to plant stems in great numbers. They feed by inserting their long, thin mouthparts – the stylet – into the sap-carrying vessels of the plant. The pressure of the sap within the plant is sufficient to drive the liquid up the aphids’ mouthparts and into their stomach, so they don’t even need to suck. Sap is little more than sugary water, so to stop themselves inflating and eventually popping, the aphids must constantly get rid of the excess fluid via their anus.

The aphids are the ‘cattle’, producing this liquid – called honeydew – for their farmers. And who might their farmers be? Ants, of course. Ants are no strangers to farming (some species even grow crops of fungi) and they ‘milk’ the aphids by stroking their abdomens with their antennae, stimulating the release of the sweet liquid. It is then eagerly lapped up by the ants directly from the aphids’ rear ends.

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Photo: Jmalik

Farmer or Harmer?

The ants, like any respectable farmers, take good care of their herds, protecting them from predators. But whereas human farmers might protect sheep from wolves, ants protect the aphids from ladybirds and lacewings. A ladybird would make short work of a small group of aphids, but it stands little chance of success with the ant shepherds around. And their care doesn’t end there – during bad weather, the ants may construct temporary shelters for their aphids using soil and leaves. Nothing is too good for their little aphids, it seems.

But make no mistake; the ants are in full control here. Just as human farmers ensure that their dairy herds get the best pastures to eat from, the ants drive their aphid flocks to places on the plant that are particularly good for sap collection, and may even transfer them completely from wilted plants to healthy ones – not for the aphid’s wellbeing, of course, but to guarantee a steady supply of honeydew for the ants. To ensure that the aphids don’t go wandering away, the ants release a special chemical in their footprints, which acts like a fence to contain their flock within the ‘farm’.

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Photo: ViaMoi

Sometimes, however, the best interests of aphid and ant start to conflict. When an aphid colony gets too big, winged individuals are produced that are able to leave the plant to avoid overcrowding. For the ants, the fragmentation of the colony is bad new – less aphids, after all, means less honeydew. The ants therefore secrete a hormonal substance from a gland in their mouths, which is transferred to the aphids as the ants lap the honeydew from their rear ends. This substance prevents the onset of sexual maturity and wing development within the aphids, ensuring that they cannot easily disperse. If the wings do grow, the ants may bite them off.

In this way, the ant farmers can increase both the productivity and size of their herds to levels they would otherwise be unable to reach. It’s only one step down from true selective breeding. That’s probably next in the pipeline for the ant shepherds.

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