This article is reposted from the old Wordpress incarnation of Not Exactly Rocket Science. The blog is on holiday until the start of October, when I'll return with fresh material.

In your garden, there's a fair chance that a farmer is currently tranquilising her livestock with a chemical cocktail she secretes from her feet. Don't believe me? Look closer...

Humans aren't the only species that farms other animals for food - ants do it too and their herds consist of aphids. They feed on plant sap and excrete a sweet and nutritious liquid called honeydew, which the ants drink.

In return, the ants run a protection racket, defending the aphids from predators like ladybirds. It seems like a nice two-way partnership that suits both partners, and aphid colonies tended by ants tend to be larger than unattended ones. But new research from two London universities suggests that ants are manipulating their herds more than previously thought.

Aphid-farming ants similar problem to human farmers - their herds are likely to wander away and it's in the ants' interests to prevent this. Earlier work showed that they sometimes bite the wings off aphids that have them or produce chemicals from glands in their jaws that subdue the development of wings in the first place.

None of that stops the several wingless individuals from just walking away, so the ants use another trick. Thomas Oliver from Imperial College London found that the black garden ant (Lasius niger) secretes chemicals in its footsteps that effectively tranquilise aphids.

Ants use chemical secretions to recognise colony members and to mark trails that their sisters can follow to sources of food. In this case, their trails sedate the aphids and stop them from walking off to another plant.

Oliver and colleagues placed aphids onto pieces of filter paper in a Petri dish and filmed their movements for five minutes. Some of the paper had previously been walked on by ants and carried traces of the chemicals they secreted.

Compared to the controls, the aphids that walked in the footsteps of ants moved about 30% more slowly than those walking on untouched paper. On average, they also dispersed more slowly, taking longer to reach the edge of the Petri dish. The ant chemicals only hindered species of aphids that are farmed by ants - a third species, with no ant partners, was unaffected.

Sedating the aphids could also stop them from leaving the host plant in indirect ways. Aphids only develop winged forms if the colony gets crowded and individuals brush against each other frequently. If they are sedated, they brush against each other less, and are less likely to develop forms that could fly off.

You could argue that it's in the aphids' interests to respond to the ants' chemicals in this way; after all, it keeps them close to their protectors. But it also risks increasing the colony size to the point where the host plant suffers, along with the aphids' health. And staying put leaves multiple generations vulnerable to attacks from parasitic wasps that can avoid the ants' attacks.

While it pays the aphids to have ant guardians, Oliver thinks that the ants are definitely manipulating the relationship to get the most out of it. They get honeydew-on-tap and they can keep their living larders away from the territories of their competitors. When herded aphids do disperse, it's at the whim of the ants, who carry individuals to new host plants within the colony's territory.

More on ants:

Reference: Oliver, Mashanova, Cook, Leather & Jansen. 2007. Ant semiochemicals limit apterous aphid dispersal. Proc Roy Soc B doi:10.1098/rspb.2007.1251