I was browsing SciTech Daily Review (always a good source of breaking science stories) when this headline caught my eye: Ants drug their aphid slaves. What a tantalising title! It led me to a just-published article (T.H.Oliver et al. 2007) looking at how ants control the aphids that they 'farm'.
You probably know that some species of ants 'milk' aphids for the honeydew that they excrete. The ants keep the aphids close by and may defend them against predators. One issue for the ants is the need to prevent their aphids from wandering off – they may produce hormones that inhibit wing development in aphid nymphs, so that when these develop into adults they can't fly away – or they may even bite the aphids' wings off! The result: the aphids are less likely to disperse, so that the aphid colony gets larger, and the ants get more honeydew.
However, this wouldn't stop the aphids simply walking away to somewhere else on the plant, or to a different plant altogether. Oliver & his colleagues had read that simply having ants around can reduce aphids' movements, and designed a study to see whether this was due to chemicals released by the ants. After all, ants use hormone trails to lead others to a food source, and possibly to delineate territories.
The researchers filmed aphids' movements in each of three treatments: 1) the control, where they were placed on filter paper in a petri dish; 2) placed on filter paper that ants had walked on; and 3) straight after direct contact with ants. (And they used aphids from a species known to be controlled by ants, with aphids from a non-farmed species as a comparison.) The results: aphids walked more slowly in the presence of ant chemicals, compared to the control treatment, and this difference was statistically significant.
A second experiment looked at whether ants affected the speed at which aphids moved away from low-quality habitat – again, ant presence significantly reduced dispersal.
So what are the benefits (and the costs) of this behavior? Obviously the ants do quite well out of it, as they are able to maintain larger 'herds' of aphids and obtain more honeydew as a result. The aphids may gain protection from the ants – in this case, you can see how reduced dispersal in response to ant hormones might have evolved. (But if aphid dispersal is a way of reducing competition, then individual aphids in the herd may lose out if they're unable to walk away from the crowd.)
References: T.H. Oliver, A. Mashanova, S.R. Leather, J.M. Cook & V.A.A. Jansen (2007) Ant semiochemicals limit apterous dispersal. Proceedings of the Royal Society B (Biological Sciences) [published on-line October 2007 doi:10.1098/rspb.2007.1251]