Science: How to restrain your cattle

One thing you might notice when walking through your garden, is big patches of small insects on the stem of a plant. These insects, known as aphids, feed on plant sap. However, this sap mostly consist of sugars, while containing only a small amount of amino acids, which are what the aphids actually need. The only way of getting enough of these amino acids is by drinking lots and lots of plant sap. This leaves them with a large supply of sugars, which they don’t need, and are actually harmful for them. The way they deal with this, is by excreting so-called honeydew, which are basically sticky droplets of sugar.

When you look closer at this patch of aphids, you might notice ants crawling over them.  Adult ants, as they don’t have to grow anymore, don’t really need many amino acids, but they do need lots of sugar. Needless to say, honeydew is a perfect solution for them!

Crematogaster cerasi tending aphids. Picture by Alex Wild (www.alexanderwild.com)

Crematogaster cerasi tending aphids. Note the drop of honeydew excreted by the aphid.  Picture by Alex Wild (www.alexanderwild.com)

These ants have taken to ’milking’ these aphids. However, a conflict of interest arises. The ants would like as many aphids as possible stacked together for maximum efficiency (just like we want to have our cows all in one place), however, for the aphids this is not good at all. If they stack up, they are a very easy target for potential predators, as they are way more easy to spot.

The ants deal with this problem in several interesting ways. First of all they offer the aphids protection, by aggressing any intruder, so the danger of stacking up is reduced (although a recent post by Alex Wild suggests some ants don’t take this job very seriously). Second, they cut off the wings of the winged individuals, so they can ’t just fly off.

A new mechanism of keeping the aphids together has been discovered by Oliver and coworkers in 2007. Apparently, ants release chemicals that actually stop the aphids from moving as much as they normally would.

They investigated this by putting ants on a paper for a while, so that this paper started to smell like ants. Afterwards they put the aphids on the paper and videotaped the movement of the aphids. The aphids moved much less when walking on paper smelling of ants than on normal paper.  However, whether this is because the chemicals actually prevent the aphids from moving a lot, or that they just feel more safe remains unknown. So while we still build a fence to keep our cows from escaping, aphids don’t escape from the ants just by smell. Nifty that!

Source:
Oliver, T.H., Mashanova, A., Leather, S.R., Cook, J.M., Jansen, V.A.A. (2007). Ant semiochemicals limit apterous aphid dispersal. Proceedings of the Royal Society B. 274, 3127-3131

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2 Responses to Science: How to restrain your cattle

  1. sedeer says:

    I’m not sure if you’ve come across this, but there’s an old idea that aphid herding may have come about because an aphid’s rear and an ant’s head look somewhat similar. I think it’s mentioned in The Ants, but I also managed to track down a blog post with a figure.

    • antyscience says:

      Thanks for that! Never heard that theory, it’s definitely interesting! I’ll chase down the part in The Ants when I have access to it again (it’s in my office, but ‘unfortunately’ I’m on fieldwork). Thanks again :)

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