Dear Readers, I have been watching the aphid colony that is slowly developing on my Cosmos plants with some interest. It started with a group of about a dozen black bugs, but as you can see, the numbers are growing, and this is because they are being ‘farmed’ by the black ants (Lasius niger) who live under the doorstep.
A high proportion of the food of a black ant colony is not chopped-up insects as you might expect, but honeydew (up to 92% of a wood ant (Formica sp.) colony for example), and this is secreted in abundance by the aphids. To get the protein that they need, these bugs have to drink prodigious quantities of plant sap, which is mainly sugar and water – it contains only about 1.8% protein. The result is that aphids are more or less continuously producing honeydew. Sometimes they flick it away from their bodies and it rains down in a sticky shower – my buddleia were particularly afflicted earlier in the year to the extent that my green wheelie bin, which was under the shrub, had its lid stuck shut. But some aphids have made a relationship with ants: the ants ‘tickle’ the aphids with their antennae, and the aphids secrete a globule of honeydew. In the little film below you can just about make out an ant walking amongst the aphids and ‘tapping’ them in search of food.
The relationship with the ants is not altogether benign: the ants sometimes also ‘cull’ their herds, eating the new young aphids. Interestingly, in his book ‘Ants’, Richard Jones explains that ants are more likely to ‘milk’ aphids that have already been milked by a colony mate, and to eat one that has not: ants may place a chemical marker on an aphid, to indicate productivity or even ownership.
Overall, though, the aphids benefit greatly from the presence of the ants, who will pick them up and move them to a new stem or even new plant if the supply of sap seems to be drying up. I dead-headed some of the Cosmos and as a result, the stem that one flowerhead was on started to dry up. There are far fewer aphids on it now, and far more on a juicier, productive stem. As these black aphids cannot fly, I suspect that the ants have been relocating them. Furthermore, the ants are ferocious predators of hoverfly larvae, ladybirds and lacewings, and indeed any other invertebrate that threatens their ‘herd’. In some species of aphid, the relationship between them and their ‘farmers’ has become so tightly-knit that the aphids can’t survive without the ants.
Perhaps the most sophisticated form of aphid ‘farming’ occurs between the Japanese ant (Lasius japonicus) and the Japanese Mugwort Aphid (Macrosiphoniella yomogicola ). As per usual, the ant ‘farms’ the aphid for its honeydew, but it goes a step further. The aphid comes in two colour forms – green aphids and red aphids. The green aphids produce more honeydew, but the red ones produce a chemical that inhibits the mugwort on which they feed from flowering. Once the plant flowers, it will die, and so will the aphids. The ants ‘cull’ the aphids until they’re at a ratio of 2 green aphids for every red aphid. In this situation, the ants get enough honeydew, but the flowering of the plants is delayed or prevented, which means that the aphid colony will survive. This is an extraordinary manipulation, almost akin to selective breeding.
I have been fascinated by ants ever since I was a child, when I used to try to mark individual ants with a blob of paint, much to their irritation. I love the way that they can communicate so effectively with one another when there’s a food source – they discovered the tinned food that I put down for our cat earlier this year, with the result that about 300 ants turned up every feeding time. Now the cat is being fed upstairs, and I’m just hoping that the ants can’t work out how to get to the first floor. They are tidiers-up par excellence, and their lives are complex and compelling. I am really enjoying Richard Jone’s book ‘Ants’, in the Bloomsbury British Wildlife series, and I recommend it to myrmecophiles everywhere.