The walnut aphid - Puceron des nervures and The four spotted ant - La fourmi quatre taches.

There is a large walnut tree beneath which we often have a meal. This pleasure has become blighted by ants. They fall out of the tree and onto us and our food. We sought a solution, but there was none. Nevertheless the story is interesting. The ant is a mediterranean species called Dolichoderus quadripunctatus. I have invented the common names. It has two particular characteristics. Its abdomen is decorated with four cream coloured spots, which I believe is unique among ants. And then its nest is not in the earth but high up in the branches in a rotten cavity or under the bark. I was in no way going to be able to remove them. Ants will eat almost anything from meat to honey. This species has whopping jaws at least a third the size of the head. What, I wondered, were they eating? I have not answered that to my satisfaction, but part of their diet is the honeydew from aphids. Widely scattered amongst the leaves are small colonies of the walnut aphid. Never, in spite of determined searches, have I found a colony greater in number than twenty eight. Always there are some ants with each colony. The ants are never keen to leave them. I have waited half an hour to see if any move away. They constantly stroke the aphids with their antennae. Occasionally an aphid excretes a drop of sugary waste and this is quickly sucked up by an ant. Other ants wander around the other leaves and I must suppose that they must be finding other food, for the aphids cannot produce enough food for all of them and of course also for the larvae in the nest. One must suppose that something is checking the reproduction of the aphids for these insects generally are known to be able to produce another by actually giving birth every hour or so. Now and then an aphid develops wings and I have seen these forms sitting on a leaf motionless for several days, sucking at the plant juice and being 'milked' by the ants. The aphids have a peculiar habit of only sucking juice from the main vein of the walnut leaf and also arranging themselves so that they are facing physically upwards in the direction of the stalk of the leaf, the leaf itself drooping down. I wondered if they could be persuaded to re-orientate themselves. To that end I pinned up some leaves, still attached to the stems and all with colonies, so that they pointed upwards. In each case the aphids became disorganised. They wandered from their posts and scattered themselves on both sides of the leaf. I never saw them regroup and in each case in about 24 hours the leaf had lost its aphids. Ants were still moving about apparently looking for their lost 'sheep'. I have no idea where the aphids went. I also wonder what happens to the hundreds of ants that fall from the tree. Do they ever get home again?

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