Aphids will protect their colonies at the cost of their own lives
APHIDS are not, you might think, the sort of creatures that would lay down their lives for their friends. But you would be wrong. A study just published in the Proceedings of the Royal Society by Utako Kurosu, of Tokyo University of Agriculture, and her colleagues shows that aphids are, indeed, capable of making the supreme sacrifice.
The aphids in question are not the common or garden greenfly that infest people's roses, but rather a species called Nipponaphis monzei that hides itself away in large galls which its activities form on the twigs of the trees that it parasitises. Unfortunately for the aphids, these galls are attractive to other animals, in particular the caterpillars of certain moths. The caterpillars like the food and shelter the gall provides.
To examine the aphids' reaction to such invasions, Dr Kurosu collected a number of galls and caterpillars from nearby woods, brought them to her laboratory, cut holes in the galls, and introduced the caterpillars. In every case the aphids attacked the caterpillars with sticky white fluid, killing them. In doing so, however, the aphids discharged about two-thirds of their body weight as fluid, behaviour likely to lead to death in the wild. Other aphids busied themselves patching up the holes with similar discharges. In this case, not only did the aphids sacrifice a lot of their body weight, many of them sacrificed their lives directly since they were trapped in the gum as it solidified.
The explanation of this self-sacrifice is that the evolution of such behaviour depends not on the moral sophistication of the creatures, but on the genetic relationship between sacrificer and beneficiary. In ants, bees and wasps, the insects best known for collaboration and self-sacrifice, a peculiar genetic system causes females to be more closely related to their sisters than to their own daughters. This results in the evolution of sterile worker females, whose role is to help their mother (ie, the colony queen) reproduce. Aphids go one better than this genetically. The aphids in a gall are a clone—all the individuals have the same genes. In these circumstances the concept of individuality is even more blurred than in an ant nest or a beehive, and the evolution of self-sacrificial behaviour is easy. Losing some “individuals” does not matter if the overall reproductive capacity of the colony is thus enhanced. In that context, it is noteworthy that the self-sacrificing individuals were the youngest nymphs. Adult aphids—that is, those old enough to reproduce—stayed out of the fray. Thus the loss to the clone's reproductive potential was minimised.
This article appeared in the Science & technology section of the print edition under the headline "Galling behaviour"
From the May 10th 2003 edition
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