In praise of greenfly

God may have a special preference for beetles but, frankly, aphids (greenfly to you, squire) are more my thing. If that seems a barmy thing for a gardener to say, rest assured I get just as irritated as everyone else by their vigour-sapping, leaf-curling, virus-transmitting presence on my flowers, fruit, vegetables and greenhouse plants. When they stick their hollow feeding tubes (stylets) into soft stems, the pressure in the plant pumps sugary sap into their bodies and they then excrete it, dripping sticky honeydew on to leaves below; this attracts small fungi called sooty moulds. What could be more annoying than that? But I also understand that, as the plant ecologist Ken Thompson puts it, they are the garden equivalent of krill: ‘small, extremely abundant and the basis of a whole ecosystem of predators and parasitoids’. In other words, our gardens would be very different, and poorer, without them.

Aphids, of which there are many distinct species, are mainly insects of north temperate regions. Their biology is fascinating. They are obligate symbionts, in other words, they require a bacterium or fungus to provide them with the amino acids, which the plant sap lacks. Some have more than one host plant, migrating from a tree or shrub, whose young leaves they suck in the spring, to herbaceous plants in the summer. (The peach-potato aphid, Myzus persicae, is the best known of these, but there are plenty of others.) Most intriguing, however, and with the greatest ramifications, is the fact that almost all aphids are female and breed asexually (parthenogenetically) most of the time, producing generation after generation of live females in the course of a season. This makes them highly efficient at increasing their numbers, and means that the colony is genetically more or less identical; they are a bunch of clone-mates, in fact.