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. Only in autumn do they breed sexually to shuffle the genes; the young can then overwinter as eggs, thus protected from the worst of the weather. How smart is that?
It’s pleasing to know that the clever men of Cambridge, who know all that there is to be knowed, are even keener on aphids than I am. One of them, Dr William Foster, senior zoology lecturer at the university, described to me how a number of species stimulate their hosts to form galls around their colonies; many of these galls are rather beautiful when examined carefully. He cited the example of Pemphigus bursarius, which inhabits a swollen, purse-like gall on the leaf petioles of Lombardy poplar. The aphids emerge in summer and migrate to the roots of garden lettuce.
According to Dr Foster, many gall-dwelling aphids exhibit altruism; in other words, if the colony is threatened, young females stab attackers with their stylets or squirt out ‘glue’, tussling and sticking themselves to invaders, often dying in the process. They do this because they are all closely related. In animals, kin selection is what promotes altruism: think of bees, wasps or ants, all bred from one queen.
Even if you don’t share my admiration for aphids, don’t spray them with insecticides. Aphid numbers increase so quickly that spraying may have the effect of actively encouraging them at the expense of slower-to-reproduce, insecticide-sensitive aphid predators. Squish them or spray the colonies with water under pressure instead. Greenfly and blackfly are far less of a problem in my garden since I gave up using insecticides a decade ago. Pemphigus bursarius, a focus of Dr Foster’s research, can be thwarted in summer by laying horticultural fleece on the lettuce crop; it is impenetrable to aphids, but not to water or sunlight. However, now that I know ‘soldiers’ guard the gall colonies, sacrificing themselves for the general good, even that benign stratagem makes me feel a bit queasy.