"POPULATION," Thomas Malthus pointed out 200 years' ago, "increases in a geometrical ratio when it is left unchecked", and his simple observation has been a source of worry ever since.
Gardeners and farmers will be aware that greenfly and other aphids are enthusiastic followers of Malthus: indeed, it has been calculated that a mere 10 generations bred from a single aphid, assuming all survived, would collectively weigh in at almost 40 tonnes.
Fortunately, however, Mother Nature has ways to limit such fecundity. The Ladybird, as we saw yesterday, does its best to eat as many aphids as it can, and the weather, too, puts a periodic brake on population growth.
Aphids are particularly sensitive to cold. This is not to say that in this respect they are very fragile creatures - some can survive at -20C; but nonetheless, sub zero temperatures decimate a population, and even a slight frost severely limits their ability to move. If an aphid cannot move, it cannot reach its food, and quickly dies.
An explosion of the population typically occurs when a mild winter is followed by a warm spring. Reproduction continues through the winter months, so numbers increase unless checked by freezing temperatures. The importance of a warm spring lies in the insects' need to migrate to sources of new food at that time of year, and flight is not possible for them unless the temperature climbs to well over 10C: a few warm days in early spring, however, give the aphids early access to abundant food, and this in turn facilitates their reproduction rate.
In recent years, scientists have noted the appearance of a new breed of "superaphids" resistant to insecticides. At first, it was feared this survival advantage would lead to a take over by the "superaphids" of the entire population - but it never happened: it was obvious the genetic change which worked to their advantage was balanced by another which made them, in some other way, more vulnerable.
The most likely answer is that an insecticide resistant aphid is more likely than his relatives to perish from the cold. Apparently a normal aphid sitting on a leaf can detect a chemical change in the vegetation when the leaf is just about to drop, and so he shifts himself.
It is a useful capability, since with low temperature inhibiting his movement, an aphid thus marooned will find it difficult to scramble back aboard the plant. Mutant aphids, it seems, are unresponsive to the signal - to their cost.