Outdoors: Nature Note

IN THIS warm, wet weather, thousands of gardeners are no doubt being irritated by the proliferation of blackfly on their broad beans; but probably few of them realise what phenomenal powers of reproduction the aphids possess. One female, settling on a beanstalk, can give birth parthenogenetically (without being fertilised by a male) to 50 wingless offspring. Seven days later, each of those 50 is capable of breeding, and in a single year the insects will go through about 18 generations.

Dr Richard Harrington, an entomologist at the Institute for Arable Crops Research at Rothamstead, has calculated that if they all survived, we would end up with the entire surface of the earth covered by blackfly to a depth of 150 kilometres. Fortunately, a high proportion of the aphids die before they can breed.

There are more than 500 species of aphids in Britain, but last week suction traps in Hertfordshire caught a type hitherto reported only in southern Italy. Global warming? A side-effect of El Nino? For the moment, no one can tell.