The recent warm weather has already caused alarming increases in greenfly numbers. Scientists from the Rothamsted research station in Hertfordshire say that one species of greenfly - the peach potato aphid - is more common than at any time since monitoring began 30 years ago.
A plague of sawfly larvae is reported to have chomped its way through the willow trees on a West Midlands estate, keeping residents awake a night with the noise. ''It's like a scene from a horror film,'' one resident complained. Meanwhile, hundreds of ladybirds descended on a beach near Goring by Sea in West Sussex at the weekend, making life irritable for bathers.
It is all a feature of insect metabolism. The higher the temperature, the faster they work through their life cycles and the greater the amount of reproduction, both sexual and asexual, that takes place, says Alan Watt, an entomologist at the Institute of Terrestrial Ecology in Edinburgh.
In warm summers insects of all species can go through a huge rise and fall in numbers, far greater than in the cool, wet weather that is such a feature of a typical British summer. What determines if one particular species gets out of balance is often whether its natural predator can keep pace with the growing number of its prey.
If global warming does make British winters milder and summers hotter, scientists are expecting an increase in the number of ''exotic'' insects, such as the Colorado beetle, which is a serious potato pest on the Continent. Medical entomologists are also worried that some species of mosquito which can carry malaria may also make a comeback to Britain. It is possible, for instance, that a mosquito common in India, Anopheles stephensi, which is a carrier of malaria and is frequently brought to the UK on board aircraft, could survive in Britain if the climate became warmer by just a few degrees.
Another feature of warmer summers and milder winters is a rise in the number of household pests, such as the cat and dog flea. A corresponding rise in the rat flea could herald a return to the UK of plague, which was last occurred here as a significant outbreak in the Twenties.
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