Black fly plague hits southern England's rivers

Parts of Britain are being terrorised by black fly, a small insect with a ferocious bite said to be far worse than that of a mosquito.

The insect saws into the skin causing bleeding and intense soreness, and health centres have been inundated with people suffering bites.

The fly, spreading throughthe UK, is at present hitting southern England the hardest, particularly Dorset and Oxfordshire. But the pesticides needed to control it are so expensive that some councils are refusing to use them and the problem appears certain to escalate.

Last week, Dr Doreen Werner, from Berlin's Humboldt University and a world authority on black fly - the species Simulium - visited Oxfordshire rivers and concluded that the plague there was among the worst she had seen. She is studying the UK infestation here with Dr Adrian Pont of Oxford University.

The problem is worsening along other European rivers, too. For three weeks in spring people cannot walk much of the 950 miles of the River Oder, which mainly forms the German-Polish border, for fear of bites.

Ironically, the presence of this voracious insect - not to be confused with black aphids - often indicates a healthy waterway since it does not tolerate pollution. Yet with few predators - they include caddis fly - it can build up huge populations.

Usually, black fly feed on blood in summertime, but UK victims this year were bitten as early as March.

The flies' larvae pupate under water, and it is the female which injects victims, the fluid acting as an anti-coagulant. In the US, calves not previously exposed to the fly were reported to have died within 24 hours of being bitten hundreds of times.

There are several species of black fly, and they include the Blandford fly, which used to infest the River Stour near Blandford Forum, Dorset.

Use of a biological pesticide which kills larvae has helped reduce bites,but it isexpensive, costing up to £800 a litre.

In Oxfordshire, infestation was recorded along the Cherwell and Glyme valleys. Cherwell District Council has resisted spraying the 25 miles of rivers in its area, owing to "the high cost of spraying" which "could not be justified" given the present infestation.

But, said Dr Pont, it might be better to act now rather than wait for natural predators to arrive and let infestation rates rise to the levels now affecting Dorset.