One man's bid to save a river ruined as pesticide kills his million mayflies

It's not a tanker disaster. There are no international TV crews swarming over oiled beaches and filming row after blackened row of oiled seabirds. But in its own micro-world - below the surface of the river Wey in Surrey - a wildlife catastrophe has taken place.

In a remarkable piece of wildlife restoration, Cyril Bennett, a freshwater biologist, had returned a million mayflies to his local river after it had been stripped of its aquatic insects by pollution, But now there has been another pollution incident and they have all been killed.

His bold attempt to restore a whole invertebrate community to an English river, reported in The Independent in May, was not only an astonishingly ambitious piece of conservation. It was also aimed at drawing attention to the steady and steepening decline over recent decades of the insects in our watercourses, which has been noticed by few people other than anglers.

The latest event, which involved an insecticide entering the river through a sewage treatment works, is a vivid illustration of just how that decline is happening - besides being the devastation of a vast range of tiny water life, and the ruin of a huge amount of Dr Bennett's own work.

"You could say I feel gutted," he said. "It makes me despair."

A former BT engineer from Farnborough in Hampshire, Dr Bennett, 62, switched careers to become a biologist and entomologist. He specialises in river insects, especially the ephemeroptera or upwinged flies, the artificial imitations of which form the basis of fly-fishing.

The mayfly, Ephemera danica, is the loveliest of them, with a long arched creamy-yellow body and translucent wings, bigger than some butterflies and beloved of anglers because of the feeding frenzy it induces in trout in late spring when it lays its eggs on the water.

Dr Bennett is one of Britain's leading mayfly experts, and through years of study and observation he has succeeded in breeding them in captivity, in "artificial streams" - tanks with water running over gravel beds. He was able to put his knowledge to formidable effect after a pollution incident last year on the south branch of the Wey killed the mayflies and other invertebrates over a long stretch of the river, which is a beautiful trout stream in its upper reaches above Farnham.

Under licence from English Nature, he collected the eggs from hundreds of female mayflies on the river Test in Hampshire; as each insect carries up to 10,000, he collected well over a million. He then returned these to the south Wey just before they hatched, with several thousand fully hatched nymphs for good measure.

He thought he had re- established a whole insect community. It may not be saving the giant panda, but in conservation terms it was still quite a feat. Then, weeks later, he received a call from an observant angler: the mayfly nymphs appeared to have gone. Dr Bennett confirmed himself that they had vanished, and tests by the Environment Agency showed the presence in the water of chlorpyrifos, a powerful insecticide.

Its concentration was very low - too low to kill fish - but it was still high enough to kill much more delicate invertebrates such as insect larvae and freshwater shrimps, which it had done with a vengeance.

Further tests showed that it had entered the river via a sewage treatment works at Bordon, Hampshire, just upstream from where Dr Bennett had put the mayfly eggs and larvae into the river.

Chlorpyrifos, which is under examination by the European Commission to see if it should be banned, is the active ingredient in many pesticides for the home and the garden as well as for farming, and it is likely that someone disposed of it by simply pouring it down the drain.

A million dead mayflies was the result. "It was coming along absolutely beautifully," Dr Bennett said. "It's hundreds and hundreds of hours of work gone out of the window. I couldn't believe it had happened again.

"The point is, this is probably happening all the time, all over the country, without people noticing. Anglers everywhere are saying fly life is declining, and this is why."

Yesterday Dr Bennett took The Independent to both arms of the Wey. Samples of the gravel in the south Wey below the pollution source showed very few invertebrates: no freshwater shrimps and certainly no mayfly nymphs. The gravel in the north Wey, by contrast, was absolutely teeming with invertebrate life.

Colin Chiverton, from the Environment Agency's local office, said the agency had spent hundreds of hours trying to trace the source of the pollution, without success. It is now running a local publicity campaign, asking people to take great care about how they dispose of pesticides.