One man's mission to put millions of mayflies back into British rivers

In the long tradition of bringing back wildlife to places from where it has vanished, Cyril Bennett's project may be one of the most ambitious ever. He is returning a million mayflies to his local river.

Dr Bennett, a freshwater biologist, is attempting the first mass restoration of one of Britain's loveliest and most charismatic insects, which is now in decline in many places, and on his local stream, the South Wey in Surrey, has vanished entirely after a serious pollution incident last year.

A successful breeder of the mayfly in captivity, Dr Bennett aims to put it back in the sort of breathtaking numbers that only nature itself could normally produce. If his experiment succeeds it may be widely followed on other rivers.

The mayfly, ephemera danica, is the largest of the upwing river flies or ephemeroptera, the artificial imitations of which, used by anglers, form the basis of fly-fishing for trout. They are attractive fluttering insects with evocative names, such as blue-winged olive, iron blue and pale watery dun.

The mayfly itself is remarkable for its size (it is bigger than some butterflies), for its beauty, with its long, slender, elegantly arched creamy-yellow body and translucent wings, and most of all for its life cycle.

After one or two years living in the gravel of the river bed, the mayfly larvae, or nymphs, swim to the surface, usually in mid-May, and change into winged insects. On the river bank they mate – the males perform a "parachute dance" to attract the females, rising up and floating down – and then the females flutter back on to the water to lay their eggs, and die.

Their existence as winged insects lasts not more than two or three days. Mayflies are not overly common, but where they are found, they emerge in such huge numbers that they fill the air, and the thousands of dying females carpet the whole surface of the river, often sending the trout into a feeding frenzy.

It is one of Britain's great natural wildlife spectacles, and it will begin in about a week from now, with different rivers having different dates for the first emergence of the adult insects, usually the same each year. The spectacle will last until mid-June.

But not on the south branch of the river Wey, the tributary of the Thames that flows through Farnham, Godalming and Guildford. In its upper reaches in the Surrey hills the Wey is a trout river with a large mayfly population. Last year, however, a serious pollution incident, perhaps from a sewage works – the origin was not definitely traced – wiped out most of the invertebrates in the water, such as mayfly and freshwater shrimps.

The complete disappearance of fly life was noticed by anglers, confirmed by the Environment Agency – and seized upon by Dr Bennett as a unique opportunity.

He is a 62-year-old former BT engineer from Farnborough, Hampshire, who took early retirement and has built a second career as a biologist and entomologist. His lifelong passion for fly-fishing sparked a particular interest in upwing flies – he is now recognised as one of Britain's leading experts – and his speciality is the mayfly (his doctoral thesis was on the mayflies of the Wey).

Through years of study and observation of their life cycles, Dr Bennett has succeeded in breeding flies such as the blue-winged olive and the mayfly from their eggs, in captivity, in "artificial streams" – long tanks with a current running through. He believes that this can be scaled up to produce a million mayfly nymphs or more.

Later this month he will go to Hampshire's river Test, Britain's most celebrated trout river, and under licence from English Nature collect the eggs from about 500 female mayflies. As the eggs each insect holds number between 5,000 and 10,000, he aims to collect perhaps two million or more.

The eggs are tiny whitish-grey dots: several thousand can fit on a microscope slide. In Dr Bennett's artificial streams they will hatch in about 24 days into microscopic nymphs, which will grow until in September or October, when they are ready to be put into the river. He hopes at least a million will be returned.

Dr Bennett has already give the Wey a start, so to speak, by putting in 1,000 fully-grown mayfly nymphs, the size of prawns, which should emerge as adults later this month and lay their eggs naturally.

He hopes his project will not only restore the mayfly to the Wey, but draw attention to the decline in upwing flies, which is now known to be taking place all over Britain, perhaps because of pollution. It's not like vanishing butterflies; in the nature of things, only anglers have noticed it.

"It's a massive problem," Dr Bennett said. "Fly life is declining on rivers throughout the country, there's no doubt about that at all. It's been an uphill struggle to get the Environment Agency to accept it, though they seem to now.

"But the public aren't really aware of it. These beautiful insects are some of the most attractive elements of our river wildlife, and we should do anything we can to halt their decline and bring them back."

OLDER THAN DINOSAURS

¿ There are about 2,000 species of upwing flies, or ephemeroptera; Britain has about 50 of them, of which the mayfly ephemera danica is the largest.

¿ Mayflies and their relatives are, with the dragonflies, the world's oldest insects, predating the dinosaurs.

¿ Mayfly larvae, or nymphs, spend up to two years living in a river; as adult winged insects they live three days at most.

¿ The mayfly is unique in having two adult stages: the sub-imago, or dun, is a duller-coloured insect; the imago, or spinner, is brighter, livelier, and lays the eggs.

¿ Mayflies that emerge from the river (the duns) first make for bushes and trees to shelter, partly to conserve body fluid (the winged insects can neither eat not drink) and partly to avoid frosts and birds.

¿ Mayflies need clean, oxygen-rich water and their presence is an indicator of river health. They are mainly concentrated on the chalk streams of southern England.

¿ The time of the mayfly's emergence is known to anglers as "duffer's fortnight", as the trout are so intent upon feeding that they can be caught by novice fly-fishers.