'It's somewhere between a flute and an opium pipe," explains Fredrik Sjöberg, unfolding a small tubular contraption in his wild flower-filled garden. More alarming than this "pooter" is a jar decorated with a skull-and-crossbones. "Cyanide," nods the Swedish writer, clearing his throat as we stand by his bleached wooden jetty leading into a dark, limpid lake. "I have a dealer. I'm not totally sure if this is legal ..."
Pfffft. In a flash, Sjöberg bends over a flower, sucks on the pooter and catches a microscopic bronze fly. If the general public regard butterfly collectors as "breathless twits", reasons Sjöberg, then a hoverfly hunter is "absurd". Perhaps the only thing crazier than a hoverfly obsessive would be to write a genre-defying memoir about it and expect to find a publisher and readers. This, of course, is exactly what the writer, translator and biologist has done with The Fly Trap, and a small book about an obscure branch of entomology has become unexpectedly big.
Sjöberg's work has taken on a life of its own since its publisher vowed to sell its 1,600 print run over five years. Instead, it's sold more than 30,000 copies in the small Swedish market and thousands more in translation in Germany, France, Russia and Norway. This year, 10 years after its publication, The Fly Trap has just come out in Britain, and will be published in Italy and Spain later in the year, and in the US next year. Perhaps it has taken so long to reach the UK because the book leaves marketeers scratching their heads: is it travel, natural history, popular science, biography, autobiography or poetry?
As Sjöberg estimates there are 25 hoverfly enthusiasts in Sweden, his writing has not simply ensnared a secret audience of fly-lovers. The Fly Trap tells of Sjöberg's decision to start collecting flies on his home island of Runmarö, 15 square miles of paradise an hour east of Stockholm, also previously the home of René Malaise, an eccentric Swede who was the last of a generation of polymath naturalist-explorers. Recounting Malaise's adventurous life and his own defiantly sedentary existence on Runmarö, Sjöberg cites everyone from Shelley to Darwin to Bruce Chatwin and explores some urgent themes – natural literacy, the appeal of slowness, the need for limits in an era of unrestrained freedom and the fallacies of the modern environmental movement. A sly challenge to virtually every contemporary orthodoxy, The Fly Trap is also charming, witty and original.
"The flies had a special quality of craziness," says Sjöberg after he meets me off Runmarö's small ferry on his "flakmoped", a three-wheeled moped with a metal tray on the front for carrying loads, including me. "It was so far out that as a reader you didn't have to fancy flies yourself, you could just concentrate on the island and the joy of following all these biographical tracks. I realised if I'm going to write this book I have to write it for readers who are not interested in flies. Then you have to tell stories about people. Quite a lot of people say they are interested in nature but all people are interested in people."
Sjöberg hails from a family of collectors – his father and grandfather both collected stamps – and he caught butterflies and beetles as a boy. "Then I stopped because of the difficulties of making a good impression on girls with dead insects. I usually call this the entomologist's reproductive phase." Aged 25, Sjöberg quit biology, telling his "terrified" parents he wanted to be a writer. He spent five years learning how via environmental journalism and love letters. "The best way to learn to write is to write love letters to girls who don't want you," he says. "I did a lot of that."
He fell in love with Runmarö when he visited it to research a story about orchids in 1984 and, two years later, he and his wife, Johanna, a book binder, moved into a derelict summer house overlooking a lake, where they lived in penury for 10 years with three young children and no running water. "We were young and stupid and that was our luck otherwise we would've never done this," says Sjöberg. When his children were teenagers, and the family could finally afford to build themselves a long low wooden house on their land, he took up hoverfly hunting.
He has found 202 species of hoverfly in seven years, 180 in his garden. Runmarö is "a hotspot. It's not only flies, it's butterflies and beetles, whatever. It's a wonderful place when it comes to biodiversity. It's just lovely nature."
Sjöberg offers many droll reasons for deciding to stay in one place and record hoverflies including "vanity and ambition", a hatred of rainforests and escapism. "I really love this because it was a room of my own," he declares. "I could just go out. With three teenagers, you go mad. You have to make money, you have to build houses, you have to do everything at the same time."
He also likens his pursuit to "learning a language", and reading nature is a key idea for Sjöberg. It leads him to some surprising conclusions. All of us can appreciate the beauty of, say, Sjöberg's small Swedish island, with its birches and scots pines, its overgrown meadows, the calm grey Baltic and the way the leaves whisper in the rain. But it's like music or language, he explains: the more refined your awareness of its complexities, the greater your pleasure. As he writes: "I want to understand the fine print of the only language that's been mine for as long as I can remember."
Growing fluent in this language, Sjöberg wrote books about the environmental movement, translated Edward O Wilson into Swedish but only found his literary style when, in 2001, he wrote a slender volume about the insect collection of Tomas Tranströmer, a 70th birthday present for the future Nobel prize-winning Swedish poet, who spends his summers on Runmarö. Over the next few years, Sjöberg wrote The Fly Trap, which is a bit like dinner with a witty European intellectual – wry, digressive and packed with fantastically clipped observations. Linnaeus, the father of taxonomic classification "managed to sell an operating system, a bit like Bill Gates". In April, "the sun shines and the grass grows so fast that the dry leaves rustle on the ground". Conservationists are "gentle self-flagellants who hunker down beside ill-smelling compost piles and rest easy in the certainty that much of life on earth has run its course".
As this last comment implies, Sjöberg is at odds with most of his peers: he is an optimist. "I know these guys, I was raised among them, I'm the biggest environmentalist in the world but I've seen it's not always as bad as they say in the newspapers," he says. He too was pessimistic in the 1970s because of industrial agriculture and pesticides such as DDT, but in wealthy western countries at least, the environment is in a better shape now.
Becoming obsessed with a tiny detail in the landscape – the hoverfly – has given Sjöberg a broader perspective than many naturalists. "You realise there's a lot of habitats disappearing, even on this island, but it's not as easy as you think to destroy nature." To take one example, there are no farms left on Runmarö (such smallholdings are not "economic") and its flower-rich meadows are turning to forest. This is bad for sun-loving species such as butterflies, but Sjöberg points out it will benefit hundreds more wildwood-loving species of beetle.
Things may not be so bad in affluent, responsible Sweden but what about the environmental destruction in the rest of the world? Sjöberg agrees with Wilson's theory of a great mass extinction, driven mostly by habitat loss. "But I think in many countries we've been through the worst and it will get better. When the dodo became extinct, no one knew anything about it. At least we know a lot about what we're doing today."
Optimism, feels Sjöberg, is not the easy option. "Optimism takes time and deep knowledge. If you know a lot about flies you have at least one area where you can read nature and realise it's not a dystopian novel you are reading." Optimism is also more constructive. "If the prognosis is too gloomy, you just don't want to think about it," he says. "If you want to save the world, the best thing is to create some sensitivity towards nature and the joy of it. You cannot go on about CO2 all the time. No one can be angry or afraid forever. If you want to change the world, you have to build it on some kind of joy."
Another big idea Sjöberg explores is how a limited pursuit such as collecting hoverflies on a small island is a blessed relief. "We're living in a boundaryless time. If you're born in a rich country like Sweden, anything is possible – you can travel wherever you want, do whatever you want, be almost whoever you want." He believes that "a state of limitation can give you some relief. It's a calmness."
Surprisingly, Sjöberg reveals he has virtually stopped hunting hoverflies now (like his hero Malaise he has started collecting art, the subject of the second of his Fly Trap trilogy). Our conversation turns to butterflies. Sjöberg has logged 58 species on the island, and his eyes glint when I mention that that's one less than the total number of Britain's native species. Later, in the long Swedish summer evening (Sweden has the best summer nights in the world, he says), we walk across some of Runmarö's overgrown land. "If my book was a hit in Britain and I made a lot of money I would buy this land and turn it into a meadow," declares Sjöberg. He pauses. "Because I want to beat the British and get more butterflies than them."
• The Fly Trap is published by Particular Books (£14.99). To order a copy for £11.99 with free UK p&p call Guardian book service on 0330 333 6846 or go to guardianbookshop.co.uk.
