a purple weed, Lythrum salicaria , that was brought from Europe and released in North America 200 years ago is threatening wetlands across the us and Canada. Bernd Blossey of Cornell University in Ithaca, New York has been trying to discover why plants that pose no problems in Europe have became a pest in North America. In parallel experiments in the us and Britain, Blossey and Tony Wills, formerly a researcher at Silwood Park, compared the vigour of native European and escaped American varieties of loosestrife. They found that on average, the escapees were 30 per cent taller than their European forebears, had larger leaves and weighed 30 per cent more overall. The experiments suggest that the American plants grew larger because they left behind their natural enemies, allowing them to reallocate the resources they would normally have spent defending themselves against insects. But this leaves them vulnerable when they are reintroduced to their former foes. In experiments where American and European loosestrife were exposed to beetles, the insects preferred the American varieties.