Settlers introduced possums to New Zealand for fur farming at the turn of the twentieth century. But there are now 60 million possums in the country. The animals have become a formidable pest, chewing their way through 20 000 tonnes of foliage a night, gorging themselves on birds' eggs and spreading bovine tuberculosis. The traditional way of controlling the animals has been to scatter carrots laced with poison. But now scientists at the Marsupial Cooperative Research Centre (MCRC) based in Sydney have created a genetically modified carrot designed to sterilise females. They hope this could bring possums under control. The carrots are engineered to contain a protein that sabotages fertilisation by binding to a key protein in the coating on possums' eggs. The main risk is that the carrots may sterilise other animals.