Most of the benefits of growing genetically modified cotton, the only commercial GM crop in China, have disappeared after seven years. The problem is that new pests have appeared to replace the bollworms eliminated by the genetic modification.

A joint study by Cornell University and the Centre for Chinese Agricultural Policy in Beijing looked at the economic impact of "Bt cotton", which makes a bacterial toxin that kills leaf-eating bollworms, the crop's most important pest. After three years the 481 Bt cotton farms in the survey had cut pesticide use by 70 per cent and were earning 36 per cent more than comparable farms growing conventional cotton.