In the US, this is the third year of farmers successfully growing crops that are genetically engineered to make their own pesticide, known as Bt toxin, a natural poison obtained from the soil bacterium * Bacillus thuringensis *. Even organic farmers approve of Bt-crops as the toxin is highly specific to particular pests without harming other animals and does not build up in the environment. But an expert panel has told the EPA that the biotechnology companies selling these crops must take steps to stop insects developing resistance to the poison. Some entomologists argue that as the plants produce the toxin continuously, it is possible that the pests will develop resistance. The best way to stop this, they suggest, is to set aside "pest refuges'