Bt cotton has been loaded with insect-killing genes taken from a bacterium called Bacillus thuringiensis (hence “Bt”). This species lives in soil and the surface of plants, and it produces crystals of proteins that are toxic to hungry insects. If they are swallowed, they stick to molecules in the pest’s gut, breaking down its lining and allowing both B.thuringiensis spores and colonies of normal gut bacteria to invade. It’s this wanton spread of bacteria that kills the animal.

For decades, the protein crystals have been sprayed over crops as a pesticide. The chemicals degrade easily and pose no risk of contaminating groundwater. They are also highly specific – toxins from a specific strain of Bt will usually only affect a