A STRAIN of genetically modified cotton cuts the numbers of a devastating moth pest and could eventually eliminate it, the longest trial of the GM crop to date has found. But for this to happen farmers need to ensure they also plant enough conventional cotton.
In 1992, Arizona farmers began planting cotton that produces a toxin from a soil microbe called Bacillus thuringiensis. The cotton kills pink bollworm, the larva of the moth Pectinophora gossypiella. It was designed as an alternative to pesticide sprays, which allow pests to bounce back between each application.
Since then Yves Carri猫re, an entomologist at the University of Arizona, and his team have tracked pink bollworm populations and correlated them with the amount of Bt cotton planted. They found that numbers of the pest fell in areas where 65 per cent of the cotton fields were growing the Bt variety. Any less than that and enough of the moth larvae survive to breed, driving the population up to previous levels.
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While pesticide sprays break down quickly, Bt cotton remains toxic and also kills off the larvae inside the plant. Carri猫re says mathematical models show that if farmers planted 80 per cent of their fields with Bt cotton they would eradicate the pest completely in several years.
However, he warned that large 鈥渞efuges鈥� of conventional crops are essential to prevent the spread of resistance. The idea is that resistant individuals will have to breed with pests from standard cotton fields, stopping the spread of the resistant trait. The US Environmental Protection Agency says farmers must plant 5 per cent of their fields with conventional cotton. Carri猫re says that as pest numbers fall, farmers will be more willing to increase the size of the refuges, but it will be a careful balancing act: if they reduce Bt cotton too much, pest numbers are likely to bounce back.