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Nature 01860.txt

Sterile moths wipe out cotton pest: Nature Newsbetween May and October for four consecutive years, aeroplanes crisscrossed the morning skies above Arizona's cotton fields, dropping millions of tiny moths onto the croplands below. The little grey insects are among the world's most notorious agricultural pests: their larvae are the pink bollworms (Pectinophora gossypiella), also known as'pinkies'.'However, the moths released from the planes were different from those responsible for the caterpillars munching their way through the state's cotton crops. They were sterile. The moth-drops were part of a programme to wipe the dreaded pinkie off the Arizona map for good. State officials hoped that the combination of sterile moths and genetically modified cotton crops, engineered to produce a toxin deadly to pinkies, would put an end to farmers'costly struggle against the caterpillars. The strategy was intended to restrict the spread of toxin-resistant pink bollworms by flooding the population with sterile moths. When rare resistant moths emerged as they inevitably would, they would probably encounter a sterile partner, and their genes would be erased from the population. It was a risky approach. To test the plan, Arizona farmers had to give up the strategies normally used to suppress toxin-resistant bollworms so if the sterile-moth strategy failed they could be faced with a bigger pinkie problem than ever. When Bruce Tabashnik, an entomologist at the University of Arizona in Tucson, first heard about the scheme, he was worried. My gut feeling was that this wouldn't work, he says. But as Tabashnik and his colleagues report today in Nature Biotechnology1 the risk seems to have paid off: four years into the programme and almost a century after pinkies were first found in the United states, in 1917, Arizona infestation rates had dropped by 99.9%.%The pink bollworm was gone all but. Sterile insect releases have already been used to drive down populations of the Mediterranean fruit fly, or Medfly, in Guatemala, Mexico and the United states; screw-worms in the United states, Central america and Libya; and tsetse flies on the island of Zanzibar, off the coast of Africa. The technique works best on pests that are not particularly populous. If you have something like aphids or thrips, where there are thousands on a plant, it's kind of hard to release enough sterile insects to do any good, says Fred Gould, an entomologist at North carolina State university in Raleigh, who wasn't involved in the study. But where pinkies were concerned, genetically modified cotton crops had driven already down the population one million-fold, says Tabashnik. The crops produce a toxin that is made naturally by the bacterium Bacillus thuringiensis called Bt toxin. To prevent the spread of Bt resistance, farmers are required to plant nearby'refuges'of conventional crops. The idea behind the refuges is to keep a population of non-resistant moths close at hand as potential mates for any resistant moths that arise. Unfortunately, however, refuges also guarantee a steady local population of pink bollworms. After a while, farmers came to resent the refuges that allowed the bollworm to persist year after year, costing them millions of dollars annually in crop losses and insecticide sprays. They asked the US Environmental protection agency for permission to dispense with the refuges and instead begin releasing sterile moths. Still sceptical Tabashnik and his colleagues developed computer simulations to predict the consequences of the farmers'proposed strategy. To his surprise, the models suggested that the combination of Bt cotton and sterile-moth releases could wipe out pest populations and stave off Bt resistance for at least 20 years. In 2005, the Pink bollworm Rearing Facility in Phoenix began cranking out pinkies for the Arizona experiment. The factory treated the moths with just enough radiation to damage the chromosomes in their reproductive cells without causing injuries that would prevent their survival in the wild. Over the course of each growing season for the next four years about 2 billion pink bollworm moths were released into Arizona's cotton fields. By 2009, a survey of 16,600 cotton bolls from conventional crops yielded only two pink bollworm larvae, and farmers had stopped using insecticide sprays to keep the pinkie population in check. So far, no live pink bollworm caterpillars have been found in bolls of cotton this season, says Tabashnik. The results are tremendous says entomologist William Hutchison at the University of Minnesota in St paul, who was involved not in the programme. But the experiment is still relatively short-term. It will be interesting to see how this plays out over another three or four years he says. Ideally, the same approach could be used in regions where poor farmer compliance with the refuge rule has contributed to widespread Bt resistance. Unfortunately sterile-insect release plans require extensive resources, Hutchison cautions. Plans to eliminate the pink bollworm from the United states and northern Mexico cost $30 million a year between 2006 and 2009. In Arizona, says Tabashnik, it's conceivable that farmers will someday no longer have a use for Bt cotton at all. For that to happen, it's not a question of whether pink bollworm is eradicated or not, it's a question of how economically damaging it is, he says. And in 2009 and 2010, pink bollworm was no longer an economically damaging pest in Arizona.

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