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Farmers in Arizona, United States, have all but eradicated a major pest from their land using a combination of genetically modified cotton and billions of sterilised versions of the pest’s parent moth.
The farmers had been growing Btcotton for several years. The cotton is genetically engineered to produce Bt toxin, which kills pink bollworm, a serious cotton pest. Bt cotton had reduced the pest population one-million-fold.
To prevent the spread of resistance to Bt among the bollworm, farmers are required to plant refuges of conventional cotton nearby. These harbour moths that will mate with any resistant moths that emerge among the Bt plants to prevent resistance developing.
However, farmers have started to resent the refuges because they also allow the bollworm to persist — costing them millions of dollars annually in crop losses and insecticide sprays.
They therefore asked the US Environmental Protection Agency for permission to dispense with the refuges and instead begin releasing sterile moths, which then conducted mathematical modelling that showed such a strategy could indeed work.
The strategy of releasing large numbers of sterile insect has previously been used to drive down populations of the Mediterranean fruit fly in Guatemala, Mexico and the United States; screw-worms in Central America, Libya and the United States; and tsetse flies in Zanzibar.
In 2005 Arizona’s Pink Bollworm Rearing Facility began treating moths with just enough radiation to damage the chromosomes in their reproductive cells without causing injuries that would prevent their survival in the wild. Two billion sterile pink bollworm moths were released over the next four years.
The scheme seems to have worked: last year only two larvae were found in the crop, and this year none have been found so far.
In Arizona, says Bruce Tabashnik, an entomologist at the University of Arizona in Tucson, it is conceivable that farmers will someday no longer have a use for Bt cotton at all, as sterile insects drive the pest populations to economically negligible levels.