From Scientific American:

Cotton genetically engineered to produce a natural pest killer not only reduces the spraying of pesticides, but has also boosted the populations of beneficial insects, according to a new study. The study monitored the impacts of so-called Bt cotton over more than 20 years and 2.6 million hectares of farmland in northern China and found that transgenic pest control produced knock-on benefits for surrounding fields of different crops—a benefit for the more than 10 million small-scale farmers in the region who grow these crops.

To assess the impacts of this natural pesticide genetically engineered into cotton, scientists monitored 36 sites in six provinces of northern China from 1990 to 2011. They found that Bt cotton fields endured less organophosphate or pyrethroid insecticide spraying and thus boasted higher populations of ladybugs, lacewings and spiders—insect predators.

As a result, Bt cotton fields had fewer aphids—an insect pest not poisoned by Bt but nevertheless controlled thanks to the boosted populations of predators. That effect also spread to adjacent fields of maize, peanuts and soybeans.

For more, see: GMO Bonus: Genetically Engineered Cotton Benefits Farmers, Predatory Insects