In 2006, a small airplane started buzzing each cotton field in Arizona, a thin, dust-like cloud trailing behind it. The dust was millions of insects called pink bollworms, and the flights were part of an audacious scheme to kill them off.Pink bollworms are insidious foes of cotton. Their larvae burrow into the plant’s seed pods, called bolls, destroying the fluffy fiber within. Where bollworms infest a field, farmers may spray insecticides many times a year to limit the damage. But the air-dropped insects released in Arizona had been exposed to radiation that left them sterile, so any pink bollworms on the ground that mated with them would produce no larvae. The sterile insects were only the mopping-up part of an eradication campaign. The cotton plants themselves had struck the first, most vital blow. Genetically modified with genes obtained from an insect-killing bacterium called Bacillus thuringiensis, or Bt, the plants churned out proteins that are toxic to pink bollworms, making cotton bolls a deadly meal for the larvae.By 2013, pink bollworms had vanished from Arizona’s cotton fields. In 2018, the US Department of Agriculture declared that the pest had been eradicated from the United States.