Pink bollworms are a terror to the cotton-growing industry and in fighting these invasive pests, growers have foregone chemical sprays and started planting genetically engineered cotton called Bt cotton which are infused with proteins that kill said caterpillars.
At first, Bt cotton worked well against pink bollworm in all three countries, but this adaptable pest harbors mutations that confer resistance to Bt toxins. These mutations were rare before Bt cotton was commercialized. However, when two resistant caterpillars develop into moths and mate, their offspring are also resistant.
So, one strategy is to plant some non-Bt cotton—a refuge—allowing normal caterpillars to survive, become moths, and mate with the resistant moths, Tabashnik says. This makes it less likely that two rare, resistant moths will find one another in the crowd. Today, most countries require refuges and use Bt cotton that contains two toxins, so caterpillars need two mutations to survive.
Three of the largest cotton-growing countries - the US, China, and India - used this tactic with varying results. For the first two, Bt cotton was able to suppress the pink bollworms.
But in India, refuges weren't planted which caused a growth in resistant bollworms. Since they were already resistant to the first toxin, when a second toxin was introduced, they quickly evolved to produce resistance to that as well.
(Image credit: Peggy Greb/USDA, ARS; Wikimedia Commons)