Trouble on the horizon for GM crops?
Tabashnik said China is the world's top cotton producer, with about 16 billion pounds of cotton per year. India is number two, followed by the U.S., which produces about half as much cotton as China.
In 2011, farmers worldwide planted 160 million acres of Bt cotton and Bt corn. The percentage of cotton planted with Bt cotton reached 75 percent in the U.S. in 2011, but has exceeded 90 percent since 2004 in northern China, where most of China's cotton is grown.
The researchers report that resistance-conferring mutations in cotton bollworm were three times more common in northern China than in areas of northwestern China where less Bt cotton has been grown.
Even in northern China, however, growers haven't noticed the emerging resistance yet, Tabashnik said, because only about 2 percent of the cotton bollworms there are resistant.
"As a grower, if you're killing 98 percent of pests with Bt cotton, you wouldn't notice anything. But this study tells us there is trouble on the horizon."