"Dominant resistance is more difficult to manage and cannot be readily slowed with refuges, which are especially useful when resistance is recessive," Tabashnik said.

Refuges consist of plants that do not have a Bt toxin gene and thus allow survival of insects that are susceptible to the toxin. Refuges are planted near Bt crops with the goal of producing enough susceptible insects to dilute the population of resistant insects, by making it unlikely two resistant insects will mate and produce resistant offspring.

According to Tabashnik, the refuge strategy worked brilliantly against the pink bollworm in Arizona, where this pest had plagued cotton farmers for a century, but is now scarce.

The dominant mutations discovered in China throw a wrench in the refuge strategy because resistant offspring arise from matings between susceptible and resistant insects.

He added that the study will enable regulators and growers to better manage emerging resistance to Bt crops.

"We have been speculating and using indirect methods to try and predict what would happen in the field. Only now that resistance is starting to pop up in many places is it possible to actually examine resistance in the field. I think the techniques from this study will be applied to many other situations around the world, and we'll begin to develop a general understanding of the genetic basis of resistance in the field."

The current study is part of a collaboration funded by the Chinese government, involving a dozen scientists at four institutions in China and the U.S. Yidong Wu at Nanjing Agricultural University designed the study and led the Chinese effort. He emphasized the importance of the ongoing collaboration for addressing resistance to Bt crops, which is a major issue in China. He also pointed out that the discovery of dominant resistance will encourage the scientific community to rethink the refuge strategy. 

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 per cent in the U.S. in 2011, but has exceeded 90 per cent 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."