Cotton genetically modified to produce a pesticide from the microbe Bacillus thuringiensis does not guarantee longterm financial benefit. Cornell University researchers and their colleagues interviewed 481 Chinese farmers in five major cotton-growing provinces. For years Bt farmers there reaped at least a third more profit than conventional growers from the money saved by slashing pesticide use up to 70 percent. But then Bt-resistant insects, such as mirids, proliferated (though for at least one season, an unusually cool, wet summer led to a large mirid outbreak). The new pests forced the farmers to spray crops as much as conventional growers did. That ate away the profits, because Bt seed costs three times more than conventional seed. The scientists, who presented their findings at the July 25 American Agricultural Economics Association meeting in Long Beach, Calif., suggested targeting secondary pests with natural predators or modifying cotton even further to resist them.