BEIJING—When scientists in China girded cotton with an insect-killing toxin in the late 1990s, they knew it would be a boon for cotton farmers. But in an unexpected twist, researchers have now found that genetically modified (GM) cotton that produces the natural pesticide casts a protective net over other kinds of crops in nearby fields--and not because introduced genes have spread beyond the cotton field. The findings, reported in tomorrow's issue of Science and at a press conference here yesterday, "are of major importance," says Huang Jikun, director of the Center for Chinese Agricultural Policy of the Chinese Academy of Sciences in Beijing. The success story, he says, should boost commercialization of a new generation of transgenic plants in China, including rice producing a toxin (Bt) from a gene from the Bacillus thuringiensis bacterium--one target of the country's new $3.5 billion R&D effort on GM crops (Science, 5 September, p. 1279). For years, farmers in China used brute force and chemistry to subdue the cotton bollworm (Helicoverpa armigera), drenching fields--and themselves--with organophosphate and pyrethroid pesticides. Because most Chinese farmers lacked protective clothes, many were poisoned or even succumbed to their exposures, says Zhao Jian-Zhou of Pioneer Hi-Bred International in Johnston, Iowa, co-author of the Science report. By the early 1990s, bollworms that survived the chemical onslaught had developed resistance, and cotton yields plummeted. Northern China was particularly hard hit in 1992 and 1993, when bollworms spoiled about 30% of the crop. "It was a biological disaster," says Zhao. The farmers began to gain momentum in 1997 when China approved commercial planting of GM cotton equipped with the gene to make Bt, a protein that kills select insects without harming animals or people and breaks down quickly in the environment. By 2001, the majority of Chinese farmers had adopted Bt cotton, and the bollworm was quickly brought to heel. Since 1992, a team led by Wu Kong-Ming of the Institute of Plant Protection of the Chinese Academy of Agricultural Sciences in Beijing has charted the rise and fall of the bollworm in six provinces in China that grow 3 million hectares of cotton and 22 million hectares of other crops such as corn, peanuts, and soybeans. The bollworm, they found, migrates easily from crop to crop over long distances. But it prefers to lay eggs on cotton plants, and when the eggs hatch on Bt cotton, the larvae succumb to the toxin. Wu's team observed a steady decline in bollworm eggs in Bt cotton fields, as expected. But to the researchers' surprise, larvae numbers also faded on nearby fields of unmodified crops other than cotton. Because bollworm moths prefer to lay eggs in cotton, Bt cotton appears to act as a dead-end trap for the pest, says Wu. "Bt cotton controls the whole population of cotton bollworm in northern China," he says. As a result, he says, "the ecological space for bollworm is getting smaller and smaller." Just as the bollworm threat is receding, however, another villain is gaining strength. In an article in press at Agricultural Sciences in China, Huang and colleagues report that with the use of broad-spectrum chemical pesticides down sharply, mirids--a secondary insect pest--are infesting more and more cotton fields. Combating mirids, Huang says, will be fertile ground for China's new GM initiative.