This is the VOA Special English Agriculture Report.

　　Bollworms are a traditional threat to cotton crops. Young ones feed on the boll, the part of the cotton plant that contains the seeds.

　　A bacterium known as Bt is able to kill bollworms and some other pests. Bt is short for Bacillus thuringiensis. It was discovered in the early twentieth century. Farmers began using Bt as a natural pesticide.

　　Then, in the nineteen nineties, researchers found a way to grow cotton plants that contain a Bt gene. The genetically modified cotton plant produces toxins that kill bollworms. Farmers around the world are now growing an estimated fourteen million hectares of Bt cotton.

　　Cotton bollworms also attack other crops. But scientists in China recently reported that Bt cotton may help suppress bollworm in other crops growing nearby.

　　The study involved crops grown in six provinces of northern China between nineteen ninety-two and two thousand seven. The study area contained three million hectares of cotton and twenty-two million hectares of corn, peanuts, soybeans and vegetables.

　　The researchers say the study suggested that Bt cotton not only controls bollworm on the transgenic cotton, but also may reduce its presence on other crops. And that reduction, they say, may decrease the need for insecticide sprays in general.

　　But the researchers also pointed out concerns. They say bollworms could develop resistance to the cotton plants designed to resist them. And they noted that insects called mirids have become "key pests" of cotton in China. They said this is because of a decrease in pesticide use made possible by Bt cotton.

　　The researchers say they do not believe that Bt cotton alone can solve all insect problems. Instead, they say farmers in China should consider it just one part of pest management systems.

　　Kong-Ming Wu of the Chinese Academy of Agricultural Sciences in Beijing led the study. The findings appeared in Science magazine.

　　A British group, the Institute of Science in Society, said the findings were presented in a way that could mislead readers. The group noted that the only part of the study available online at no cost, a short abstract, said nothing about the concerns.

　　And that's the VOA Special English Agriculture Report, written by Jerilyn Watson with additional reporting by Jessica Berman. I'm Bob Doughty.