Bollworm is susceptible to an insecticidal toxin made by the bacteria Bacillus thuringiensis, and China approved the commercial growth of cotton plants modified to produce this toxin in 1997. This Bt cotton is now grown on 4 million hectares in the country. Researchers from the CAAS Institute of Plant Protection in Beijing have monitored bollworm populations in an area of northern China since 1992. Their study area now contains 3 million hectares of Bt cotton and 22 million hectares of various other crops which the bollworm can infect.

The researchers report that, after Bt cotton was introduced, bollworm populations gradually declined not just in Bt cotton, but also in other crops. Using statistical analyses, the researchers found that the fall in bollworm populations correlated better with the amount of land devoted to Bt cotton than with patterns of temperature or rainfall.