|Science. 2008 Sep 19;321(5896):1676-8. |
Suppression of cotton bollworm in multiple crops in china in areas with bt toxin-containing cotton.
Wu KM, Lu YH, Feng HQ, Jiang YY, Zhao JZ.
State Key Laboratory for Biology of Plant Diseases and Insect Pests, Institute of Plant Protection, Chinese Academy of Agricultural Sciences, Beijing, 100193, People's Republic of China.
Transgenic cotton that has been engineered to produce insecticidal toxins from Bacillus thuringiensis (Bt) and so to resist the pest cotton bollworm (Helicoverpa armigera) has been widely planted in Asia. Analysis of the population dynamics of H. armigera from 1992 to 2007 in China indicated that a marked decrease in regional outbreaks of this pest in multiple crops was associated with the planting of Bt cotton. The study area included six provinces in northern China with an annual total of 3 million hectares of cotton and 22 million hectares of other crops (corn, peanuts, soybeans, and vegetables) grown by more than 10 million resource-poor farmers. Our data suggest that Bt cotton not only controls H. armigera on transgenic cotton designed to resist this pest but also may reduce its presence on other host crops and may decrease the need for insecticide sprays in general.
|However, a major challenge for planting Bt cotton for pest control is the potential for insects to evolve resistance to Bt. Continuous monoculture of varieties that express the same Bt toxin could select for resistance, particularly when the amount of Bt toxin decreases as the plants age (9, 10). A promising resistance management strategy entails the use of plants with a high dose of toxin in combination with the maintenance of "refuge" crops that encourage proliferation of Bt-susceptible insects within the pest population (11–13). To this end, the U.S. Environmental Protection Agency requires that each cotton farm set aside some land for cotton that does not produce Bt if farmers plant transgenic Bt cotton producing Cry1Ac toxic protein (14–16). Although successful in the United States (17), this strategy is difficult to implement in China because of the challenges associated with educating and monitoring millions of small farmers. In China, a multiple cropping system consisting of soybeans, peanuts, corn, and vegetables is common. These crops also serve as hosts for H. armigera, and, because they do not express Bt toxin, they serve as refuges for nonresistant insects (10). Because cotton is not the only host crop, Bt cotton comprises about 10% of the major host crops in any province or throughout northern China. This accidental approach to refuge management appears to have, so far, warded off the evolution of resistance (10). Nevertheless, as a result of decreased spraying of broad-spectrum pesticides for controlling cotton bollworm in Bt cotton fields, mirids have recently become key pests of cotton in China (18, 19). Therefore, despite its value, Bt cotton should be considered only one component in the overall management of insect pests in the diversified cropping systems common throughout China. |