Insect Resistance Management Assessment of Transgenic Plants Expressing Two Bt Toxins

Transgenic plants expressing insecticidal proteins from the bacterium Bacillus thuringiensis (Bt) were grown on over 22.4 million ha worldwide in 2004, nine years after their first commercial use in 1996. Insect resistance management (IRM) is critical for the sustainable use of Bt plants. Theoretical models suggest that plants expressing two dissimilar Bt toxins in the same plant have the potential to delay insect resistance more effectively than single toxin plants used sequentially or in mosaics. Transgenic cotton plants expressing two Bt toxins are now registered for use in the US and Australia, and several companies are developing new varieties of Bt corn, Bt cotton and Bt brassicas with dual Bt genes for commercial use in the coming years. We conducted two greenhouse studies using a unique model system composed of broccoli plants transformed to express different Bt toxins (Cry1Ac, Cry1C or both) combined with populations of diamondback moth, Plutella xylostella, carrying resistance to each Bt toxin, with over 20 generations of selection in each study. The results from the first study indicated that resistance to two-gene plants was significantly delayed compared to single gene plants deployed sequentially or in mosaics. The second study proved that using two-gene plants alone is better for sustainable IRM than concurrent use of one-and two-gene plants in the same region. These results have important implications for the development, deployment and regulation of transgenic insecticidal plants.

Impact of insect-resistant transgenic rice on target insect pests and non-target arthropods in China

Progress on the research and development of insect-resistant transgenic rice, especially expressing insecticidal proteins from Bacillus thuringiensis （Bt）, in China has been rapid in recent years. A number of insect-resistant transgenic rice lines/varieties have passed restricted and enlarged field testing, and several have been approved for productive testing since 2002 in China, although none was approved for commercial use until 2006. Extensive laboratory and field trials have been conducted for evaluation of the efficiency of transgenic rice on target lepidoteran pests and potential ecological risks on non-target arthropods. The efficacy of a number of transgenic rice lines currently tested in China was excellent for control of the major target insect pests, the rice stem borers （Chilo suppressalis, Scirpophaga incertulas, Sesamia inferens） and leaffolder （ Cnaphalocrocis medinalis）, and was better than most insecticides extensively used by millions of farmers at present in China. No significantly negative or unintended effects of transgenic rice on non-target arthropods were found compared with non-transgenic rice. In contrast, most of the current insecticides used for the control of rice stem borers and leaffolders proved harmful to natural enemies, and some insecticides may directly induce resurgence of rice planthoppers. Studies for developing a proactive insect resistance management of transgenic rice in the future are discussed to ensure the sustainable use of transgenic rice.