Mechanisms Of Action Of Transgenic Bt Cotton On Pest Insects And Their Natural Enemy

Laboratorial raising, field sampling and investigating, and enzyme liked immunological assay (ELISA) were used to evaluate the effects of transgenic Bt cotton, 33B (Monsanto, USA; along with its parental cultivar DP5415) and GK-12 (Institute of Biological Technology of Chinese Academy of Agricultural Sciences; along with its parental cultivar Simian 3), on target and secondary target pest insects, non-target pest insects, and their natural enemy. The interrelationships and mechanisms of action for the tritrophic of transgenic Bt cotton, pest insects and natural enemy were studied. The main results are as follows.No acute detrimental effects of the transgenic Bt cotton, 33B and GK-12, were detected on hatching (first-generation), preimaginal development and survival between these two transgenic Bt and respective non-Bt cotton cultivars during the first and second generation development of Bemisia tabaci. And the number of eggs laid on cultivar GK-12 was about twice higher than on 33B.The weight of 24 h old Helicoverpa armigera larvae reared on transgenic Bt cotton (Gk-12 and 33B) was lessened by 24.0 - 47.2% when compared with non-Bt parental cultivars. The mortality of 48 h old H. armigera larvae reared on transgenic Bt cotton was 80.4 - 92.0% higher than on non-Bt isogenic cultivars. Moreover, higher mortality was observed on 33B than on GK-12. In the case of Spodoptera litura larva, the survival rate and the time required to reach the second instar, and the weight upon reaching the second instar reared on 33B were significantly affected when compared with reared on both non-Bt cotton cultivars. Further, both H. armigera and S. litura larvae could obtain and pile up Bt toxin by feeding on Bt cotton. A significant decreases in increased weight of Propylaea japonica young larvae were observed when they were fed with 24 h old S. litura larvae reared on Bt cotton, GK-12 and 33B, when compared with on Simian 3 and DP5415, however, there were no differences when the mortality and the percentage of developing to the second instar of the ladybird were evaluated.The ELISA tests indicated that there was some Bt toxin in Aphis gossypii reared on transgenic Bt cotton, however, the developing time of P. japonica larva, prepupa and pupa, the mortality, the weight of each larval stage and newly imaged adult were no differences when P. japonica larvae fed with aphis reared on transgenic Bt cotton since their hatching, compared with on non-Bt cotton. And so were the percentage of deformity of eclosed adult, the percentage of ovipositing in mated female, the preperiod of oviposition and the number of eggs laid by individual female adult. But the preperiod of mating was shortened and the times of mating were increased. Moreover, the Bt toxin was detected in 5 - 20 day old P. japonica adults fed with aphis reared on transgenic Bt cotton, 33B and GK-12 from their hatching to emerging, and in newly hatched larvae which were the progeny of above adults.In addition, the ELISA tests indicated that the concentration of Bt toxin in 33B cotton plants was higher than in GK-12. Also, the concentrations of Bt toxin were highest in anther and stigma (33B and GK-12) of cotton plants, moderate in petal (GK-12) or petal and ovary (33B), and least in young square and boll, tender tip and leaf (33B and GK-12), and ovary (GK-12). Moreover, in cotton fields, cotton aphis and cotton bollworm could digest Bt toxin by feeding on Bt cotton plants. Meanwhile, more Bt toxin was digested when they fed on GK-12 than on 33B. Further, the larva and male and female adults of P. japonica could receive the Bt toxin in 33B cotton field via the preys, cotton aphis and cotton bollwrom young larva.As stated above, transgenic Bt cotton can effectively suppress the development of target pest insects, H. armigera, secondary target pest insects, S. litura, but no acute detrimental effects on non-target pest insect, Bemisia tabaci. Also, they were not harmful to P. japonica, an important predator in Bt cotton fields. H. armigera, S. litura and A. gossypii can digest Bt toxin by feeding on transgenic Bt cotton plants, and this toxin can be transferred to their predator, P. japonica. However, further work will be urgent to clarify the effects of transgenic Bt cotton on natural enemies in Bt cotton fields.