Characterization of resistance to Helicoverpa armigera in three lines of transgenic Bt Upland cotton

In the present study, different Bt cotton lines, Shanxi94-24, Zhongxin94 and R19, showed good efficacy against populations of Helicoverpa armigera, in both laboratory and field assays. Laboratory bioassays, however, indicated that there was a declining level of efficacy with plant age, as the mortality (%) of it Helicoverpa decreased gradually when fed leaves sequentially from the first to the last developing node from the main stem. The mortality of neonate insects fed leaves from the three transgenic lines was as high as 100% before flowering, but only 50%-70% or so by peak flowering and boll set. The temporal difference in resistance of these transgenic Bt cotton lines was also demonstrated in a second set of bioassays of transgenie leaves simultaneously collected from the transgenic plants planted at different times from March 1 to September 11, 1999 at intervals of twenty days. No difference in efficacy was observed over 3 or 4 successive generations of three transgenic lines, i.e., resistance was stable in different generations. This confirms the stable transmission and expression of the Bt gene after being integrated into the genome of the cotton plant. The relationship between the temporal difference in efficacy of transgenic cotton lines and the possible risk of resistance development in bollworm larvae to Bt cotton are also discussed.