The Analysis Of Bt Cotton Tolerance Evolution For Field Population Of Helicoverpa Armigera

Genetically modified cotton expressing the Cry1Ac toxin has been commercially cultivated in China since 1997. Bt cotton provides an efficient tool for controlling cotton bollworm (CBW) Helicoverpa armigera (Hübner) in many areas of China, but little is know about their long-term ecological consequences. During 2006-2008, the sensitivities of isofemale lines of H. armigera collected from Xiajin County, Shandong Province and Anci County, Hebei Province in northern China to Cry1Ac toxin protein were monitored systematically by using both diagnostic concentration and DNA screen methods, and population dynamic models were established under different planting system in northern China. In addition, interactions between Cry1Ac and Cry2Ab in the field H. armigera populations were investigated. The main results were summarized as follows.1. During 2006~2008, a total of 2306 isofemale lines from the Xiajin population and a total of 1270 isofemale lines from the Anci population were successfully screened on Cry1Ac diets. For each year, it was estimated that the major resistance gene frequency in Xiajin population in 2006, 2007, and 2008 was 0, 0.00022, and 0.00033, respectively. No major alleles conferring resistance to Cry1Ac were found in Anci population, the frequency of resistance alleles for Cry1Ac was 0. Based on the relative average development rates (RADR) of the H. armigera larvae in F1 tests, no substantial increase in Cry1Ac tolerance was found in either location over the 3-yr period. There were also significantly positive correlations between RADR of the lines in F1 generation and the RADR of their F2 offspring, indicating that the tolerance was genetically based.2. We used polymerase chain reaction (RCR) primers that specifically amplify one mutant allele of a cadherin and one mutant allele of a APN gene linked with resistance to Cry1Ac in H. armigera, a major pest. We screened DNA of 2,570 insects derived from Cry1Ac cotton fields in Xiajin and Anci during 2006~2008. No resistance alleles were detected despite a decade of exposure to Bt cotton.3. There was a negative correlation between moth densities of H. armigera and planting years of Bt cotton for the Bt cotton planting regions, which indicated that population density of H. armigera were reduced with the introduction of Bt cotton in northern China. There are three generations of moth occurred between early June and late September in the Bt cotton planting regions. Interestingly, the 2nd generation moths decreased and seemed to vanish in recent years in two intensive Bt cotton planting regions (Xiajin County and Linqing County of Shandong Province), but this tendency was not found in other two less intensive Bt cotton planting regions (Anci County of Hebei Province and Fengqiu County of Henan Province). This indicated that deployment of Bt cotton, especially in the intensive Bt cotton planting areas, have deeply suppressed the 2nd generation significantly.4. A total of 572 isofemale families of H. armigera from Xiajin county of Shandong Province and 124 families from Anci county of Hebei Province were screened with both Cry1Ac- and Cry2Ab-containing diets in 2008. The bioassays results indicated that RADR of F1 full-sib families from field collected female moths on Cry1Ac and Cry2Ab containing diet were positively correlated. The same correlation was found in the F2 generation, indicating cross-tolerance between Cry1Ac and Cry2Ab in field populations of H. armigera in Yellow River cotton-farming region of China.In conjunction with data from bioassays and DNA based-screen, the present results reported here indicated that the frequency for Bt resistance allele are still rare in the current field population of H. armigera. The complete control of second generation moths minimizes the impact of minor resistance genes and would be important factors contributing to the continued susceptibility to Cry1Ac in field population of H. armigera in Yellow River cotton farming region of China. Furthermore, the cross-tolerance must be considered in evaluating the utility of pyramiding Bt genes (Cry1Ac/Cry2Ab) in cotton for delaying evolution of resistance.