| Helicoverpa armigera (Hiibner) is an important cotton pest in China. There were serious outbreaks in the cotton area of Northern China in 1990's because of resistance to synthetic chemical insecticides. Transgenic Bt cotton expressing CrylAc gene has been planted commercially since 1996. Due to its excellent agricultural characters, good performance of resistance to the target pest and reducing insecticides used against the cotton bollworm, Bt cotton planting has expanded rapidly in US and China. The present transgenic cotton cultivars express a single CrylAc toxin protein and the toxin is expressed through all growth season, which leads to the concern on that H. armigera could evolve resistance to cry1Ac very soon. In the present study, the inheritance mode and cross resistance pattern of a laboratory-selected strain of H. armigera (with high level of resistance to CrylAc) were investigated. The frequency of resistant individuals of H. armigera from Hebei Province was detected using discriminating dose of CrylAc. It is expected to provide some theoretic bases on Bt resistance management in China.A field population of H. armigera, collected from Bt cotton in Gaoyang City, Hebei Province, China in 2001, was divided into two sub-populations. One has been reared on artificial diet in laboratory for 28 generations without exposure to any insecticides and Bt toxins and designated as GY strain. Another was continuously selected with activated CrylAc toxin for 28 generations and designated as GYBT28 strain. The GYBT28 strain has developed 564 fold resistance to CrylAc compared with the GY strain. The inheritance mode o f CrylAc resistance w as examined b y reciprocal crosses (GYBT28 GY and GY GYBT28 ) and backcross (F1 GYBT28). The LD-P lines of GY, GYBT28, F1 and F1' progeny to CrylAc were constructed. The LC50 of GY, GYBT28, F1 (GYBT28 GY ) and F1' (GY GYBT28 ) was 0.1, 56.4,0.51 and 0.41 g/cm2 respectively. The LC50 of F1 and F1 ' progeny was not significantly different, showing the resistant gene was located on the euchromosome. According to formula of Stone, the value of dominance level of F1 and F1 ' was -0.49 and -0.55 respectively and it showed the resistance was incompletely recessive. An apparent plateau around the 50% mortality in theLD-P Line of backcross progeny suggested the resistance was controlled by a single locus and this was further confirmed by a goodness of fit test on single gene model. It was then concluded that the resistance to CrylAc in the GYBT28 strain was autosomal, incompletely recessive and controlled by a single locus.The toxicities of CrylAa, CrylAb, CrylAc, Cry2Aa and Btk were compared between the GY and GYBT28 strains in order to identify the cross resistance pattern of CrylAc resistance. Compared with the control GY strain, GYBT28 strain showed high level of cross resistance to CrylAa (103-fold) and CrylAb (>46-fold), and low level of cross resistance to a Bt formulation (Btk) (5-fold), while no cross resistance to Cry2Aa (1.4-fold) was detected.By discriminating dose assay, the frequencies of resistant individuals to CrylAc of//. armigera from Gaoyang and Handan cities, Hebei Province, China in 2003 were detected. The frequencies of resistant individuals in Gaoyang and Handan city were 0.7%o and 4%o respectively. The frequency of resistant individuals to CrylAc is still very low in this insect although the Bt cotton has been planted for over 5 years in the both sites. But there are significant variations of Bt resistance frequency in between the two sites. It is crucial to build a network to systematically monitor CrylAc resistance of field strains of H. armigera in China.
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