Abstract: An improved F₁ screen technique was used to estimate the frequency of alleles conferring resistance to Bt cotton producing the Cry1Ac toxin in field populations of the cotton bollworm Helicoverpa armigera (Hübner) collected from Northern China in 2005. Eggs were collected from Bt cotton in Anyang of Henan Province and Cangxian of Hebei Province, respectively. The collected eggs were reared with artificial diet in the laboratory. The 2nd instar larvae were subjected to a primary selection with 1 μg/cm² activated Cry1Ac, and the survivors were individually mated with moths from a laboratory-selected resistant GYBT strain. After screening F₁ with a discriminating dose of 2.5 μg/cm² activated Cry1Ac, we estimated the field resistance frequency of 1.4×10^-3 for Anyang population from Henan Province, and 1.5×10^-3 for Cangxian population from Hebei Province. Susceptibility of five field populations of H. armigera collected from Northern China cotton area and Xinjiang cotton area during 2004-2005 was investigated using a surface contamination bioassay. Geographic variations (<8-fold) were found between these populations. In general, the field populations of H. armigera in Northern China have not evolved prominent resistance to Cry1Ac, and their resistance frequency to Cry1Ac is at a normal level. But Bt resistance risk still exists, and the early resistance detection and alarming system should be initiated in China as early as possible.

Key words: Helicoverpa armigera, Bt cotton, Btδ-endotoxin, Cry1Ac, resistance allele, resistance detection