Evidence of field-evolved resistance to Cry1Ac-expressing Bt cotton in Helicoverpa armigera (Lepidoptera: Noctuidae) in northern China - Liu, Fengyi; Xu, Zhiping; Zhu, Yu Cheng; Huang, Fangneng; Wang, Yanhua; Li, Huiling; Li, Hua; Gao, Congfen; Zhou, Weijun; Shen, Jinliang