The two transgenic Bt cotton cultivars ZMSJ (expressing Cry1Ac/Cry2Ab) and ZMKCKC (expressing Cry1Ac/EPSPS) used in this study were gifts from the Institute of Cotton Research, the Chinese Academy of Agricultural Science. The local cotton variety, Emian 24 (non-GM cotton), was a gift from the National Key Laboratory of Crop Genetic Improvement, Huazhong Agricultural University. The ZMSJ cotton expresses two Bt proteins for the control of lepidopteran pests, such as the cotton bollworm Helicoverpa armigera. The ZMKCKC cotton expresses one gene for insect resistance and one gene for herbicide tolerance. All cultivars were cultivated under recommended agronomic practices at the experiment field at Huazhong Agricultural University in early May 2011 without exposure to any pesticide. Pollen samples of each cultivar were collected using the multi- point field sampling method [18] on June 20th, July 20th, and August 20th (early bloom, mid-stage bloom and late bloom respectively). The freshly collected cotton pollen samples were sieved (830 mm mesh size) and stored at –80uC until they were used for experiments or analyses.