Abstract:Cotton ballworm, Helicoverpa armigera (Hübner), is an important insect pest of maize. The developments of transgenic Bt maize provide a new tool for corn borers control. Monsantos YieldGard Bt transgenic maize expressing Cry1Ab protein and a non-Bt control were evaluated through laboratory bioassays by exposing neonates directly to silks, husk cover, young kennels and cobs that are the tissues initially attacked by neonates in the field. Field trials were also conducted with artificial infestations cotton ballworm at silk stages, respectively. Neonate of cotton ballworm fed on the silk, young kennels and cobs, and husk cover could lead to 53.0％, 90.0％ and 100% mortality within 5 days respectively and all could not survive longer than 23 days. These were significantly difference compared with only 14.0％, 46.0％, and 16.0% mortality within 5 days and 51.0％ (on silk) and 22.0% (young kennels and cobs) of larvae were pupaed finally for the negative control. There were only 3.5% ears with 0.007 rating scale of damage and 14.3 larvae survived per hundred plants after artificial infestation at silk stage in field trial. These were significantly less than that of negative control that performed 82.5% ears with 2.51 rating scale of damage and 79.6 larvae survived per hundred plants. These results indicated that YieldGard provided good controls of cotton ballworm in reproductive stage.