Abstract:The Oriental armyworm, Mythimna separata (Walker), is an important insect pest of maize. Planting transgenic Bt maize is a new way for the control of corn borers. Monsanto’ s YieldGard transgenic Bt maize expressing Cry1Ab protein and a non-Bt control were evaluated through laboratory bioassays by exposing 1st-3rd instar larvae of Oriental army worm directly to whorl leaves. The 1st-3rd instar larvae fed on Bt maize leaves could lead to 39.0% -92.0%, 96.0% -98.0%, and 97.0% -100% mortalities within 3, 7, and 10 days, respectively. All larvae could not survive longer than 14 days. There were significant difference compared with only 3.0% -8.0%, 6.0% -20.0%, and 11.0% -28.0% mortalities for the non-Bt control. The development of larvae was also severely inhibited when feeding on Bt maize. Larvae survived 10 days on Bt maize leaves weighted only 9% of that on non-Bt control. Field trials were also conducted with artificial infestations of 1st-6th instar larvae into the maize whorl at V4-V7 stage, respectively. There were only 0.0-6.7% of survival on Bt maize. In contrast, there were 8.3%-80.0% of survival on non-Bt control, which were significantly higher than Bt maize. Damage ratings of Bt maize were 0.1-0.7, which were significantly lower than 2.1-7.3 of non-Bt control. These results indicated that YieldGard could provide good controls to Oriental armyworm and protect maize well from the damage.