Abstract:Control effects of the seedlings, whorl leaves and the leaves of transgenic Bacillus thuringiensis corn on the 1st instar, 4th instar and 5th instar larvae of Mythimna separata (Walker) were evaluated in laboratory. Two Bt corn hybrids of events MON810 and Bt11 expressing Cry1Ab protein and their non-Bt controls were employed in the bioassays. Results showed the survival was 4.17% and 0 at the 3rd day when the 1st instar larvae feeding on the seedlings of MON810 and Bt11, compared with 91.66% and 88.99% in corresponding non-transgenic isogenic hybrid, respectively. The survival rate was 13.89% and 19.45% for the 1st instar larvae when feeding on whorl leaves of MON810 and Bt11 at the 2nd day, respectively, and none survived at the 7th day. The mortality was 100% for 4th instar larvae of M. separate when feeding on the leaves of MON810 and Bt11 at the 11th day, compared with 0 mortality in non- transgenic isogenic hybrid. The 4th and 5th instar larvae exhibited weight loss when fed on MON810 and Bt11 leaves, and showed weight gain when fed on two non-Bt controls. It was shown that the Bt corn expressing Cry1Ab toxin had good control effects on M. separate, not only the 1st instar larvae, but also the 4th and 5th instar larvae.