Abstract: As a species of migratory pest,Mythimna separata （Walker） is prone to outbreak,and it is one of the most harmful grain pests in China.In this paper,the pathogenicity of nine Bacillus thuringiensis （Bt） strains to M.separata was determined by bioassay,and a strain Bt46 with high pathogenicity to M.separata was screened.The strain had the weakest pathogenicity to the 1st instar larvae.The strongest pathogenicity to the 2nd instar larvae,and the median lethal concentration (LC₅₀)was 5.25×10~6spores·m L^-1,the adjusted mortality rate of the 2nd instar larvae at 1×10~8spore·m L^-1concentration reached 81.08%at 24 h and 100%at 96 h.The strain Bt46 had pathogenicity to adults of M.separata,and its LC₅₀was 4.66×10~8spores·m L^-1,the corrected mortality of adults treated with 1×10¹⁰spores·m L^-1for 168 h was 80.77%.The results showed that the median lethal concentration of the strain had a negative effect on the growth and development of M.separata.Although the larvae treated by this strain had little effect on the growth and development of the contemporary survival population,the hatching rate of the offspring eggs was significantly reduced.After treated with the strain,the life span of the adults was significantly shortened,the number of eggs laid was reduced by about 2/3,and the mortality of the first instar larvae was 24.29%.The results provide theoretical basis and experimental evidence for the application of Bt in biological control of M.separata,and help to promote the development of green control technology of M.separata.