It was the first time to make clear the difference between two strains in the super-cooling point of diapause prepupae was significant（P＜0.05）.Before diapause,the super-cooling point and freezing point of prepupae of the unvoltine strain were lower 2.03℃ and 2.89℃ than that of the bivoltine one,respectively.The difference became lower and lower as the treatment time at low temperature increased.The difference of prepupae super-cooling point between two strains’ was not significant after they were treated at low temperature for 12 months.（P＜0.05）.