The brassica leaf beetle Phaedon brassicae Baly, one of the serious pests infesting cruciferous vegetables, had a complex life history. The incidence of diapause of the beetles fed with old and young bakchoi leaves, and the survival period of adults under starvation, were measured in this study. The results showed that the incidence of diapause of the beetles fed with old bakchoi leaves was significantly higher than that of the beetles fed with young bakchoi leaves at 25℃ and 12L∶12D. By transferring the adults obtained at 25℃ and 12L∶12D with 0～7 days' fed with young bakchoi leaves to the conditions of 30℃, 14L∶10D without offering food, the adult survival period increased with the increasing of feeding period before. Survival period of adults increased significantly when they fed 5 days or more, and the mean survival period of adults exceeded 75 days. Finally, possible mechanism of adaptation to aging and absence of host-plant in the brassica leaf beetle was discussed.