Environmental factors that regulate the induction, maintenance and termination of adult diapause and postdiapause oviposition were examined in a phytophagous insect, Dybowskyia reticulata (Heteroptera: Pentatomidae). When insects were reared from eggs at 25°C in the laboratory, all adults under a short-day photoperiod of 12:12 (L:D) hr and most adults under a long-day photoperiod of 16:8 (L:D) hr entered diapause. Overwintering adults started oviposition after being transferred to the long-day or short-day photoperiod at 25°C in March. Under the short-day photoperiod, however, they entered diapause again after a short oviposition period. Transferral to a long-day photoperiod after a long exposure to a short-day photoperiod synchronously terminated diapause. After exposure to a low temperature during diapause, the adults began to lay eggs under both long-day and short-day photoperiods. Low temperature in winter probably plays a dominant role in the termination of diapause in nature. In addition, the females started oviposition only after feeding when transferred from outdoor conditions to the laboratory in spring, and the onset of oviposition after starvation in the females was more synchronous than that without starvation. The regulation of the seasonal life cycle by various environmental factors in D. reticulata is discussed as an adaptation to the phenology of host plants.