Photoperiod influences diapause induction in the grape berry moth, Endopiza viteana (=Polychrosis viteana), and eggs and neonates (<12 h) were the most sensitive to decreasing photoperiod. When held at photoperiods of 7–14 h, eggs produced mostly diapausing pupae, whereas at 15 h, development proceeded without diapause. Neonate larvae also showed sensitivity to short photoperiods, but responsiveness of larvae decreased rapidly with age, so that instars greater than 12 h old exposed to a short photoperiod of 8 h developed into adults without diapause. Eggs held at a high temperature (30°C) and 8-h photoperiod also produced diapausing pupae, suggesting that diapause induction in E. viteana is independent of temperature. Field studies showed considerable population variation in diapause induction, and an exponential saturation model predicted that 50 and 90% of individuals entered diapause at photoperiods of 14 h, 45.5 min and 14 h, 21.1 min, respectively.