Pidorus euchromioides Walker has two separate diapauses during its life cycle, a winter diapause as a fourth larval instar and a summer diapause as a prepupa in a cocoon. Winter diapause was induced by short photoperiods. The critical day length of 13 h 20 min to 13 h 40 min at 23.3°C under insectary conditions was very similar to the critical day length (13 h 32 min) observed in the field. Sensitivity to day length was restricted to the first 9 days, corresponding to the first and second instars. The duration of larval diapause was not only strongly correlated with the inducing photoperiod, but also dependent on the number of short days experienced during the larval growth period. The shortest diapause occurred at the critical day length, or when the larvae were exposed for 6 days (a critical number of short days) to a photoperiod of LD 12 h:12 h. The summer diapause appeared to be primarily determined by a genetic factor and only occurred in part of the overwintering generation and the first generation. The two different diapause mechanisms play important roles. Winter diapause ensures that P. euchromioides synchronizes its development and activity with the phenology of specific host plants, and summer diapause provides the species with a high degree of adaptation to an unpredictable environment.