When Scolytus ventralis was reared under relatively constant temperatures 50-70% of the brood developed rapidly and emerged within 70 days. The remainder emerged gradually over the 130 days following the first emergence peak. Exposure to field conditions resulted in retarded emergence of the rapidly-developing proportion of the population and increased synchrony in the emergence pattern. Increasing exposures to cold temperatures in the field resulted in increased emergence synchrony, and a shorter developmental time when exposed to warmer temperatures in the laboratory. It was concluded that the rapidly-developing portion of the population may enter a facultative diapause while the remainder enters an obligatory diapause under normal field conditions
