Pupal diapause in the blueberry maggot was studied to characterize the relationship between temperature and development during different phases of diapause and determine the traits responsible for the late adult emergence period in some populations present in blueberry fields in southern New Jersey. Pupae in the refractory phase of diapause were exposed to 1, 4, or 7°C for 45, 90, or 135 d, and in a separate experiment, to 11 or 15°C for similar periods of time, to determine temperature thresholds and rates of diapause development at constant temperature. Results showed that pupae took much longer to reach the activated phase of diapause when exposed to 1°C than 4 or 7°C and were able to reach the activated phase when exposed to up to 15°C. In the range of 4–15°C, diapause development was largely independent from temperature. Pupae in the activated phase of diapause were exposed to 7, 15, 20, 25, 30, 25:35 (12:12 h), and 35°C for 10 d. There was a linear increase in the development rate in the range of 7–30°C, and the development rate was similar at 30, 25:35 (12:12 h), and 35°C. The proportion of pupae reaching the activated phase of diapause from August to February in a normal early population and from October to February in a late population was compared by keeping pupae in the refractory phase outdoors in a screenhouse and bringing samples to 20°C monthly. Data showed that pupae of both early and late populations reached the activated phase from October to January. The duration of the activated phase of diapause in pupae of early and late populations kept at 20°C after exposure to cold was compared by measuring changes in their respiration rates. Results showed that the activated phase lasted 10–20 d in the early population versus 60–70 d in the late population. In the blueberry maggot, and possibly in other Rhagoletis species, adaptation of the adult emergence period to the fruiting period of different host plants seems to be mediated by changes in the duration of the activated phase of diapause.