Overwintering pupae obtained from early and late emerging blueberry maggot populations were kept at similar temperatures to determine if phenological differences are caused by initial exposure to different temperature conditions in the soil after pupation. The effect of temperature and duration of exposure on time to emergence was determined by dividing pupae of each population into two cohorts placed at 20 or 25°C. Subsequently, at periodic intervals, samples were first transferred to 5°C for 6 mo to ensure diapause completion, and later kept at 20°C to measure time to emergence. In addition, we studied postdiapause pupal temperature-dependent development of the late population under laboratory and field conditions, and compared rates of development at constant temperatures and outdoors using degree-day emergence models. Longer exposure of pupae to similar temperature conditions did not decrease differences in time to emergence between populations. In the early population, there were no differences in postdiapause development with relation to temperature exposure, but in the late population, pupae exposed to 25°C emerged slower than pupae exposed to 20°C. In both populations, longer exposures to high temperature after pupation resulted in slower postdiapause development. Also in the late population, 52% of individuals emerged after 60 d exposure to 25°C, without going through diapause. Field pupal development of the late population was characterized by longer postdiapause than the early population. The presence of a transient change in the temperature-development rate relationship, observed during outdoor postdiapause development in July, was also apparent from degree-day model predictions. This developmental slowdown had not previously been observed in any Rhagoletis species.