Pupae of the blueberry maggot, Rhagoletis mendax Curran, from populations with late flight period develop much faster when placed at constant temperature than when kept outdoors through summer, suggesting that exposure to extreme temperature outdoors may delay adult emergence by causing a heat-induced quiescence. We evaluated the effect of heat exposure on time to adult emergence with pupae of both early and late populations in three experiments. Heat exposure consisted of 12 h at 25°C followed by 12 h at 35°C for different periods of time (10–30 d). In the first experiment, the time to emergence at 20°C was sharply reduced in most samples of pupae of the late population kept in an incubator and exposed to heat relatively early in diapause. However, regardless of the duration of exposure, pupal development always stalled at a threshold of ≈50 d to emergence. In the second experiment, heat exposure increased time to emergence in most samples of pupae of the late population brought from outdoors immediately before the onset of high soil temperature. These pupae also took ≈50 d to emergence after the end of the heat exposure, despite being more advanced in diapause than the pupae in the previous experiment. In the last experiment, short heat exposure accelerated, and longer exposure delayed development of pupae of the early population. As in the previous experiments, emergence occurred ≈50 d after the end of the heat exposure. These results suggest that a heat-induced quiescence is a likely cause for the developmental delay observed in pupae of the late population through summer. Furthermore, the similar time to emergence after heat exposure of both pupae of early and late populations suggests that a discrete quiescence-sensitive period occurs during a specific stage of pupal development in the blueberry maggot. This is the first report of a heat-induced quiescence interrupting diapause development in the blueberry maggot or any Rhagoletis fruit flies.