The effect of temperature on prepupal diapause development in the hymenopteran Megachile rotundata (F.) was investigated by storing prepupae at various temperatures from −20° to 28 °C prior to final incubation at 28 °C. Preincubation temperatures of 5°, 10°, and 15 °C proved equally effective for diapause development and for the synchronization of adult emergence. Lower temperatures resulted in less synchronous emergence and higher mortality. When prepupae were exposed to more moderate temperature regimes prior to storage at lower temperatures, adult emergence became more synchronized and mortality declined. A high percentage of bees emerged after storage at 20 °C but their emergence was poorly synchronized. Over 50% of the prepupae stored at 28 °C developed without prior treatment at lower temperature but emergence was even less synchronized. The results of these experiments show that all prepupae diapause but the rate of diapause development becomes increasingly variable above and below an optimal range of 5° to 15 °C.