Cerceris fumipennis (Say, 1837) (Hymenoptera: Crabronidae) is a solitary ground-nesting wasp that uses adult buprestid beetles to provision nests. It is being used as a detection tool for the invasive buprestid, Agrilus planipennis F., the emerald ash borer. Modeling the degree days necessary for emergence of the wasp will aid in that effort. During 2010 and 2011, 27 and 23 soil temperature probes, respectively, were deployed at C. fumipennis colonies in three US states and Ontario, Canada. Sites were monitored daily for wasp emergence. Degree-day models were constructed using the soil temperature data from the probes and air temperature data available on the internet from nearby weather stations. Our base temperature for degree-day accumulation for C. fumipennis was 10°C. The average soil degree days needed for emergence was 696.2 ± 16.8. The average air degree days needed for emergence was 463.73 ± 14.01. To measure the performance of the air degree-day model, we tested the model against a set of 24 distinct sites for which emergence dates were known from 2010–2012. The average difference in date at which the predicted degree days were accumulated, and that at which wasps emerged at a site was—0.15 d, with a median value of 1 d. This provides a narrow window to monitor for emergence using readily available data, air temperature, which is more efficient and cost effective than driving to colonies weekly or daily to ascertain wasp activity.