AbstractTemperature‐based degree‐day models describe insect seasonality and to predict key phenological events. We expand on the use of a temperature‐based process defining timing of reproduction through the incorporation of female reproductive physiology for the invasive pentatomid species Halyomorpha halys, the brown marmorated stink bug. A five‐stage ranking system based on ovary development was able to distinguish between the reproductive statuses of field‐collected females. Application of this ranking method described aspects of H. halys’ seasonality, overwintering biology, and phenology across geographic locations. Female H. halys were collected in the US from NJ, WV, NC, OR, and two sites in PA in 2006–2008 (Allentown, PA only) and 2012–2014. Results identify that H. halys enters reproductive diapause in temperate locations in the fall and that a delay occurs in developmental maturity after diapause termination in the spring. Modification of the Snyder method to identify biofix determined 12.7‐hr photoperiod as the best fit to define initiation of reproduction in the spring. Applying the biofix, we demonstrated significant differences between locations for the rate at which the overwintering generation transition into reproductive status and the factors contributing to this difference require further study. For example, after including abiotic variables influencing development such as temperature and photoperiod (critical diapause cue), reproduction occurred earlier in OR and for an extended period in NJ. This data describe a method to investigate insect seasonality by incorporating physiological development across multiple regions that can clarify phenology for insects with overlapping generations.