The western tarnished plant bug (Lygus hesperus Knight) is an important crop pest in the western United States that overwinters in an adult diapause. However, knowledge of L. hesperus diapause is incomplete. Eggs from field-collected adults were reared under photoperiods of 10:14, 11:13, 12:12, and 13:11 (L:D) h at 26.7 ± 1°C, and the diapause response for each gender was modeled by a logistic function. Incidence of a recently reported “glass bead” fat body type also was examined. Validation studies using the same methods were subsequently conducted using photoperiods of 10.5:13.5, 11.5:12.5, and 12.5:11.5 (L:D) h. No effects of bug gender, photoperiod, or diapause status on occurrence of the novel fat type were detected. Estimates of diapause in validation studies were within confidence intervals for initial predictions, but systematic deviations from initial predictions prompted refitting of the models to include validation data. Refitted functions estimated critical photoperiods of 11h44min for females and 11h21min for males. The maximum incidence of diapause was lower for males than for females and was <100% for both genders. Refitted functions also predicted 50% of the population-specific maximum diapause response corresponded to photoperiods of 11h54min (females) and 12h7min (males). These results, combined with other recent findings, suggest heterogeneity in diapause response likely enables L. hesperus populations to adapt to local conditions. The estimated functions relating photoperiod to diapause incidence provide baselines to facilitate future studies of environmental and geographical influences on diapause in this species.