Development of management tactics that reduce reliance on conventional pesticides for control of the western tarnished plant bug, Lygus hesperus Knight, will require improved understanding of Lygus ecology. One poorly characterized aspect of Lygus ecology is the induction of adult diapause. Although photoperiod is a key regulator of diapause, previous research has reported inconsistent results, likely because of the different criteria used to distinguish diapause. To establish standardized criteria, we reared L. hesperus under photoperiods of 10- and 14-h and a constant temperature of 26.6°C. Adults were dissected at ages of 3, 7, 10, 14, and 17 days to evaluate associations among characteristics of the fat body and reproductive organs. Whereas most characters examined were at least weakly associated with others, we concluded that seminal vesicle condition, presence of a fatty sheath on the testis, and fat body color were not reliable indicators of L. hesperus physiological status. The most appropriate criteria for distinguishing diapause appeared to be hypertrophied fat bodies combined with undeveloped medial accessory glands (for males) or undeveloped ovaries (for females) in adults that were at least 10 days old. We also evaluated less stringent criteria, permitting some development of accessory glands or ovaries to accommodate the delayed reproductive development observed in some specimens reared under the 10-h photoperiod. The descriptions and illustrations of the morphological characters, combined with our suggested sets of criteria, should permit a more standardized assessment of L. hesperus physiological status, and thereby facilitate meaningful comparisons of research by different investigators.