Inconsistencies among results of previous studies on the induction of adult diapause in the western tarnished plant bug, Lygus hesperus Knight, could be either attributed to differences in the criteria being applied to assess diapause status or to differences in the sources of the bugs. To determine the cause of variation and to refine the criteria used to define diapause status, we obtained eggs from adults collected from the field and from long-term laboratory colonies. Concurrent experiments were conducted at two locations. L. hesperus were reared from egg to adulthood under photophases of either 10 or 14 h and at a constant temperature (26.6°C, Shafter, CA; 27.5°C, Maricopa, AZ). Adults were dissected at 10-d posteclosion to measure diapause status using both stringent and relaxed morphological criteria. Under both sets of criteria, offspring of field caught parents were more likely to enter diapause with short day exposure than those offspring from stock colonies. The relaxed criteria appeared appropriate for distinguishing diapause in males, whose reproductive organs can appear undeveloped after mating, although the stringent criteria appeared appropriate for females. Additional analyses were conducted to determine whether extended laboratory rearing influenced other behavioral or physiological traits. Bugs from stock colonies were more likely to mate, and to oviposit as virgins, than were bugs originating from the field. The results demonstrate a pronounced effect of colony rearing on L. hesperus physiology and behavior, and also have led to refinement of the criteria used to assess diapause status.