The sensitive stage for switching migrant oriental armyworm, Mythimna separata (Walker), into residents was determined under the stresses of starvation and cold (5 degrees C) in a laboratory population. Presumed migrants developing from gregarious larvae oviposited significantly earlier when adults were starved or shocked by cold only during the first 24 h after emergence. In contrast, preoviposition periods for migrants starved or cooled on days 2-6 of adult life were not significantly affected. The preresponse period of male migrants starved on day 1 of adult life was also significantly shortened, whereas those treated on following days were not significantly influenced. Similarly, total lifetime fecundity of migrants, which were starved or cooled on day 1 after emergence, was significantly increased; however, the same effect was not observed in migrants that were stressed later. Although preoviposition periods of presumed residents, developing from solitary larvae, starved or shocked by cold during the first several days of adulthood were significantly prolonged, this delay was not related to migratory strategy, because lifetime fecundity was not significantly reduced. The results suggest that the first day of adult life in this species is a sensitive stage during which migrants can shift into residents, although residents cannot shift to migrants. Thus, starvation and cold experienced in this stage induces migrants to modify their developmental path into reproduction and residency.