The southwestern corn borer, Diatraea grandiosella, has expanded its range during this century so that it is now distributed between about 18 and 38°N latitude in Mexico and the United States. The species survives from year to year by entering a facultative larval diapause, and over most of its range it is bivoltine or trivoltine. Fully grown pre-diapausing larvae prepare an overwintering cell in the stalk crown of their host plant, typically maize, and then ecdyse from a spotted to a pigment-free morph at the onset of diapause. Southwestern corn borers originally obtained from southcentral Mexico (19°N latitude) and the United States (33–38°N latitude) were reared on an artificial diet under standardized conditions to compare several of their life history traits. Information is presented about voltinism, photoperiodic responses, growth and reproduction, larval behaviour and ecdyses, and the diapause-associated protein in an attempt to evaluate characteristics that have enabled the species to disperse from a tropical to a temperate region.