Mortality data from the literature on the southwestern corn borer, Diatraea grandiosella Dyar, were analyzed to determine which factors have the most impact on populations of this insect. Adult mortality, presumed to occur primarily as a result of dispersal, appears to have been the largest source of mortality, as well as the key factor in the annual variance of mortality. Nonpredation mortality during diapause was also an important factor. Of the density-dependent mortality factors, predation by the flicker, Colaptes aruaus (L.) appears to be the factor that limits the size of overwintering populations in the southeastern United States, while cannibalism among prediapausing larvae limits the size of overwintering populations in the western Great Plains. Based on the analysis of mortality rates and historical field data, the annual finite capacity for increase (multiplication rate) was estimated to be ≈10 in the central United States. This analysis suggests that to comprehend more clearly the factors that influence population variation in D. grandiosella, future research efforts should be focused on understanding adult dispersal, and the interaction between flicker predation and nonpredation mortality in the diapausing larvae stage.