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Field studies over 3 yr demonstrated that overwintering populations of the Colorado potato beetle in upstate New York and on Long Island are composed of adults from both the first and second summer generations. The two populations from the climatically different regions differ in their responses to environmental factors that influence voltinism. The critical photoperiod for aestival diapause induction is longer, and the induction of diapause by low temperature is greater, in the population from the cooler, inland locality (upstate New York) than in the population from the warm coastal area (Long Island). Under the long days of early summer, temperature appears to influence aestival diapause induction and voltinism at both localities. A large proportion of first-generation adults enters diapause either without ovipositing or after ovipositing for a brief period. These responses have significance for the evolution of the beetle's life history and for its population dynamics and management.