The Giant Swallowtail butterfly, Papilio cresphontes Cramer (Papilionidae), has been reported in New York State for nearly 150 years. In recent years there has been an unexplained increase in P. cresphontes occurrences along the northeastern periphery of its geographical range. This study examined historical records to describe the movement of P. cresphontes populations into New York State and adjacent Ontario. Climate data and field studies were used to identify environmental factors that may influence the range expansion, which was found to correspond with an absence of September frosts beginning in 2001. Field studies indicated that some P. cresphontes larvae were capable of withstanding multiple frosts and descended to pupate normally into late October in the range expansion area. Although the larvae may have adapted to endure cooler temperatures to some degree, the effects of warming temperatures on other factors such as natural enemies and larval host plant quality in autumn may influence the spread of P. cresphontes populations at least as much as larval frost tolerance.