The mechanisms and adaptations involved in the "obligate" third instar larval diapause of Colias alexandra, a native Rocky Mountain and intermountain region butterfly, are examined. Generally univoltine throughout its distribution, scattered, isolated bivoltine populations occur. Factors influencing continuous development were investigated under laboratory conditions on a single generation of offspring from a univoltine population each year from 1976-1979. If exposed to mean temperatures >24°C during the second instar, a significant number of larvae fail to diapause. These results are interpreted in the context of the actual environmental conditions experienced by the univoltine source population and a bivoltine population. A high degree of individual variability in response to continuous development conditions is shown. Variation in certainty of diapause, within an originally univoltine population, could be the evolutionary starting point for evolving a genuine multivoltine cueing system and provide the mechanism for ecological range expansion.