Climate change is implicated as one contributor to insect declines. Insects may respond to warming by advancing phenology and increasing the number of generations each year (voltinism). However, if earlier phenology changes cue-response relationships, then late-season generations might lack time to complete diapause development before winter and result in doomed “lost generations”. Using 27 years of monitoring of 30 multivoltine butterfly species, we find the opposite, as larger late-season generations (voltinism shifts) are associated with higher overwinter population growth rates. The potential threat of lost generations is limited to late-season species at cooler sites in years with early frosts. Overall, long-term population trends are positively correlated with larger late-season generations, suggesting that they are an adaptive response to climate warming. Still, overwinter population growth rates and long-term population trends have declined over time as the benefits of voltinism shifts have been insufficient to reverse population declines.