Climate change is implicated as a leading cause of insect declines. One way that insects respond to the warming climate is by advancing phenology and increasing voltinism (adding generations). However, if earlier phenology changes cue-response relationships, then late season generations might lack time or resources to complete development before winter and insects attempt doomed “lost generations”. Using 27 years of monitoring of 30 butterfly species, we find the opposite, as added generations increase population growth rates of multivoltine butterflies. We find lost generations are rare and occur at cooler sites in years with cold winters or early frosts. Overall, long-term population trends are positively correlated with increasing voltinism over time, suggesting additional generations as one mechanism by which species adapt to the changing climate. Long-term monitoring programs can test mechanistic hypotheses about biotic responses to warming while simultaneously tracking if population consequences match the predicted outcomes.