Temperature fluctuations interact with means to impact life history traits in Spodoptera frugiperda (J.E. Smith) (Lepidoptera: Noctuidae)

Temperature variability associated with climate change is expected to impact insect performance and alter the ecological and economic impacts of both beneficial and pest species. However, our current understanding of how temperature impacts insect performance is not based on the fluctuating temperatures insects experience in their natural environments or the impact of these fluctuations over the lifespan. Yet, thermal performance curves vary for different organismal traits and life stages, and, thus, understanding the impacts of temperature fluctuations requires measures of multiple traits across the full life cycle. Using fall armyworm Spodoptera frugiperda, a global insect pest, we designed a full factorial experiment with a wide range of temperature treatments to study the effects of temperature mean and fluctuations on survival, development and/or reproduction in each life stage. Our results show interactive effects of mean temperature and the magnitude of daily fluctuations on survival to maturity, development time, and pupal mass as well as on the number of eggs laid by females and subsequent egg hatching. However, the impacts of temperature fluctuation varied across life stages: at the warmest mean temperature, larger fluctuations significantly reduced larval and pupal survival but not adult survival relative to constant temperatures. Our results demonstrate the importance of explicitly considering the biological effects of fluctuating temperatures across traits and life stages since these effects may become more salient under climate warming. Understanding how temperature fluctuations impact the entire life cycle of insects can support more accurate forecasts of insect populations under climate change.