Evaluating the effects of temperature variability on a lepidopteran agricultural pest: from life history traits to host-pathogen interactions

Temperature variability associated with climate change is expected to impact insect performance and alter the impacts of pest species and the efficacy of biocontrol strategies. However, our understanding of insect thermal biology is mostly based on measurements of insect performance under constant temperatures, which may not reflect the fluctuating temperatures insects experience over their lifespans in their natural environments. Moreover, thermal performance curves vary for different organismal traits and life stages; thus, understanding the impacts of temperature fluctuations requires measures of multiple traits across the life cycle. Here, I explored the impacts of temperature variability on the life history and demography of Spodoptera frugiperda, a widespread insect pest, and its susceptibility to a pathogen commonly used in biocontrol. Using a full factorial experiment with a wide range of temperature treatments, I systematically investigated the effects of temperature mean and fluctuation magnitude on fitness-related traits in each life stage. I then used these data to parametrize stage-structured matrix population models to understand the overall effect of temperature means and fluctuations on expected population growth rates. I found interactive effects of mean temperature and the magnitude of daily fluctuations on survival to maturity, development rates, pupal mass, reproductive output, numbers of viable offspring and population growth rates. Larger fluctuations significantly reduced larval and pupal survival but not adult survival relative to constant temperatures. Effects of variability on individual traits and population growth rates were generally larger and more negative at the highest mean temperature. Changes in expected population growth with increasing variability are primarily driven by differences in reproductive parameters and egg hatch rates. Lastly, I investigated the effects of temperature fluctuations at intermediate and elevated mean temperatures on outcomes in S. frugiperda larvae exposed to sublethal doses of Bacillus thuringiensis subsp. kurstaki. Mean temperature largely predicted survivorship following infection. However, temperature fluctuations modified delays in larval development in those surviving Bt infection, including at the intermediate mean temperature. Together, these results demonstrate the potential consequences of temperature fluctuations for multiple organismal traits and in the context of biotic stressors, which may become more salient under climate warming.