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While both daytime and nighttime temperatures are increasing with climate change, few studies have experimentally investigated their differential effects under field conditions. We conducted a factorial field experiment examining how day- and night-warming impact the growth, survivorship, and behavior of cabbage white caterpillars (Pieris rapae). In this experiment, the night-warming only treatment showed the highest rates of caterpillar growth, but also showed the highest mortality, the shortest maximum caterpillar lengths, the least accumulated herbivory and reduced pupation. Caterpillars in the treatments that were not warmed during the day showed daytime-shifted growth, and caterpillars in the combined day- and night-warming treatment showed strongly night-shifted herbivory. Both biotic (e.g., predation risk) and abiotic (e.g., thermal) factors could have contributed to these results. Broadly, these results show the importance of temperature-mediated behavioral changes in diel activity for caterpillar development and survival. These results also support the emerging hypotheses that periods of reduced activity may be important for successful development, that warmer nighttime conditions could limit a temporal thermal refuge for caterpillars, and that increasing temperatures could increase the likelihood of metabolic meltdown. By documenting organismal responses in the context of more complex microclimates and communities, this experiment also illustrates the value of field studies to provide insights into how ectotherms might respond to ongoing climate change. 

While both daytime and nighttime temperatures are increasing with climate change, few studies have experimentally investigated their differential effects under field conditions. We conducted a factorial field experiment examining how day‐ and night‐warming impact the growth, survivorship, and behavior of cabbage white caterpillars (Pieris rapae). In this experiment, the night‐warming only treatment showed the highest rates of caterpillar growth, but also showed the highest mortality, the shortest maximum caterpillar lengths, the least accumulated herbivory, and reduced pupation. Caterpillars in the treatments that were not warmed during the day showed daytime‐shifted growth, and caterpillars in the combined day‐ and night‐warming treatment showed strongly night‐shifted herbivory. Both biotic (e.g., predation risk) and abiotic (e.g., thermal) factors could have contributed to these results. Broadly, these results show the importance of temperature‐mediated behavioral changes in diel activity for caterpillar development and survival. These results also support the emerging hypotheses that periods of reduced activity may be important for successful development, that warmer nighttime conditions could limit a temporal thermal refuge for caterpillars, and that increasing temperatures could increase the likelihood of metabolic meltdown. This experiment also illustrates the value of field studies to provide insights into how ectotherms might respond to ongoing climate change.