D3074: Diet C:N ratio and temperature interact to affect caterpillars' color thermoregulation

Animals can respond to variable temperatures along their range by plastically altering their body coloration, with darker coloration usually associated with colder environments and improved thermoregulation. However, this plastic response can be costly, thus potentially affected by their diet’s nutritional quality. We used tobacco hornworm (Manduca sexta, Sphingidae) to analyze how diet quality and temperature can interact to affect their fitness, color morphology, and thermoregulation. We reared individuals from egg to pupa at 27℃ or 18℃, representing extremes in M. sexta geographic range. We fed them a low and high Carbon:Nitrogen artificial diet to simulate the ratio found on plants associated with elevated CO2. Caterpillars had 3x faster development at 27℃ than 18℃. Survival and pupal mass were influenced by diet, with caterpillars fed a low C:N diet having higher survival and mass gain, confirming that diet with relatively more nitrogen to carbon was of better quality. We found that the diet quality resulted in most dissimilar coloration when caterpillars were growing at 18℃. Lastly, the temperature treatment influenced thermoregulation, in which caterpillars reared at lower temperatures heated up faster than caterpillars reared at warmer conditions. Our results show that diet quality and temperatures can interact to affect the fitness of caterpillars and alter their coloration, potentially allowing them to better thermoregulation at colder temperatures. These results can inform how insect herbivores may respond to changes in temperature and plant C:N ratio that may be associated with future climate change.