Heat stress and the temporal dynamics of insect growth
Temperature affects rates of ingestion, growth and other key processes in insects and other ectotherms, a relationship described by the thermal performance curve. But the history of temperature exposure can cause stress or acclimation responses, altering the relationship between current temperature and performance. The temporal dynamics of such time-dependent effects for ectotherm growth are poorly understood. We quantify how growth and ingestion change over time at different constant temperatures during the final larval instar for two thermal-generalist insect species, Pieris rapae L. and Manduca sexta L. Initial growth rates were greatest at higher temperatures for both P. rapae (29-35 {degrees}C) and M. sexta (34-38 {degrees}C); but growth rates at higher temperatures declined over time. As a result, the optimal and maximal temperatures for growth declined over time (2-4 days) in both species. Unlike P. rapae, M. sexta could maintain growth and survival to wandering and pupation at temperatures above 35 {degrees}C. For M. sexta larvae at 40 {degrees}C, ingestion continues even as growth declines, suggesting that post-ingestive processes limit growth rates at these high temperatures. Our results show how time-dependent effects at higher temperatures develop over hourly to daily time scales, with important consequences for understanding and modeling organismal responses to heat waves and extreme temperature events.