Effects of temperature on metabolic rate during metamorphosis in the alfalfa leafcutting bee

Authors: EARLS, KAYLA - North Dakota State University; CAMPBELL, JACOB - North Dakota State University; Rinehart, Joseph; GREENLEE, KENDRA - North Dakota State University

Submitted to: Biology Open
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 11/15/2023
Publication Date: 12/15/2023
Citation: Earls, K., Campbell, J., Rinehart, J.P., Greenlee, K. 2023. Effects of temperature on metabolic rate during metamorphosis in the alfalfa leafcutting bee. Biology Open. https://doi.org/10.1242/bio.060213.
DOI: https://doi.org/10.1242/bio.060213

Interpretive Summary: Insects rely on air temperatures of their habitat to determine their body temperatures. This can cause a variety of negative effects when temperatures encountered are either lower or higher than optimal conditions. However, while the harmful effects of thermal stress are well known for many species, the key physiological traits of an insect during the thermal stress event are often not well described. This limits our ability to fully understand the effects to stressful events, and to predict how economically important insects, such as pollinators, are likely to respond to an expected increase in stressful events caused by climate change. Therefore, using the alfalfa leafcutting bee, we established a thermal performance curve that measured respiration rates over a broad range of temperatures from 6°C to 48°C. We discovered that the respiration response to temperature changes was not linear, with rates increasing dynamically with increasing temperatures. Interestingly, at temperatures well above what other experiments have shown to cause significant injury, respiration rates continued to climb, rather than reaching a peak and then declining as we expected. These data represent a first step in characterizing this insect’s response to a wide range of temperatures, and will serve as a basis for future studies that will investigate the physiological and ecological effects of thermal stress.

Technical Abstract: Spring conditions, especially in temperate regions, can fluctuate more abruptly and drastically. Environmental variability can expose organisms to temperatures that are outside of their optimal range. For ectotherms, sudden changes in temperature can cause short and long-term physiological effects, including changes in respiration, morphology, and reproduction. The alfalfa leafcutting bee, Megachile rotundata, resumes active development in the spring after overwintering as prepupae. During this time, developing bees can be exposed to changes in temperature. Oxygen consumption of M. rotundata pupae were measured across a large range of temperatures (6-48°C) using a closed optical oxygen sensor system. Absolute and mass specific metabolic rates were calculated for bees that were extracted from their brood cells and those remaining in the brood cell and between sexes. Metabolic response to temperature created a non-linear slope, which is an assumption of a thermal performance curve; however, the slope descent at higher temperatures was not observed. Despite sexual dimorphism in body mass, sex differences only occurred in mass-specific metabolic rates. Understanding the physiological and ecological effects of thermal environmental variability on M. rotundata will help to better predict their response to climate change.