Size, sex, & survival: water balance across temperatures in the alfalfa leaf cutting bee

Water balance is critical to insect survival. Megachile rotundata, a solitary bee, is used for alfalfa pollination across the United States, necessitating performance across a range of environmental conditions. We investigated sex-specific effects of temperature on desiccation tolerance, cuticular permeability, and water loss rates in adult M. rotundata. Females were larger than males in both wet mass (42.9 ± 8.2 mg vs. 32.5 ± 6.9 mg) and body water content (27.9 ± 7.1 mg vs. 20.8 ± 5.2 mg), with body water content scaling hypermetrically with dry mass while male body water content scaled hypometrically. For all bees, survival time decreased with increasing temperature and was positively associated with body water content, with the effect strongest at lower temperatures. Cuticular permeability was 1.8-fold higher in males than females. For both sexes, mass-specific water loss rates increased with temperature, but was 1.6-fold greater in males. Comparisons of water loss routes revealed that respiratory water loss substantially exceeded cuticular water loss in live bees, and the relative contribution of respiration increased with temperature. Overall, these results indicate that sex, body size, and route of water loss interact to determine desiccation risk in M. rotundata, and that males may be more vulnerable to desiccation at high temperatures due to higher cuticular permeability and respiratory water loss rates. These findings provide insight into the physiological adaptations of M. rotundata to water stress and contribute to a broader understanding of insect water balance mechanisms.