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

Abstract

Spring conditions, especially in temperate regions, may fluctuate abruptly and drastically. Environmental variability can expose organisms to temperatures outside of their optimal thermal ranges. For ectotherms, sudden changes in temperature may cause short- and long-term physiological effects, including changes in respiration, morphology, and reproduction. Exposure to variable temperatures during active development, which is likely to occur for insects developing in spring, can cause detrimental effects. Using the alfalfa leafcutting bee, Megachile rotundata, we aimed to determine if oxygen consumption could be measured using a new system and to test the hypothesis that female and male M. rotundata have a thermal performance curve with a wide optimal range. Oxygen consumption of M. rotundata pupae was measured across a large range of temperatures (6-48°C) using an optical oxygen sensor in a closed respirometry system. Absolute and mass-specific metabolic rates were calculated and compared between bees that were extracted from their brood cells and those remaining in the brood cell to determine whether pupae could be accurately measured inside their brood cells. The metabolic response to temperature was non-linear, which is an assumption of a thermal performance curve; however, the predicted negative slope at higher temperatures was not observed. Despite sexual dimorphism in body mass, sex differences only occurred in mass-specific metabolic rates. Higher metabolic rates in males may be attributed to faster development times, which could explain why there were no differences in absolute metabolic rate measurements. Understanding the physiological and ecological effects of thermal environmental variability on M. rotundata will help to better predict their response to climate change.

Keywords: Megachile rotundata; Closed system respirometry; Oxygen consumption; Solitary bees; Thermal performance curve.

Fig. 1.

Comparison of mass-specific metabolic rate calculations from both respirometry systems. Oxygen consumption was not statistically different between systems (two-tailed t-test; t=−0.682, d.f.=25, P=0.502; n=20 bees per system). Carbon dioxide was measured concurrently with oxygen in the fuel cell system but not in the microplate system.

Fig. 2.

V̇O₂ for bees left in their brood cells (red triangles) and those that were extracted from their brood cells (blue circles) during measurements in objective 1. No statistical differences (P>0.05) were detected by two-way ANOVA in absolute (A) and mass-specific (B) metabolic rates between bees that were extracted (n=16) or left in their brood cells (n=16). Open colored circles indicate individual bee measurements, while the black circles and triangles represent the mean±s.e.m.

Fig. 3.

Absolute V̇O₂ between the first (n=32 per temperature) and second (n=40 per temperature) objectives. There was a significant difference in absolute V̇O₂ between objectives 1 and 2 (two-way ANOVA, F_1,673=49.3, P<0.0001). Additionally, pupae used for objective 1 weighed 4 mg more than those used for objective 2 (objective 1: 0.0487±0.0005 g; objective 2: 0.0444±0.0004 g). Open colored circles indicate individual bee measurements, while the black circles and triangles represent the mean±s.e.m.

Fig. 4.

V̇O₂ for female and male bees measured by the optical oxygen sensor system in objective 2. Pupae at 48°C were unable to complete development are represented by black open circles, because we were unable to determine sex. Those bees are not included in the regression. There was a significant difference in both absolute (A; two-way ANOVA, F_11,445=1160, P<0.0001) and mass-specific (B; two-way ANOVA, F_1,434=239, P<0.0001) V̇O₂ across temperatures. (A) No statistical differences (F_1,445=0.0464, P=0.830) were detected between absolute V̇O₂ of female and male pupae across T_a (n=40 bees per temperature). (B) Mass-specific V̇O₂ between sexes was significantly different across temperatures (n=40 bees per temperature). The number of males and females differed across temperatures (see Materials and Methods). Open colored circles indicate individual bee measurements, while the black circles and triangles represent the mean±s.e.m.

Fig. 5.

Six of the temperature treatments resulted in significant regressions of oxygen consumption versus mass. Individual points represent a single M. rotundata pupa (n=40 per temperature treatment). Individual regressions were performed on all the temperatures to determine which are significant (P<0.05). An ANCOVA was performed on the significant regressions (F_5,244=255, P<0.0001). All slopes differed significantly from each other except for 33°C and 40°C.