Most of the world’s pollinator species are wild, solitary bees. Recent declines in bee populations have been attributed to habitat loss, pesticides, parasites, and currently climate change is another looming threat. During the fall, solitary bees undergo diapause where their metabolic rate is internally reduced. However, in the late winter, they enter a state of post-diapause quiescence where metabolism is regulated by environmental factors. As winter and spring temperatures rise with climate change, bees are put at risk of higher respiration rates leading to reduced fat storage and possible mortality. We evaluated the physiological response to warmer temperatures during their period of overwintering. We measured oxygen consumption at 4°, 12°, and 22° C during diapause and post-diapause quiescence in the native bellflower resin bee (Megachile campanulae) and the non-native alfalfa leafcutter bee (Megachile rotundata). We also overwintered bees in Columbus, Ohio; we used control sites and sites where we experimentally raised ambient temperature approximately 5° C using heating pads, to simulate climate warming. We compared body mass change, mortality rates, and emergence time of bees between the control and raised temperature sites. Results from this study can help predict how other solitary bee species will react to the changing climate.
Results/Conclusions
Survivorship across species and treatments was not different. Metabolic rates of native and non-native Megachile were not significantly different from another. However, oxygen consumption during diapause of both bees were significantly higher at 22° C that at 4° and 12° C, which did not differ. These results indicate that during diapause both bee species can save significant amounts of energy if ambient temperatures are kept relatively low. In preliminary analyses, M. rotundata overwintered at a higher temperature emerged later than bees overwintered at a lower temperature. The timing between male and female emergence did not differ between temperature treatments. Higher metabolic rates combined with longer periods of time until emergence could result in lower energy upon emergence or lower fitness. Overall, these results predict some of the future struggles that Megachile bees and potentially other solitary bees will face because of the changing climate.