Exposure to Suboptimal Temperatures during Metamorphosis Reveals a Critical Developmental Window in the Solitary Bee, Megachile rotundata.

Metamorphosis is an important developmental stage for holometabolous insects, during which adult morphology and physiology are established. Proper development relies on optimal body temperatures, and natural ambient temperature (Ta) fluctuations, especially in spring or in northern latitudes, could result in interruptions to development. It is unclear how low-Ta exposure may affect insects that are actively developing. To understand how suboptimal Ta may affect metamorphosing insects, we used the alfalfa leafcutting bee, Megachile rotundata (Fabricius), a solitary, cavity-nesting bee that spends its juvenile and pupal stages within a brood cell. We characterized suites of physiological traits, rather than just using a singular parameter to determine effects of sublethal Ta stress. Metamorphosing M. rotundata were exposed to either constant or fluctuating low-Ta stress and compared to control bees allowed to develop normally. All bees survived and emerged as adults, but the constant low-Ta-stressed bees were affected most severely. Male constant low-Ta-stressed bees had decreased flight performance (lower metabolic rate, shorter flight bouts, decreased wing length), suggesting that the stress altered muscular or neurological development. Constant low-Ta-stressed bees also had altered activity levels, providing more support for the hypothesis that low-Ta stress causes long-term neurological defects. Exposure to fluctuating low Ta also delayed development time for both sexes; males had decreased adult life span, and both sexes had shortened wings. Together, these results provide evidence for a critical developmental window during metamorphosis and suggest that there may be severe implications for bees in the wild that are exposed to low-Ta stressors.