Exposure to suboptimal temperatures during metamorphosis reveals a critical developmental window in the solitary bee, Megachile rotundata

Authors: BENNETT, MEGHAN - North Dakota State University; COOK, KEELEY - North Dakota State University; Rinehart, Joseph - Joe; Yocum, George; Kemp, William - Bill; GREENLEE, KENDRA - North Dakota State University

Submitted to: Physiological and Biochemical Zoology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 4/16/2015
Publication Date: 9/1/2015
Publication URL: http://handle.nal.usda.gov/10113/61431
Citation: Bennett, M.M., Cook, K.M., Rinehart, J.P., Yocum, G.D., Kemp, W.P., Greenlee, K.J. 2015. Exposure to suboptimal temperatures during metamorphosis reveals a critical developmental window in the solitary bee, Megachile rotundata. Physiological and Biochemical Zoology. 88(5):508-520.

Interpretive Summary: The alfalfa leafcutting bee is an important alternative pollinator in the United States. This bees spend much of its life in brood cells constructed in cavities by adult females, and spends the winter in these cells as well. Development resumes in the spring with warmer temperatures, eventually leading to adult emergence. While the overwintering bees are tolerant of cold temperatures, bees developing in the spring are more vulnerable to sudden cold spells. This may become increasingly important since global climate change is predicted to increase temperature variability, including the occurrence of spring frosts. Although survival after a cold shock has been studied in the past, this study was designed to investigate the quality of bees that survived a prolonged cold exposure and those that were given potentially protective warm temperature pulses during the cold treatment. Both warm temperature pulsed and cold stressed bees showed delayed adult emergence, and males and females were delayed differently. The cold stressed bees showed additional damage as well, including shorter lifespans, and the fact that 50% of bees cold stressed during development were unable to fly as adults. Other effects of the cold stress included reduced feeding activity and a reduction in wing size. Thus, this study shows that cold exposure during development significantly affects the resulting adult bees, and that multiple measurements beyond simple mortality must be used to fully appreciate the effects of cold stress.

Technical Abstract: 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.