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Thermoprofile Parameters Affect Survival of Megachile rotundata During Exposure to Low-Temperatures

George D. Yocum, Joseph P. Rinehart, Arun Rajamohan, Julia H. Bowsher, Kathleen M. Yeater, Kendra J. Greenlee
Integrative and comparative biology 2019 v.59 no.4 pp. 1089-1102
Megachile rotundata, chilling injury, cold stress, temperature
Insects exposed to low temperature stress can experience chill injury, but incorporating fluctuating thermos profiles increases survival and blocks the development of sub-lethal effects. The specific parameters required for a protective thermoprofile are poorly understood, because most studies test a limited range of thermoprofiles. For example, thermoprofiles with a wave profile may perform better than a square profile, but these two profiles are rarely compared. In this study, two developmental stages of the alfalfa leafcutting bee, Megachile rotundata, eye-pigmented pupae, and emergence-ready adults, were exposed to one of eight thermoprofiles for up to 8 weeks. All the hermoprofiles had a base of 6C and a peak temperature of either 12C or 18 C. The duration at peak temperature varied depending on the shape of the thermoprofile, either square or wave form. Two other treatments acted as controls, a constant 6C and a fluctuating thermal regime (FTR) with a base temperature of 6C that was interrupted daily by a single, 1-h pulse at 20C. Compared with constant 6C, all the test thermoprofiles significantly improved survival. Compared with the FTR control, the thermoprofiles with a peak temperature of 18C outperformed the 12C profiles. Bees in the eye-pigmented stage exposed to the 18C profiles separated into two groups based on the shape of the profile, with higher survival in the square profiles compared with the wave profiles. Bees in the emergence-ready stage exposed to 18C profiles all had significantly higher survival than bees in the FTR controls. Counter to expectations, the least ecologically relevant thermoprofiles (square) had the highest survival rates and blocked the development of ublethal effects (delayed emergence).