Ed Southwick investigated a number of parameters and variables concerning thermoregulation and metabolic rates, reported in a series of publications through the 1980s. Can't find the exact reference but I think he took colonies down to -40 (where the Fahrenheit and Centigrade numbers coincide). Nice summary of the whole topic in a Bioscience 1987 article by him. And he even addressed the issue of the potential range of Africanized bees in the US and got it mostly right (in 1990, when the first swarms were detected in south Texas):
https://www.sciencedirect.com/science/article/abs/pii/0300962990907133
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