Starvation rather than winter cold is the main cause of wintertime colony losses. Here's how to prepare your bee colonies for winter and what you can do to help make sure that they'll be around to produce honey next year.
Moorman, Robert B.
"Help Your Bees Survive the Winter,"
Iowa Farm Science: Vol. 17
, Article 5.
Available at: https://lib.dr.iastate.edu/farmscience/vol17/iss4/5