One of the members of our beekeeping association used a heating pad on
one of his hives during winter.  He checked the hive during the annual
"January Thaw" in Wisconsin.  His bees were doing poorly so he ran an
extension cord out to the hive and set the heating pad on the top bars
along with some menthol (he suspected tracheal mites).  After three
weeks (now well into February) the hive was recovering so he
discontinued his treatment.  He claimed that the hive was one of his
strongest that summer.