This is a very interesting question. I am convinced if its too mild, bees continue to consume more stores, they dont go in to a dormant state, they keep working and they wear themselves out more over the winter months. So, come next spring when the queen really gets going, bees tend to die earlier, as they have worked harder over winter.
Subsequently the transition between the old overwintering bees and the new bees being born (the increase if you like) is "jaded", as the support staff bringing on the young brood struggle because they are at the end of their life and therefore the colonie struggles to get going.
Last year for us was a classic example. We had warm mild weather up until now,Then we had a period of much colder, with daytime temperatures of between 2 and 8 degrees for about 6 weeks, so bees were very quiet, hardly moving, almost catatonic. if you looked under duck tape, you could see them, if you bother them enough they would stick their butts in the air, but thats all. They were very strong , early in the spring as soon as temperatures rose in early march.
In colder climates they are silent! if you stick your head against the hive you hear nothing, stone cold silence and your sure their dead.
Yesterday we had 16 degrees here, i undid the duck tape on one of my nucs and a splurge of bees came out the feeder hole!!
So Yes personally i think cold is good!! it helps the bees stay stronger for longer. Providing their well stocked up, their used to cold weather and have inbuilt method of overwintering on minimal food!! We always overfeed and always worry, but most of the time, their just fine!! its how they have evolved, long before we started sticking them in boxes and wearing medals for doing so!!!!
