This is a great discussion! The fact that bees initiate brood rearing in the cold dark of winter, at least here in the frozen  north, is "intuitively obvious to the most casual observer". IOTTMCO is one of my favorite sayings although I don't know who to credit it to. We know that they do it but we don't know why or how. If it was easy to figure out science would have known long ago. If it were really easy even we beekeepers would have figured it out.

For each hypothesis we have both supporting and conflicting evidence. 

Maple bloom is correlated with degree days but sap flow seems to be circannual. I observe and agree with both. 

Photoperiod changes little around the solstice and temperatures can vary greatly with location but brood initiation  is pretty consistent at around early Jan., circannual? Colonies stored indoors in the dark and with controlled atmosphere and temp (40 f) have zero brood when they leave storage in mid Jan. but have eggs when they arrive in Calif. 1 day later.  (Personal communication with WSU researchers) Photoperiod or at least light? Don't discount truck vibration.

Mike observed that the only bee to be old enough to be effected by circannual effect would be the queen. Fair enough, bur Monarchs somehow find a particular grove of trees in central Mexico five generations and 1 year after leaving on their northern migration. Multi-generational circannual information transfer, with a pretty good GPS to boot ?

Degree days set the fruit bloom here in WA and can be well predicted and colony placement scheduled but the general trend with climate change is for steadily earlier bloom. Migrating species like birds and insects are not adjusting fast enough and are missing the prime time in increasing observations. Degree day, photoperiod, and circannual sending conflicting signals?

I expect that the answer is a combination of these and probably other factors that we have not even considered. But it is fun to contemplate here, snug and warm, in our winter lairs.

Paul Hosticka
Dayton WA




 
 

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