> Bees certainly seem to respond to day length.  Here good pollen and nectar flows can moderate the autumn/spring response, but not eliminate it.

It is one thing to observe the extreme seasonal changes that honey bee colonies go through; it's another thing to show what triggers these changes. It is certainly possible that changes in day length, changes in temperature, etc. are cues that let the colony know whether to expand or contract. 

The fact that honey bees evolved in the tropics offers alternate explanations. Rather than hang their life cycle on an annual winter/summer dynamic -- they may be much more closely tethered to the feast/famine cycles that they encountered in their ancestral home (Africa). The ability to cut way back or expand in any season would be beneficial under a much wider range of scenarios than to simply follow lengthening and shortening days.

In a Mediterranean climate, the days are short in winter, but the temperatures stay reasonable and bees can forage all year long. In a desert region, they may have 3 or 4 months of dearth during summer, when the days are the longest. If a colony shut down in January in say Southern California, they would miss some of the best pollen and nectar flows of the year. If they did NOT shut down during an extended summer drought, they would starve.

Further, in many deserts, like the Sonora desert of Arizona, there are thunderstorms in August which causes sudden blooming and colonies can switch quickly to gather the available resources. So bottom line, I suggest the seasonal aspect of honey bee colonies is not hard wired, not tethered to photoperiodism at all, but is connected directly to resource availability. 

This, more than anything, would explain why bees brood in winter, and why they do not go into a state of hibernation like many other organisms that evolved in a cold winter climate. The honey bee colony is in a state of perpetual readiness; they have no idea how long winter lasts; to them, spring is always "coming soon", be ready for it.

PLB

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