Southwick took colonies to minus 80 C in a freezer.  Held them for 48  hrs, 
then did measurements.
 
His data shows that bees can survive brutal cold, if they have enough bees  
and sufficient food.
 
Honey consumption is highest at warmish temperatures and at extremely  cold 
temperatures.  But, at the temperatures encountered in many northern  areas 
- the cluster when settled in to the cold, actually is at its most  
efficient state, needing less energy to keep 'warm'.  
 
In Montana, we often come through a long-cold winter with colonies in  
better shape then when we get a mild winter - when they consume more  honey.
 
Sorry about the unscientific terms - but I've too much going on to run to  
ground his original papers - I had several good conversations with him about 
 this.
 
One tip, he found that you should leave an empty comb at the outside of  
each box.  When it gets really cold, the cluster moves up against that  comb, 
using the hexagonal cylinders as dead, insulating space.  So, if you  'help' 
the bees by putting combs full of honey all the way across, the outside  
frame, when full of honey, transmits the cold right through.
 
Jerry

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