> When I returned next day it was cold  
> and there were large numbers of dead
> bees on the concrete below the entrance; 
> many had pollen loads....

> Then it started raining...

Started raining AGAIN, I'll speculate.  

The first rain caught some of your bees unaware.  Wet wings don't fly.
Flying bees can get hit by raindrops.

Bottom line, if is warm enough to go out and fly around and collect all that
pollen, it is hard to imagine it not being warm/dry enough for those bees to
fly those last few feet to the entrance unless something extraordinary
knocks them out of the sky on final approach.  

But I still like the pesticide theory best, as nothing kills foragers and
makes them do counterintuitive things like a snootfull of poison.

	

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