> And if they are able to detect the problem, why do they collect it in the first place?

Of course, theories abound, but it is plausible that the pollen is contaminated with fungicides, which in turn interfere with the bees' process of converting raw pollen into bee bread. If the pollen does not undergo the changes properly, or takes on some other objectionable quality, they could decide it was untouchable foreign matter, and just propolize over it to seal it off. 

It's pretty clear from recent work that fungicides coming in on pollen have the potential to cause great harm to the colonies microbial population, which in turn could wreak havoc with their digestion of pollen. It is certainly plausible that bees require microbes to digest pollen, much as termites require them to digest wood. 

Further, foragers may be a lot less particular about what they collect than the house bees are about what they will use. Most of us have seen bees collecting sawdust early in the spring. I don't know what happens to this stuff but it wouldn't surprise me if the house bees just threw it out with the garbage.

PLB

             ***********************************************
The BEE-L mailing list is powered by L-Soft's renowned
LISTSERV(R) list management software.  For more information, go to:
http://www.lsoft.com/LISTSERV-powered.html

Guidelines for posting to BEE-L can be found at:
http://honeybeeworld.com/bee-l/guidelines.htm