Aspen leaf rust produces the yellow "dots" you describe.  These are full of
the urediaspores of the fungus.
 
It would be unusual I would think that they are collecting this material.
However perhaps in their end of the season rush to pack in stores they are
getting a little non-picky and collecting pollen-like material.
 
However, it may also be that they are collecting honeydew.  We see bees on
aspen late in the season collecting honeydew from aphids.  There often is
rust as well.
 
Whitney Cranshaw
Colorado
 
 
>the aspen trees are turning and getting ready for winter --- however they
>are alive with bees gathering what looks like dot size grains of yellow -
>on the underside of the leaf only  and are packing the brood nest with
>it... It is not honeydew or nectar --- thought at first it was sort of
>insect excreta? has anyone else seen this --- south central B.C. area..
>   any ideas?   where can I send leaf for examination??
>
>