Glyphosate, the active ingredient of Roundup, is relatively nontoxic to bees.  The oral and dermal LD50 is greater than 0.1 mg/bee.  Check out its toxicology information on Etoxnet at http://ace.orst.edu/cgi-bin/mfs/01/pips/glyphos.p54.
 
Herbicide products formulated with glyphosate also contain adjuvants to make the spray spread out and stick to the leaves of the target plant.  Some of these are surfactants, which act like detergent.  What happens when you spray a detergent solution on bees?  They die.  I would guess that any bees directly sprayed with Roundup might die because of the surfactants.  Once the spray is dry it would present no further hazard.   
 
I would look for another cause for your loss.  Are there plants attractive to bees in the neighboring orchard?  Have they sprayed the orchard lately?  Is carbaryl being used now to thin apples in your area?
 
Bob Roach