Remember that uniting bees and giving colonies brood from other colonies is the a very "unnatural" thing to do. 
 
The natural way is that colonies are single/stand alone entities, with for multiplication having a subgroup of the population leaving the brood and comb material, with its pathogens and residues, behind and start a new nest, experiencing a brood stop doing so. 
 
When you give colonies bees/brood you make horizontal transfer of all kinds of nasties very easy. 
 
Lennard 		 	   		  
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