If the this syrup is on the hive and has time to get moldy, it is because the bees don't need it.  As soon as they get sufficient nectar coming in, they will ignore the syrup.  Molding is one of the big disadvantages of using sugar syrup.  If it is not processed promptly by the bees,  it gets several types of mold.  Some people say that the bees don't seem to mind but I would not rather not have something like that on the hive.  I don't have that problem with corn syrup.  I stays good until they use it.
 
Frank Humphrey
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