>...the syrup is left over from spring feeding, can it go bad?

If anything, it should have been refrigerated although storing syrup long term is not very practical.  Always mix just the amount you can use short-term.

You can do a test.  Mix up some fresh syrup and offer it to one hive.  If they take it down eagerly, you'll know for sure your spring stuff has fermented.

Waldemar

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