>>Having mixed oxalic acid syrup for trickling for a single hive, I 
was left with half a liter of the mixture.

I always estimate how many beespaces I'll need to treat and make the 
corresponding amount of solution plus ~10%.

>>After half a year standing on the shelf...  Is this still effective 
against varroa? Or should I just throw it away and make a new batch?

It may be effective against varroa but you must consider if it's safe 
for the bees.  I don't mean the oxalic component of the solution.  
Sugar dissolved in water undergoes changes over time that are or can 
be harmful to the bees.

You don't want to keep oxalic sugar water solution for more than a 
few days even in refrigeration.  Always use fresh solution.  It's 
cheaper than having to replace colonies.

I make up my OA solution just before treating and discard whatever 
little I have leftover.

Waldemar

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