>>I haven't yet been brave enough to open a hive in December and destroy the sealed brood before applying the oxalic.

Then how do you know they are not broodless in December? :)

It won't harm the bees if you gently open the hive around 40F and sunny.  Just don't cause too much commotion and you won't alarm the bees excessively.

Waldemar
 

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