Salty, how are ya fella?!
We need to know what you plan to treat with? I am only familiar with Oxalic Acid. You can add brood before, or after. I have not noticed any I?ll effects on brood of any stage with Oxalic acid vapor using 1 gram per 10 frame langstrof deep.
To spare argument, the nurse could remove eggs or young larva treated with Oxalic acid without my notice. I have studied, detailed research papers of the chemical effects of OAV bees in detail and have little concerns for my bees when used as directed.
There are many methods of treatment of Varroa mites, Oxalic acid is one I prefer. However you should decide for yourself which treatment or lack of treatment as described in detail by M. Bush.
Blessings
Van