I had a customer call me looking for a queen. She went through her hive and could not find any brood. In a panic she was looking for a queen.
After a short conversation about queen types, I learned she had a Carniolan queen. Carniolans shut down brood rearing in early fall. Right now if you looked in a Carniolan hive you should be broodless.
If a hive is being fed syrup, that is a nectar flow and there more than likely brood in a colony that is being fed or was fed in the last two weeks or so.
Italian queens will have brood into November. Especially if they have been fed.
I have gone into Italian hives on Thanksgiving weekend and have found brood in the hive.
All of this ties together about mites. Feeding should be done early so a hive can be broodless for the Oxalic Acid treatment. This beekeeper with the broodless Carniolan hive, will be able to get an excellent treatment of Oxalic Acid and it should prove to be very effective.
Late feeders, who will have brood in the colony ( I am still feeding three colonies myself) the Oxalic Acid treatment may be a little less effective because of capped brood in the hive. But the treatment is still of great value and is worth doing.