[crofter]

Right now I am starting OAV three times every 5 days. Then monthly OAV until August. Then three times every 5 days. The winter OAV proved to be insufficient in 2018.

Where my bees are located 3 miles from my home I've seen at least 3 other beekeepers. So I expect at least drone drift if not robbing.

I would suggest that with the shorter interval that you do at least one more time to be sure you cover the whole time span of all brood. Some operators in heavy infestation areas are suggesting they have to go for 7 shots at 3 day intervals.

The failures in 2018 might have had to do with virus loads. It can take several rounds of brood AFTER the mites are all dead before the virus levels drop. Guess it depends from year to year what is going around and what your bees went to bed with besides the mites. Mites are easy to test for; the viruses, not so much.

It sounds like you have been having good success.