According to this guide http://www.biosecurity.govt.nz/files...rroa-guide.pdf there are 3 treatment programs for varroa control. They all have their advantages and disadvantages.
2) Calendar treatment
3) Treatment based on monitoring and economic thresholds (IPM)
For all, but especially for beekeepers with 500+ hives what is your preferred program? If you wish to present one or other reason for your choice even better.
In my case I am currently doing the treatments according to the calendar. After the treatment I do a brood inspection to each hive, to check the effectiveness of the treatment and learn. I have now 600 hives. I have found that my hives do not all have the same response/efficiency to the treatment, especially to the treatment of late summer. The last two years I have verified that about 10-20%, need a new treatment.