The problem of varroa reinfestation is much more of an issue than most people think. The latest 2022 Colony loss survey shows that the greatest issue when dealing with varroa is reinfestation.
What is reinfestation? After treating hives for varroa the hive is re-infected by varroa from bees from other hives. If you have multiple hives in an apiary it is important to treat them all at the same time. Also, if there are other beekeeper’s hives nearby, they should be treated concurrently. Due to the practise of over-wintering commercial hives in areas where there are established apiaries, this has become a serious health issue. All of this makes for a lot of extra work for the conscientious beekeeper. A helpful aid is to put a robbing screen on your hive to prevent unwanted visitors: https://www.hiveworld.co.nz/product/robbing-screen/
It is important to do a mite check after treating to monitor mite levels and again a month later to check for varroa reinfestation. If mite levels are high you must treat again in order to save your hive. 56146-2023-01-NZ-Colony-Loss-Survey
To purchase the Control of Varroa book – https://www.hiveworld.co.nz/product/control-of-varroa/