Counting Varroa - how many are too many?
The articles below give information on how to sample for varroa loads but also thresholds for when to treat. The gist of it all is when sampling in August a colony is best to have no more than 2% mite infestation. That's 6-8 mites per 1/2 cup of bees.
2017 - Randy Oliver treats when 6-8 mites are counted.
For my apiaries in the California foothills, I’ve found that if I keep the mite infestation rate below the 2% level (2 mites per 100 bees) that my colonies thrive. But should that rate reach 5%, then I start seeing the brood fall apart. By the time the rate reaches 15%, the colony is generally seriously on the way downhill, and even with treatment may not recover.
The most critical time to monitor and reduce the mite level is in late summer and fall, since this is when the generation of bees that form the winter cluster is raised. If there is a virus epidemic in the hive in fall, the colony will likely not survive the winter.
2017 - Honey Bee Health Coalition.
When mite levels are above 5 percent, apply mite control immediately, using a proven, effective, seasonally appropriate treatment method.