Varroa Mite Wash Calculator
Turn an alcohol-wash mite count into an infestation percentage and a monitor-or-treat verdict.
Sample nurse bees from a brood frame, and check first that the queen isn't on it before you shake bees into the wash. An alcohol wash kills the sample but gives an accurate count; a sugar roll lets the bees go back to the hive but tends to undercount mites. Check monthly during the season. Colonies can crash from varroa-vectored viruses in fall if an August count gets ignored.
How it works
A standard alcohol wash samples a level half-cup of bees, roughly 300 of them, shaken in alcohol so the mites separate and can be counted. Since 300 bees is the sample, each mite counted works out to 1/3 of a percentage point of infestation, so the calculator just divides your count by 3 and rounds to one decimal place. That percentage is compared against the widely used thresholds beekeepers use to decide whether to treat.
Worked example: you wash a sample and count 9 mites. Divided by 3, that's 3.0%, right at the treat-now line. If you'd counted only 2 mites, that's 0.67%, which rounds to 0.7% and sits comfortably in the monitor range. The difference between those two counts is the difference between doing nothing this month and starting treatment before the colony's mite load starts affecting brood health.
FAQ
Why sample from a brood frame instead of anywhere in the hive?
Nurse bees working brood carry a higher mite load than foragers, since mites prefer to be near brood to reproduce. Sampling from a brood frame gives a more accurate read on the colony's real infestation than shaking bees off a frame near the entrance. Just check carefully that the queen isn't on the frame before you shake bees into your sample cup.
Alcohol wash or sugar roll, which should I use?
An alcohol wash kills the sampled bees but gives the most accurate count, since the alcohol fully knocks the mites loose. A sugar roll lets the sampled bees return to the hive, which some beekeepers prefer, but it tends to undercount mites because a few stay stuck to the bees under the sugar coating.
What do I actually do at 2 to 3%?
That's the borderline range: not urgent, but worth a re-check in about two weeks rather than waiting for your next scheduled inspection. If the count is climbing between tests, treat before it crosses 3%. If it's flat or falling, you can usually keep monitoring.
Why does an August count matter so much?
Late-summer mite loads set up the winter bees, the long-lived generation that has to carry the colony through the cold months. High varroa loads in August spread viruses into those winter bees, and colonies can crash over winter from virus damage even if the mite count itself later drops. Don't skip the late-summer check.
For the full wash and roll procedure, see how to do a mite wash: alcohol wash and sugar roll and varroa mites: how to monitor and treat them. For your regular routine, see how to do a hive inspection.