How this calculator works
Mite load is reported as mites per 100 bees, which is also the percentage of sampled bees carrying a mite. The number of bees in the sample matters more than people expect: 7 mites is 2.3% from 300 bees but 3.5% from 200. Combwise stores the per-100 figure on every inspection and raises a flag on the hive card when it reaches the threshold.
Sources
Threshold and comparison rule copied from Combwise's BeeLogic (Models.swift) on September 17, 2026: varroaThreshold = 3.0, treat when mitesPer100 >= threshold. The 300-bees-per-half-cup convention and method notes are standard beekeeping practice rather than app data.
Questions
How do I calculate mites per 100 bees?
Divide the mites you counted by the number of bees in the sample and multiply by 100. A level half-cup of bees is about 300, so 9 mites from a half-cup is 3 per 100 bees, or 3%.
What varroa level needs treatment?
A widely used action threshold is 3 mites per 100 bees, which is the level Combwise flags. Some guidance uses 2% in spring and 3% later in the season; you can change the threshold in the calculator.
Alcohol wash or sugar roll?
An alcohol wash recovers more of the mites present and is the reference method, at the cost of the sampled bees. A sugar roll keeps the bees alive but tends to under-count, so a borderline sugar roll deserves a re-sample.
Where should I take the sample from?
From a brood frame, where mites concentrate, after checking the queen isn't on it. A sample from the honey supers will understate the load.
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