Varroa Mite Count Calculator
Calculate varroa mite infestation percent from alcohol wash or sugar roll mite count and bee sample size.
Action threshold is around 2-3 per 100 adult bees.
Varroa destructor mites are the most serious threat to honey bee colonies worldwide. Regular monitoring with an alcohol wash or powdered sugar roll is essential for integrated pest management (IPM). The standard measurement is mites per 100 bees, which gives a comparable infestation percentage regardless of colony size.
Mite Wash Formula
Infestation Rate (%) = (Mites Counted / Bees Sampled) × 100
A standard sample is approximately 300 bees, which is roughly half a cup (120 mL) of bees scooped from a brood frame. The 300-bee sample provides a statistically reliable estimate of the colony’s overall mite load.
Worked Example
You collect half a cup of bees (~300 bees) and perform an alcohol wash. You count 9 mites in the jar.
Infestation Rate = (9 / 300) × 100 = 3.0% (3 mites per 100 bees)
Treatment Thresholds
| Season | Threshold (mites/100 bees) | Action |
|---|---|---|
| Spring (Mar–May) | 1% (3 mites/300 bees) | Treat before honey flow |
| Summer (Jun–Aug) | 2% (6 mites/300 bees) | Treat promptly |
| Late Summer (Aug–Sep) | 3% (9 mites/300 bees) | Treat immediately |
| Fall (Oct–Nov) | 3% (9 mites/300 bees) | Treat before winter prep |
Important Context
The alcohol wash detects about 75–90% of phoretic mites (mites riding on adult bees). However, roughly 80% of the total mite population is hidden inside capped brood cells at any given time. So the actual colony mite load is approximately 5× higher than the phoretic count suggests.
For example, if you count 3% phoretic mites, the true colony infestation is roughly 15% when including mites in brood cells. This is why the treatment thresholds look so low. A 3% wash is not a colony with a few mites in it, it is a colony already under real strain.
Sugar Roll vs Alcohol Wash
The sugar roll method is non-lethal to bees but detects about 70–90% of the mites that an alcohol wash finds. Treat a sugar roll count as a floor, not an answer: the true phoretic count is usually 10 to 30% higher. This calculator splits the difference and scales a sugar roll up by 25%.
The alcohol wash with 70% isopropanol or windshield washer fluid is the gold standard for accuracy. It costs you the 300 bees in the jar, which sounds worse than it is: a healthy colony replaces them in a couple of hours, and an unmeasured mite load costs you all of them by March.
How we build and check this calculator
This calculator runs entirely in your browser, so the numbers you enter stay on your device. The math behind it is written by hand and tested against worked examples and standard references before the page goes live.
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