Varroa Mite Wash Calculator (Infestation % & Treatment Threshold)

How it works

infestation % = mites ÷ bees in sample × 100 (÷ recovery: alcohol 0.95, sugar roll 0.75) · phoretic mites = colony bees × % · total ≈ 3 × phoretic when brood is present · thresholds: spring 1%, summer 2%, late summer 2–3%, > 5% emergency · growth ≈ doubling every 3–4 weeks

Varroa mites ride on adult bees only between breeding cycles inside capped brood cells, so a wash of three hundred bees samples the exposed third of the population and the percentage it yields is a standardised index rather than a total count. Thresholds are set by what the colony can tolerate at each stage: mites transmit deformed-wing virus, and the bees raised in late summer must live through the winter, so that window has the strictest practical limit and is where most colony losses are decided. Because two-thirds of the mites are sealed away, single-dose treatments during brood rearing leave many survivors, which is why extended-release products, repeated applications or a broodless-period oxalic treatment are used, and why a follow-up wash is part of the job.

Worked example

Nine mites in an alcohol wash of 300 bees is 3.2% — above the late-summer threshold, so treat now. In a 30,000-bee colony that is about 950 mites on the adults and roughly 2,800 in total with brood present; a single 90%-efficacy application would leave about 1,300, so a multi-week formic or thymol course or three oxalic treatments a week apart are needed, with a re-wash three weeks later. Left alone, the count would pass 6% in a month.

Frequently asked questions