Free tool
Varroa calculator
Turn the mites you counted into an estimate for the whole colony, the National Bee Unit’s way, and see how it compares with the levels the NBU gives. Every figure and threshold here is theirs.
Type how many mites you counted to see the estimate.
How the NBU works it out
- Natural drop: divide the mites on the tray by the days it was in, then multiply the daily drop by 400 in November to February, 30 in May to August, and 100 in March, April, September and October. The factor allows for the mites hidden in sealed brood. [NBU FS 13]
- Treat before a colony reaches about 1,000 mites. [NBU FS 13] About 2,500 leads to damage that cannot be undone, and a real risk of the colony collapsing. [NBU BPG 11]
- A wash, sugar roll or drone uncapping: over 5 mites per 100 is serious and needs treating urgently. [NBU FS 13]
- Monitor at least four times a year: early spring, after the spring flow, at the honey harvest and in late autumn. [NBU FS 13]
Where these figures come from
- National Bee Unit, Estimating Varroa mite populations, fact sheet 13 (April 2024)
- National Bee Unit, Varroa, best practice guideline 11 (October 2026)
An estimate, not a diagnosis
One count is a rough guide. The same method, done the same way, several times a year tells you far more.
- The August varroa treatment covers monitoring, and why late summer is the treatment that counts.
- Oxalic acid and the broodless window is the winter treatment, if October’s count calls for it.
- Reporting varroa and foulbrood covers what the law asks of you.
Keep every count against its hive
A free Bees Knees Apiary account records each count with the inspection, works out the same estimate as you type, and charts it over the season beside your treatments.