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Varroa mite calculator

Enter your count: the calculator gives the infestation and compares it with the National Bee Unit’s figures, so you know whether to act now, plan control or simply count again.

These are guide figures. Read them with the strength of the colony and the time of year, and ask your bee inspector or association if in doubt.

Monitoring method
Every mite since the tray went in.
days
At least 7 days in summer, longer in winter (NBU).
The NBU factor that turns a daily drop into a colony estimate.

Result

  • 5mites / daydaily natural drop
  • 500mites in the colony (estimate)
  • Below the NBU threshold

No control needed on this count. Count again in a few weeks: the NBU suggests at least four times a season, in early spring, after the spring flow, at harvest and in late autumn.

How it is calculated

The formulas

daily drop = mites on the tray ÷ days
mites in the colony ≈ daily drop × seasonal factor
infestation = 100 × mites ÷ bees (or drone pupae) in the sample

Natural drop: the NBU’s seasonal factors

MonthsFactorDaily drop for 1,000 mites
November to February× 400about 2.5
March, April, September, October× 10010
May to August× 30about 33

The NBU puts the damaging level at 1,000 mites per colony: control varroa before the colony reaches it. The last column is simply 1,000 divided by the factor. The natural drop is not reliable in a colony without brood or one that is collapsing.

Bee samples and drone brood

For a sugar roll or an alcohol wash, the NBU takes about half a cup of bees, 300 to 400, from a brood frame without the queen. More than 5 mites per 100 bees is serious and needs urgent control; 3 or fewer is acceptable. The same marks apply to drone brood: more than 5 mites per 100 drone pupae calls for urgent control, fewer than 3 is acceptable. Between the two, the calculator suggests planning control and counting again.

What to do next

The NBU recommends integrated pest management: monitoring, husbandry such as drone brood removal or a brood break, and treatment when the count calls for it, following official guidance and the advice of your bee inspector. Read more in our guide to varroa mites.

Frequently asked questions

How many varroa mites is too many in a hive?

The National Bee Unit puts the damaging level at about 1,000 mites in a colony. With a sample of adult bees, it treats 3 mites per 100 bees or fewer as acceptable and more than 5 per 100 as serious, needing urgent control.

How do I work out the mite population from a natural drop?

Leave a greased or sticky tray under an open mesh floor for at least seven days, count the mites and divide by the number of days. The NBU then multiplies the daily drop by 30 from May to August, by 100 in March, April, September and October, and by 400 from November to February.

Why does the multiplication factor change with the season?

Most mites live in sealed brood. In summer, with plenty of brood, only a small share of them fall each day, so a small daily drop stands for a large population. In winter there is little or no brood, so the mites that fall are a larger share of the whole.

Is a sugar roll as good as an alcohol wash?

The NBU says a sugar roll, shaking the bees in icing sugar for about three minutes, gives results comparable to an alcohol wash, and the bees survive. Take about half a cup of bees, 300 to 400, from a brood frame, making sure the queen is not among them.

How often should I check for varroa?

The NBU suggests at least four times a season: early spring, after the spring flow, at the honey harvest and in late autumn. A single count matters less than how the numbers move from one count to the next.

Sources

  1. Estimating Varroa mite populations (Fact sheet 13, April 2024), Animal and Plant Health Agency, National Bee Unit
  2. Using IPM for Varroa control (Fact sheet 27), Animal and Plant Health Agency, National Bee Unit

Calculators