Varroa mite count calculator: mites per 100 bees

A mite count only means something next to the sample it came from. Nine mites in a jar of 300 bees is a different result from nine mites in a jar of 150, and a sticky board left out for three days cannot be compared with one left out for one. The calculator below turns a count and its sample into a rate you can compare from one visit to the next, and the sections after it show the arithmetic so you can do it with a pencil.

I develop Crownboard, an iPhone app that stores a varroa count with its sample, so I wrote this page around the same arithmetic. The calculator and the notes work without the app.

Calculator

The fields start with example values. Replace them with your own.

The arithmetic

Wash or sugar roll: mites per 100 bees = mites counted ÷ bees in the sample × 100

Two published examples give the same sum. The Honey Bee Health Coalition counts 12 mites in a sample of 300 bees: 12 ÷ 300 × 100 is 4 mites per 100 bees. A University of Florida extension publication counts 15 mites in 325 bees, which is 4.6 mites per 100 bees, and calls that a 4.6% infestation rate. With a sample of exactly 300 bees the sum shortens to dividing the mite count by three.

Sticky board: mites per day = mites on the board ÷ days in the hive

Twenty-four mites on a board left for three days is 8 mites per day. That figure is a mite fall for the whole colony, not a share of the bees. The University of Florida publication says a sticky board cannot give an infestation rate per 100 bees, and the Coalition calls it a whole-colony measure and not a per-bee one. The two quantities are different, so keep them on separate lines of a record and do not convert one into the other.

Which method, and what each one cannot tell you

• Alcohol or soapy-water wash. The bees in the sample die, but you can count them, which makes the rate more exact. The University of Florida publication says a wash gives a more accurate estimate than a sugar shake for that reason, and the Coalition calls it the more reliable of its two sampling methods.
• Powdered sugar roll or shake. The bees survive and can go back to the colony. They are never counted, so the 300 you enter is an estimate and the rate is rougher than from a wash.
• Sticky board. The University of Florida publication calls mite fall nondestructive and convenient and gives three days as the usual time. It also says mite fall is largely determined by how much brood is emerging, so unless you know the colony's population you should be cautious about treatment decisions made from it. The Coalition lists it as useful for trends or as a quick check after treatment, and notes that ants or other scavengers can remove mites from the board.

All three describe one colony on one day. The Coalition points out that even a result of zero does not mean a colony has no mites, because mites may be in capped brood cells or on bees that were not sampled. Both sources take the bees from brood combs, with the queen kept out of the jar, and the Coalition says a sample of fewer than 300 bees is not accurate. Repeating the count on a regular schedule makes the trend visible.

What the rate means

This page does not tell you when to treat, and neither does Crownboard. Published treatment thresholds are given as mites per 100 bees, but they differ between sources and are not the same in every region or every part of the season. The two sources linked here do not give identical figures, both tie them to the colony's phase of the season, and the Coalition adds that they can vary by region and advises asking your state, province, territory or university extension apiculturist. Read both sources and check with your local bee inspector or association before deciding anything about your own colonies.

Keep the count with its sample

The most common mistake in a hive record is writing "9 mites" and nothing else. A note of the method, the mite count and the sample size lets you compare May with August. Crownboard stores exactly that on each inspection: you choose a wash, a sugar roll or a sticky board, enter the mites and the sample, and the app shows a wash or roll as mites per 100 bees and a board as mites per day. A wash or roll starts at a 300-bee sample and a board at three days, and you can edit both. A count with no sample is kept, and no rate is shown for it. The app states no treatment threshold of its own.

Crownboard is free to download for iPhone with iOS 18 or later, in English. Two active hives include every feature; one optional purchase, paid once and never renewed, removes the two-hive limit.