Varroa Monitoring Methods and Thresholds

In varroa control, decisions are made on the basis of measurement, not the calendar. Count every colony several times a year using the same method; these numbers decide whether intervention is needed and whether a treatment has worked. The most reliable method is the alcohol (or detergent water) wash; the icing sugar method does not kill bees, while the floor-board count is useful for tracking trends.

Why should you measure?

Measuring has two purposes: to decide (is intervention needed?) and to verify (did the treatment really bring the mites down?). The second is the step most often skipped; a beekeeper who does not measure can repeat an ineffective treatment for years.

By the time visible symptoms appear (bees with crumpled wings, perforated cappings, scattered brood), the mite load is usually already far above the damage threshold. Measuring warns you before it gets to that point.

How to take a sample

• Take the bees from a frame with open brood. Mites are found mostly on nurse bees; bees from the honey super or the landing board give low results.
• First make sure the queen is not on the frame. If the queen ends up in the sample, you will lose the colony.
• Shake the bees into a basin or a wide container, tap the container gently to gather the bees into one corner, and scoop them up with a measuring cup.
• The standard sample is about 300 bees; this corresponds to a volume of about 120 ml (a little more than half a glass of water). Always use the same measuring cup.
• Measure each colony separately. In the same apiary one colony may be clean while the one next to it is heavily infested.

Alcohol or detergent water wash

This is the most accurate and most repeatable method. The sampled bees die; in return the result directly gives the “number of mites per hundred bees”.

1. Put about 300 bees in a lidded container with enough alcohol (or water with washing-up liquid) to cover them.
2. Shake the container vigorously for at least one minute; the mites come off the bees.
3. Pour the liquid through a strainer or wire sieve into a white container; the bees stay in the sieve and the mites pass through with the liquid. If you wash the bees once more, you also catch the mites that escaped.
4. Count the mites on the white background. Divide the number by three: the result is the number of mites per hundred bees (%).

Icing sugar method

This works on the same principle, but the bees stay alive and are returned to the colony. The bees are put into a jar with a mesh lid, about a tablespoon of icing sugar is added, the jar is rolled gently for about a minute and left to stand for a few minutes. The jar is then turned upside down and shaken over a white container; the mites fall out with the sugar. Adding a little water dissolves the sugar and makes the mites visible.

It catches somewhat fewer mites than the alcohol wash; its efficiency falls in humid weather and when the sugar clumps. It is a reasonable option if you do not want to kill bees; but always use the same method, because if you switch between methods the numbers cannot be compared.

Floor-board (natural drop) count

A white, greased or sticky board is placed under the mesh floor; after a few days the fallen mites are counted and divided by the number of days to give the average daily drop. The board must stay beneath the mesh so that ants and bees cannot reach it.

It is easy and does not require opening the colony; however, the drop varies greatly with colony size, amount of brood and season. The same number can mean different things in two colonies. It is therefore good for tracking the trend (are mites increasing?) and seeing the drop after a treatment, but weak on its own as a basis for deciding to intervene.

Uncapping drone brood

Varroa prefers drone brood because it stays capped longer. Capped drone brood at the pink-eye stage is pulled out in one movement with an uncapping fork, and the pupae are checked for reddish-brown mites. About a hundred cells are opened and the number in which mites are seen is noted.

This is a quick early warning method: if you see mites frequently in drone brood at the start of the season, measure the rest of the colony with a wash as well. It cannot be used when there is no drone brood, and its result cannot be converted into a mite rate per hundred bees.

Comparison of methods

Thresholds: when to intervene?

The intervention threshold is not a single number; it varies with the season, colony strength and region. A commonly used approach is as follows (mites per hundred bees, by the wash method):

• Spring and early summer: As the colony grows, the mites also multiply rapidly. At this time even 1–2 mites per hundred bees is a value that needs close monitoring.
• Mid and late summer: 2–3 mites or more per hundred bees is regarded in many sources as the intervention limit.
• While winter bees are being raised (late summer to autumn): The aim is to keep the mite level as low as possible; if the threshold has been exceeded at this time, do not wait.

If your regional beekeepers' association or the official veterinary service / local agriculture authority issues current recommendations, base your decisions on those.

When to measure? A practical calendar

1. First inspections in spring: Find out the starting level.
2. Midsummer (during or just after the main flow): Decide whether intervention is needed after the harvest.
3. After every treatment: Once the treatment period is over, measure again with the same method. If the expected drop is not there, investigate problems with dose, timing, reinfestation or resistance.
4. Autumn: The last check before winter; reinfestation from neighbouring colonies is common at this time.

Common mistakes

• Sampling one colony and generalising to the whole apiary.
• Taking the sample from the honey super or from a frame without brood (the result comes out lower than it really is).
• Constantly changing the method; the numbers become impossible to compare.
• Mistaking wax crumbs and pollen clumps on the floor board for mites.
• Not measuring again after a treatment.
• Not writing down the results. Note the date, hive number, method and count; a trend can only be seen from records.