A colony can look busy, well-tempered and packed with brood while varroa quietly builds underneath the cappings. The mites are small, they hide in sealed cells, and the harm they do — weakened workers, deformed wing virus, a cluster that shrinks instead of growing — tends to appear long after the numbers have crept up. By then, the season is often too far gone to put right.

Monitoring is just counting. Done regularly, it replaces guesswork with something you can act on. Twenty minutes per hive, a jar, some icing sugar and a notebook is all it takes. Here is how to sample properly, what the numbers mean, and when to think about treatment.

Why counts beat assumptions

Two hives standing side by side in the same apiary can carry very different mite loads. Drifting bees, robbing, swarms, how much drone brood is around, and how much open brood is in the nest all shift the picture. So does the calendar. While a colony is rearing brood steadily, varroa numbers can climb quickly — fast enough that a count which looked reassuring in June feels very different by late August.

Keep a simple record for each hive: date, method used, number of bees sampled, mites counted, and what you did next. After a season or two, that notebook tells you more about your bees than any general rule, because it shows how quickly pressure builds where you keep them.

Four ways to check a colony

Sugar roll

This is the method most UK beekeepers reach for, because the bees survive it.

1. Choose a frame with plenty of capped brood and nurse bees on it. Mites gather near brood, so nurse bees give the most honest sample.
2. Shake or brush roughly 300 bees — about half a mug — into a clear jar.
3. Add a heaped tablespoon of icing sugar, fit a mesh lid, and roll the jar gently for about two minutes so the sugar coats every bee.
4. Let it stand for two minutes, then shake the sugar through the mesh onto a pale tray.
5. Add a splash of water to dissolve the sugar and count the mites. Look for dark reddish-brown ovals, roughly pinhead-sized.
6. Tip the bees back at the entrance. They will clean themselves up and carry on.

Results are usually expressed per 100 bees, which makes samples of slightly different sizes comparable.

Natural mite drop

Slide a greased monitoring board under an open mesh floor, leave it for seven days, then count what has stuck to it. It is the least intrusive check you can make, and useful for spotting trends between sugar rolls, particularly in late autumn and early spring. Treat it with caution in midsummer: when a colony is rearing plenty of brood, most mites are hidden in cells, so the drop understates the real load.

Drone brood uncapping

Varroa strongly prefer drone brood, because drone larvae take longer to develop. Uncapping 100 to 200 sealed drone cells at the purple-eyed stage and counting the mites gives a quick snapshot without killing workers. It is a handy early-season check, but it only describes one corner of the nest.

Alcohol wash

Washing bees in alcohol is the most accurate method available, but it kills the sample. Many beekeepers avoid it for that reason. If you do use it, take the bees from brood comb and accept that 300 workers will not be returning.

When to monitor through the year

• April and May: first counts as brood rearing accelerates. Early numbers are low, but they set your baseline.
• June and July: sample every three to four weeks, especially before and after the main flow.
• Late July to September: the most important window of the year. Mite numbers peak while the winter bees are being raised, and those bees have to last until spring.
• Three to four weeks after any treatment: check whether it actually worked. Never assume it did.
• Midwinter: a broodless period is a good moment for a mite drop check, and for judging whether a winter treatment is needed.

Reading the numbers — and the mistakes that skew them

Thresholds are guides, not laws, and different sources quote slightly different figures. As a rough rule, many beekeepers treat when a sugar roll returns around three mites per 100 bees in late summer, or when the natural drop reaches roughly ten mites a day at that time of year. In spring, a much lower count deserves attention, because the colony has months of brood rearing ahead of it and plenty of time to build a problem.

Whichever figures you work to, avoid these common errors:

• Sampling bees from a honey super instead of brood comb, which understates the load.
• Relying on a single count. One number is a snapshot; three counts over a season are a trend.
• Changing method halfway through. Pick one approach and stick with it so the figures can be compared.
• Forgetting to record the result, then trying to remember it in September.

If you are unsure how to interpret your counts, the National Bee Unit's BeeBase resource and your local bee inspector are both sensible places to ask.

Deciding whether to treat

Treatment is a judgement call, not an automatic response to a number. Consider the size of the colony, how much brood it is rearing, the time of year, and whether the bees are producing honey you intend to harvest.

UK-licensed varroa treatments fall into a few broad groups: thymol-based products, formic acid, oxalic acid and amitraz. Each has its own temperature window, its own rules about supers, and its own place in the season. Read the label properly and follow it exactly — dose, timing and conditions all matter. Rotating between different active ingredients helps slow the development of resistance. If you are unsure which product suits your hives, ask your local bee inspector or a veterinary surgeon with an interest in bees.

Chemical treatment is only part of the picture. Good husbandry reduces the pressure in the first place: replace old brood comb on a regular cycle, remove drone brood where it is practical, keep equipment and combs within your own apiary, and be cautious with swarms and bought-in colonies, which can arrive with mites already on board. Building hives from stock that shows good grooming and hygiene behaviour is a long-term investment worth making.

Make it a habit

Monitoring works best when it is boring and regular. Put a date in the diary for a sugar roll on every colony, do it on a warm afternoon when the bees are flying well, and write the result down before you close the hive up. If the count is low, relax and check again in a month. If it is creeping up, plan your next step while you still have options — a brood break, a drone brood clean-up, or a treatment applied at the right temperature with supers handled correctly.

Then check again a few weeks later to see whether it worked. That final count is the one most beekeepers skip, and it is the one that tells you whether your bees are heading into winter in good shape.

Photo: Erik Karits / Pexels