Monitoring for varroa

Why you cannot just look

Mites spend most of their life cycle inside capped brood, feeding on developing bees where you cannot see them. The mites you spot on adult bees are a fraction of the population, and a colony can look busy and healthy while carrying a load that will kill it before spring.

Varroa also does most of its harm indirectly, by transmitting viruses. That is why mite counts in late summer decide whether the winter bees are healthy, and why a colony that looked fine in September is dead in February.

The alcohol wash

Penn State Extension is direct about this: alcohol washes are the most accurate method of monitoring mite populations. You take a sample of around 300 bees from a brood frame, wash them in alcohol, and count the mites that separate out.

It kills the sample. Three hundred bees out of a colony of tens of thousands is a trivial loss against the information, and the accuracy is what makes the number worth acting on.

Sugar roll and sticky boards

A sugar roll uses powdered sugar instead of alcohol and the bees survive. It is less accurate, because not every mite is dislodged, so it reads low. Sticky boards under a screened bottom count natural mite drop and are the least precise of the three.

Any of them is enormously better than not counting. If squeamishness about the alcohol wash is what stands between you and monitoring at all, do the sugar roll.

When to sample

Monthly, May through October, is Penn State's guidance for the mid-Atlantic, and USDA ARS publishes on timing across the season. The late-summer samples are the consequential ones because they determine the health of the bees that have to survive winter.

What the number means, and why it has moved

The direction of travel is what matters: treat earlier and at lower counts than older sources say. A count that a 2010 book calls acceptable is a count current guidance calls a problem.

References