Why Varroa kills colonies

It's a mild day in early March. Anne walks out to her strongest hive, the one that was bursting with bees last August, and puts her ear to the box. Nothing. She lifts the lid. A small cluster of dead bees, some with their heads buried in the cells. On the bottom board, a scatter of bees with crumpled, twisted wings.

How does a colony that looked so strong in August end up dead in March? In Ontario, the most likely answer is a mite smaller than a sesame seed.

Meet Varroa destructor. The adult female is about one to one and a half millimetres across, reddish brown and shaped like a flat oval. You'll see some riding on adult bees. But the real damage happens where you can't see it, inside the brood cells.

Just before a cell is capped, a female mite slips in with the larva. Under the wax capping she feeds and lays eggs. In a worker cell she leaves behind, on average, about one and a half new mites. In a drone cell, which stays capped longer, two to two and a half. So the mites multiply right along with the brood.

When there is brood in the hive, about two thirds of the mites are hidden inside capped cells, out of reach of many treatments. That is why mite numbers can climb quietly all summer while the colony looks busy and strong.

Mites also carry viruses. The best known is deformed wing virus. Infected bees emerge with crumpled wings and survive only a short time. A heavy mite load plus viruses can wear a colony down even when you don't notice many mites on the bees.

Now remember the winter bees. They are raised in late summer and early fall, and they have to last four to six months. Varroa numbers usually peak in August and September, exactly when those winter bees are being raised. Damaged winter bees die early, the cluster shrinks, and by late winter it is too small to keep warm.

Ontario's agriculture ministry says inadequate Varroa control is the primary cause of winter losses in the province. When Ontario beekeepers were asked why colonies died in the winter of twenty twenty four to twenty twenty five, mites and the viruses they carry topped the list. And one in five small scale beekeepers had done no mite monitoring at all.

You can't keep Varroa out by being careful, either. Colonies interact over three kilometres or more, through drifting and robbing, so your bees share mites with the colonies around them.

Now let's change one fact. Suppose Anne had counted her mites in the middle of August, found them too high, treated, and counted again to be sure it worked. Her winter bees would have been raised in a hive with far fewer mites, and that March, her ear to the box would have heard a steady hum.

The mistake beginners make is judging a colony by how it looks. In August, a colony heading for a Varroa collapse can look like your best hive. You only know by counting.

Here is the rule, in one breath. Varroa mites breed inside capped brood, spread viruses, and peak in late summer just as the winter bees are raised, so in Ontario poor mite control is the main reason colonies die over winter, and you have to measure mites, not guess.

Next, how to count them.