Not surprisingly, the bees that carry the colony to spring are called “winter” bees. Their longevity is due to differences in their physiology, such as a higher fat content and lower hormone levels. Winter bees are not reared all at once, but rather added gradually starting in late summer. The probability of a colony overwintering successfully is largely dependent on the health of these bees, and their health is largely dependent on mite levels.
Varroa and associated viruses reduce the bee’s lifespan by nearly half. That means winter bees that are parasitized during development may die in January, and leave the surviving cluster immobilized. Small winter clusters are unable to move to additional food stores. They die on the comb once the honey in the cells below has been depleted. Often times, colonies lost in winter due to mite infestation ultimately die of cold/starvation.
Your colony’s best chance comes by hitting Varroa hard before the first cohort of winter bees is reared. In the temperate climate of Northern Utah, that means early August. The first batch of brood containing winter bees will emerge by the end of August or early September. Nearly half of these new bees will be winter bees, and that proportion will increase with each successive batch. However, due to the decline of egg laying by the queen, over half the bees that make up the winter cluster will emerge in those first couple rounds. This is why you have to knock the mites down in late summer.
Rearing of winter bees continues until the queen stops laying, which could happen by the end of October. This gives the beekeeper about a one-month window to address Varroa. If treatment hasn’t been applied by the end of September, and mite levels are over five percent, chances of winter survival are significantly diminished. Several thousand bees are required to effectively thermoregulate throughout winter, and there is simply not enough brood reared in October to make up for lost time.
During the fall, the rate of infestation can change in a day. If you could only get a few mites to drop three weeks ago, don’t assume your colony is still okay. There is a high density of hives in the Salt Lake metro area. Bees from collapsing colonies often abandon the hive in a last ditch move of desperation, leaving behind honey, some brood and the queen (sound familiar?). Many of these bees will attempt to join other colonies, bringing their mite loads with them. Monitor Varroa early and often, and use a proven method should treatment become necessary.