Not surprisingly, the bees that carry the colony to spring are “winter” bees. Their increased longevity is the result of changes in their physiology, such as a higher protein content and lower levels of juvenile hormone. They are not reared all at once but rather produced gradually throughout late summer and fall. The probability of a colony overwintering successfully depends largely on the health of these winter bees, and their health is highly dependent on the beekeeper knocking the mites back at the right time.
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, leaving the surviving cluster immobilized and unable to reach food stores. To prevent this, hit varroa hard before the colony starts rearing winter bees. In a temperate climate, this usually means mid-August, because winter bees begin to emerge in early September. Due the decline of egg laying as winter approaches, most the winter cluster will be produced before October. This is the reason for knocking the mites back in late summer.
Rearing of winter bees continues until the queen stops laying, which could happen by the end of October. If treatment hasn’t been applied by the end of August, and mite levels are above five percent, chances of winter survival are not good at all. Several thousand healthy bees are required to effectively thermoregulate throughout winter. Only the winter bees reared after treatment are reliable. If treatment is delayed until September, there probably won't be enough brood reared the rest of the year to produce an adequate winter cluster.
Late in the season, the rate of infestation can change rapidly as the bee population continues to contract while the mite population continues to grow.. A sample that produces three mites in mid August may yield thirty mites in mid September. Don't wait for that to happen. You must keep your colony well ahead of varroa. If the mite load is higher than 1% in August, treat immediately.