Winter Varroa Management: Treatment Timing, Broodless Period OAV, and Spring Mite Levels

Winter Varroa management is where many beekeepers make their most consequential mistakes. The bees that carry a colony through winter are long-lived winter bees with fat body reserves that sustain them through 3 to 6 months of cold. Those bees develop in late summer from eggs laid in August and September. If mite loads are high during that developmental period, the winter bees are physiologically compromised at birth, their fat bodies are reduced, and the colony starts spring in a weakened state regardless of what treatment you apply in October.

The Critical Late-Summer Treatment Window

The single most impactful Varroa management action for most beekeepers in the northern United States is a well-timed late-summer treatment to protect winter bee development. This means having mite levels at or below 1 to 2 percent by mid-August when winter bee production begins. If your mite count in July is at 3 to 4 percent, you need to treat immediately, not wait until fall.

Apivar strips inserted in late July or early August complete their 6 to 8 week treatment window before October and allow winter bees to develop with low mite exposure. MAQS in early August also works and has the advantage of penetrating capped brood. Time any summer treatment to be complete before the main winter bee brood is capped.

Broodless Period OAV

In northern climates, colonies typically become broodless or nearly broodless from mid-November through January. This is the optimal window for a single oxalic acid vaporization treatment that kills all remaining phoretic mites. With 100 percent of mites phoretic during a broodless period, a single OAV dose achieves 95 to 99 percent efficacy and sends the colony into the deepest part of winter with a near-clean slate.

Confirm broodlessness before treating. In a mild fall, queens may continue laying into November. Treat during a brief warm spell (above 40 degrees F) that allows vaporizer operation, ideally on a clear day with temperatures between 40 and 50 degrees F at midday.

Evaluating Spring Mite Levels

Do your first spring alcohol wash as early as you can inspect brood, typically when daytime temperatures first hit the mid-50s to 60s in March or April. Colonies that come out of winter with mite counts above 1 to 2 percent on a small bee population need immediate treatment before brood production scales up. A small spring population with even a moderate mite load can see exponential mite growth through spring that overwhelms the colony by June.