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When to treat for varroa mites

Varroa mites are the number-one killer of honey bee colonies, mostly because they spread viruses like deformed wing virus. The hard truth is that nearly every colony has mites — so the question isn't if you have them, but how many, and when that number crosses the line. The only way to know is to monitor, not guess.

1. Monitor — don't eyeball it

You can't reliably judge mite load by looking at bees. Use a quantitative method on a sample of ~300 bees (roughly half a cup) from a brood frame:

• Alcohol wash — most accurate; kills the sample but gives a true count.
• Sugar shake — non-lethal, slightly less precise, fine for routine checks.
• Sticky board / 24-hour natural drop — easy but the least reliable as a trigger.

Express the result as mites per 100 bees (the percentage infestation). Check at least monthly through the season, and more often in late summer.

2. Treatment thresholds

A widely used guideline is to treat when you reach roughly 3 mites per 100 bees (3%) during the active season, and to act sooner — around 1–2% — in late summer, because the mites you have in August become an explosion by the time winter bees are reared. Crossing these levels means it's time to act; sitting at high counts for weeks is how colonies are lost.

3. Timing matters as much as the number

• Late summer / early autumn is the critical window. Your colony rears its long-lived "winter bees" then; if those bees are raised under heavy mite load, the colony often dies mid-winter even though it looked fine in October.
• Mind your honey supers. Many treatments can't be used while supers are on for harvest — plan the treatment for right after you pull honey.
• Temperature governs your options. Thymol and formic acid depend on ambient temperature ranges; oxalic acid works best in a broodless period (mid-winter, or after a brood break).
• Rotate active ingredients across the year to slow resistance, and always follow the label dose and timing.

4. Confirm it worked

Monitoring isn't a one-and-done. Re-test a couple of weeks after treating to confirm the count actually dropped. A treatment that didn't work is worse than knowing you still have a problem.

Related: Varroa treatment log · Preparing hives for winter · How often to inspect