Treat for varroa mites in late summer and fall

Apply an effective varroa mite treatment in August-September, timed so that the long-lived "winter bees" (born September-October) develop with low mite parasitism. Colonies entering winter with mite loads above 2-3% infestation rate face dramatically higher mortality.

Why It Works

Varroa destructor feeds on bee fat bodies and transmits Deformed Wing Virus (DWV), which shortens adult bee lifespan. Winter bees normally live 4-6 months compared to 4-6 weeks for summer bees, but DWV-infected winter bees die prematurely, causing the cluster to dwindle and collapse mid-winter. Research from the University of Maryland shows that colonies with fall mite loads above 3% are 3-5 times more likely to die over winter than colonies treated below that threshold.

Steps

1. Monitor mite levels in August using an alcohol wash or sugar roll on 300 bees -- the 2-3% action threshold applies
2. Select a treatment appropriate for the temperature window: formic acid strips (above 10°C/50°F), oxalic acid vaporization (effective during broodless periods in late fall), or amitraz strips (Apivar, 42-day treatment period)
3. Apply the treatment following label directions for dose, duration, and hive configuration
4. Re-monitor 2-3 weeks after treatment ends to confirm mite levels dropped below 1%

Tips

• Treating in September is often too late for amitraz strips because the 42-day treatment window runs into cold weather -- plan ahead
• Oxalic acid vaporization is most effective during a natural broodless period (late November-December in northern climates) as a cleanup treatment
• Rotate treatment chemistries annually to reduce resistance development