If you keep honey bees long enough, varroa will be the reason you lose a hive. Not might be — will be. Varroa destructor is the single most destructive pest in modern beekeeping, responsible for the majority of colony losses globally since it spread out of Asia in the 1980s.
The good news: varroa is manageable. Beekeepers who monitor consistently and treat when thresholds are exceeded keep healthy, productive colonies year after year.
What varroa actually is
Varroa destructor is an external parasitic mite about 1.5mm wide — roughly the size of a sesame seed, reddish-brown, and visible to the naked eye on adult bees if you know what to look for. It originated in Asia, where it co-evolved with Apis cerana (the Asian honey bee). When it transferred to Apis mellifera (our honey bee), it found a host with no evolutionary defences against it.
The mite feeds on the fat body of bees, which plays a critical role in immune function, overwintering capacity, and overall bee health. A heavily parasitised bee's immune system is compromised in ways that make it far more vulnerable to viruses.
The lifecycle: why it's so hard to control
Phoretic phase — The mite rides on adult bees, feeding on their fat bodies. During this phase, the mite is exposed and treatable.
Reproductive phase — Just before a worker cell is capped, a mite slips inside. Once capped, she lays eggs on the developing bee pupa. The first egg hatches as a male, subsequent eggs as females. The offspring mate inside the cell, and when the new bee emerges, the mated female mites emerge with her and go looking for new cells to parasitise. The mite prefers drone brood — drone cells are capped longer, giving more time to reproduce.
This reproductive cycle means varroa populations can grow exponentially. A mite load that looks manageable in May can be catastrophic by September if left unchecked.
The real danger: viruses
Varroa acts as a vector for at least 18 bee viruses, the most significant being Deformed Wing Virus (DWV). When a mite feeds on a developing pupa, it injects viral particles directly into the bee's body. Bees raised in September and October — the long-lived winter bees that need to survive until spring — are often severely compromised. This is why colonies often appear to collapse suddenly in late fall or winter: the damage was done months earlier.
How to monitor: the alcohol wash
You cannot know your mite level without testing. The alcohol wash is the gold standard:
- Find a frame of young nurse bees in the brood nest
- Shake approximately 300 bees (about ½ cup) into a wide-mouth jar
- Add isopropyl alcohol (70%) to cover the bees
- Seal and shake vigorously for 60 seconds
- Pour through a strainer into a white tray and count the mites
- Divide mite count by 3 to get mites per 100 bees
A sugar roll is a non-lethal alternative — same process but with powdered sugar. Slightly less accurate but useful if you prefer not to sacrifice bees.
Action thresholds
| Time of Year | Action Threshold |
|---|---|
| Spring (March–May) | ≥ 2 mites per 100 bees |
| Summer (June–August) | ≥ 2 mites per 100 bees |
| Late summer / fall (August–September) | ≥ 1–2 mites per 100 bees |
| Winter (no brood) | Monitor; treat if using oxalic acid |
The late summer threshold is more conservative because this is when winter bees are being raised.
The monitoring calendar
Build monitoring into your inspection schedule: January–February (before winter OA treatment), March–April (baseline spring count), June (midsummer check), August (critical late-summer check), October (pre-winter check). Five monitoring visits per year is enough to catch problems before they become disasters. The alcohol wash takes about 10 minutes.
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