Varroa destructor is the most important parasite of the honey bee and the leading cause of colony losses worldwide. Its original host is the Asian honey bee; in the middle of the twentieth century it crossed to the western honey bee and today it is found almost everywhere except Australia.

The Asian honey bee has lived alongside this mite for thousands of years and has developed defensive behaviours. The western honey bee has no such history; an untreated colony collapses within a few years.

Life cycle

The mite reproduces only inside sealed brood. That single sentence determines the whole logic of control.

A female mite enters the cell beside the larva just before capping and hides beneath the brood food. Once the cell is sealed she begins to feed and lays eggs. The first egg is male, the following ones female. The offspring mate inside the cell; when the adult bee emerges, the mature females come out with her.

Drone cells suit the mite better: because they stay sealed longer, more offspring reach maturity. This preference explains why drone brood removal works.

How it causes harm

The mite was long thought to feed on the bee's haemolymph. Current research indicates that the real target is the fat body. This distinction matters, because the fat body is responsible for immunity, detoxification and winter hardiness.

The consequences:

• Emerging bees weigh less and live shorter lives
• Immunity is suppressed
• The fat body of the winter bee is damaged and overwintering fails
• Viruses are injected directly into the haemolymph

The real danger: virus transmission

The most destructive effect of Varroa is not its feeding but its transmission of viruses. Deformed Wing Virus above all is carried directly into the bee by the mite.

For this reason colony collapse often occurs without the mite count being especially high. "The mites look few" does not mean safe.

Signs

• Reddish-brown mites visible to the naked eye on adult bees
• Young bees with deformed wings, unable to fly
• Irregular, patchy brood pattern
• Bees crawling in front of the hive
• Rapid and unexplained dwindling in autumn

Visible signs usually mean it is already late. Rather than waiting for symptoms, measure regularly.

The basis of control

Varroa cannot be eradicated; it is kept below a threshold. A working programme has four parts:

• Measuring: Regular monitoring by sugar roll or alcohol wash
• Non-chemical methods: Brood interruption, drone brood removal, trap comb
• Licensed products: When needed, rotating the active substance
• Verification: Measuring again after treatment

The last is the most neglected and the most expensive to omit.

Timing

The critical moment of the year is late summer. The winter bees are reared then; if the mite load damages their fat body the colony will not survive the winter. The mite level in August largely determines how many colonies are alive in March.

The broodless period is the second critical window: all the mites are on the bees and a single treatment reaches nearly all of them.