Varroa (Varroa destructor)

Varroa destructor is the most important parasite of the honey bee and the main cause of colony losses worldwide. The mite both weakens the bee and spreads viruses, above all deformed wing virus; a colony left untreated usually collapses within a few years. The key to success is not waiting for symptoms but counting regularly and intervening at the right time.

What is it and how does it spread?

The original host of varroa is the Asian honey bee (Apis cerana). In the mid-twentieth century it jumped to the western honey bee (Apis mellifera), and today it is found almost everywhere in the world apart from a few islands and regions; it was also detected in Australia in 2022. Because the Asian honey bee shares a long history with the mite, it has developed defensive behaviours; in our bees these defences are weak.

The mite is carried between hives by robbing, by bees entering the wrong hive (drifting), by swarms and by exchanging colonies and frames. A colony whose varroa is not under control, or one that is collapsing, can pass large numbers of mites to the surrounding hives.

Life cycle: the logic behind control

The mite reproduces only in capped brood cells. The female mite enters the cell next to the larva shortly before it is capped and hides beneath the brood food. Once the cell is capped she lays eggs: the first egg produces a male, the following ones females, and they mate inside the cell. When the bee emerges, the mature female mites leave with it and are carried on adult bees.

Because drone cells stay capped for longer, the mite raises more offspring there; this is why it prefers drone brood. It is also why drone brood removal and broodless-period treatments work.

How does it cause damage?

For a long time the mite was thought to feed only on the bee's haemolymph; current research shows that it feeds largely on the fat body. As the fat body is critical for immunity, detoxification and winter hardiness, the damage runs deeper than once believed:

• Emerging bees are lighter and shorter-lived, and their immunity is weakened.
• If the fat body of winter bees is damaged, the colony cannot survive the winter.
• The mite transmits various viruses, above all deformed wing virus, directly into the bee's body. In colony losses these viruses are often the truly destructive factor.

Seeing few mites in a collapsed hive is misleading: by that point the virus load has already risen, and the mites may have been carried to neighbouring hives by robber bees.

How to recognise it

• On adult bees: Flat, reddish-brown mites between the abdominal segments or on the thorax. Because most mites hide between the segments, counting by eye is misleading.
• In the brood: Patchy (pepper-pot) brood, perforated or re-opened cappings, mites on pupae in opened cells and white mite faeces on the cell walls.
• On young bees: Crumpled, short, useless wings and a shortened abdomen (deformed wing virus).
• In front of the hive: Crawling bees unable to fly; unexplained rapid dwindling in autumn.

How it differs from similar conditions

How to measure it in the hive

• Alcohol (or soapy water) wash: About 300 bees (roughly half a tea glass) are taken from a brood frame, shaken in a lidded container and the fallen mites are counted. This is the most reliable field method; the sampled bees die. Make sure the queen is not in the sample.
• Icing sugar method: The same number of bees is coated with icing sugar and shaken, and the mites are shaken out through a mesh; the bees can be returned to the hive. It catches slightly fewer mites than a wash.
• Natural mite drop count: In a hive with a mesh floor, the mites falling onto the tray over several days are counted and divided by the number of days. Useful for following trends and seeing the effect of a treatment.
• Uncapping drone brood: Drone pupae are lifted out with an uncapping fork and checked for mites; this gives a quick first impression.

Express the result as the number of mites per 100 bees. As a general rule, more than 2–3 mites per 100 bees in summer and late summer calls for action; thresholds vary with region, season and the amount of brood in the hive. Measure every hive if possible, not just a few in the apiary.

What to do, step by step

1. Measure: Count in spring, immediately after the honey harvest and in autumn.
2. Use non-chemical methods: In spring, drone brood removal (trap comb), a brood break when making splits and taking swarms, and creating a broodless period by temporarily caging the queen.
3. If the threshold is exceeded, treat with an authorised product: after the honey harvest and before the winter bees are reared.
4. Treat all hives in the apiary at the same time; otherwise untreated hives will carry mites back into the others.
5. Measure again after treatment. This is the step most often skipped and the one that costs the most.
6. Carry out a clean-up treatment in the broodless period (usually early winter); at that time all the mites are on the bees.

Treatment and medicine information

Use only veterinary products authorised in your country; take the product, dose and treatment period from the package leaflet or from your veterinarian. The main groups of active substances are:

• Oxalic acid: Applied by trickling or sublimation; it works best in the broodless period and has no effect on mites in capped brood.
• Formic acid: One of the few options that can also reach mites inside capped brood; its effectiveness and its harm to bees depend strongly on the outside temperature.
• Thymol-based products: Their effectiveness depends on temperature; they require a treatment lasting several weeks.
• Amitraz strips: A synthetic acaricide; it should not be used continuously, to prevent resistance from developing.

Timing and prevention

The critical moment of the year is late summer: that is when the winter bees are reared. If the mite load is high at that time, the colony may not survive the winter even if the hive looks full in autumn. August–September counts largely determine how many hives will be alive the following spring.

• Renew combs regularly, prevent robbing, and arrange hives so as to reduce drifting.
• Measure colonies and swarms you buy before adding them to the apiary, and treat them if necessary.
• Coordinating treatment times with neighbouring beekeepers reduces reinfestation.
• Record the count results and treatments for each hive.