Varroa mites and varroosis

Varroa destructor is the number one health problem in apiaries worldwide. Left unmanaged, an infested colony usually collapses within two to three years, sometimes within a single season. In Europe, every colony should be assumed to carry mites: the real question is not whether varroa is there, but how many mites there are, and at what point in the year.

This factsheet covers its biology, how to measure infestation, decision thresholds, non-chemical methods and authorised treatments. The goal is simple: reach autumn with healthy winter bees.

What varroa is

The adult female is a reddish-brown, oval, flattened mite about 1.1 mm long and 1.6 mm wide: it is wider than it is long, which sets it apart from other mites. It is visible to the naked eye on a bee or a pupa. The male, smaller and pale, never leaves the cell where it was born.

Its original host is the Asian honey bee, Apis cerana, with which it lives in balance: in that species it breeds almost only in drone brood. Its jump to the western honey bee, Apis mellifera, during the 20th century was catastrophic, because this bee had no adapted defences. Varroa was first reported in France in 1982. It is now present on almost every continent; Australia, long spared, saw it arrive in 2022.

A cycle tied to the brood

Understanding the varroa cycle means understanding why treatments work or fail.

The dispersal phase. The female first lives and feeds on adult bees, preferably nurse bees, which carry her to the brood. This phase, long called the phoretic phase, lasts from a few days to about ten days in season. This is when the mite is exposed to treatments that do not act inside capped cells.

Entering the cell. The female slips into a brood cell shortly before it is capped: about 15 to 20 hours before for a worker cell, and 40 to 50 hours before for a drone cell. This longer window, and the longer capped period of drone brood, explain her strong preference for drones¹.

Egg laying. About 70 hours after capping, the female lays a first egg, which becomes a male. She then lays female eggs roughly every 30 hours. The young females mate with their brother inside the cell, before the bee emerges.

The outcome. In a worker cell, a female produces on average slightly more than one viable mated daughter. In a drone cell, she produces more than two¹. In the main season, the mite population can therefore double in about a month. A colony with few mites in spring can be overwhelmed by August.

Why it is so serious

The mite weakens every bee

Varroa does not suck haemolymph, as was long believed: it mainly digests the bee’s fat body². This organ stores reserves, handles detoxification, provides much of the bee’s immunity and governs the longevity of winter bees. A bee parasitised as a pupa emerges lighter, lives shorter and copes worse with diseases and pesticides.

It transmits and amplifies viruses

Varroa injects viruses straight into pupae. The most important is deformed wing virus (DWV): the global epidemic of this virus is directly linked to the spread of the mite³. Other viruses, such as those of the acute paralysis complex, also become very dangerous when varroa is present. In most late-season collapses, the viruses, more than the mite itself, finish off the colony⁴.

It strikes at the worst time

The mite population peaks in late summer, just when the colony is rearing its winter bees. These bees must live for several months. If they are parasitised and infected during development, the colony enters winter with workers doomed to die early. It then dwindles in November or December, often with untouched stores.

What we have learnt from Samuel Ramsey’s work

For nearly fifty years, textbooks repeated that varroa fed on haemolymph, the bee’s “blood”. The idea rested on observations from the 1970s that had never really been tested. In 2019, the American entomologist Samuel Ramsey, then a PhD student at the University of Maryland, published with the US Department of Agriculture’s microscopy unit (USDA-ARS) a study that overturned this dogma².

Where varroa hides on the adult bee

By freezing parasitised bees and examining them with low-temperature scanning electron microscopy, the team showed that varroa does not wander at random on adult bees. It wedges itself under the plates of the abdomen, between the segments, often on the left side, where the membrane is thin and where it escapes grooming^{2, 11}.

A wound right above the fat body

Under the mite, the researchers found a wound in the membrane, surrounded by the marks of its legs. Sections through the abdomen show that this wound opens directly onto the fat body, and that nearby tissue is damaged².

The proof inside the mite’s gut

The team then stained the bees’ fat body with a fluorescent dye. Mites feeding on these bees had guts full of the dye: they had digested fat body, not haemolymph².

Finally, mites were fed artificially. Those given fat body lived longer and laid eggs; those given only haemolymph did barely better than starved mites^{2, 12}.

What it means for beekeepers

• The damage is worse than previously thought. The fat body manages reserves, detoxification and much of the immune response. A parasitised bee copes worse with viruses and pesticides, and a parasitised winter bee lives shorter.
• Adult bees are genuinely parasitised. During the dispersal phase, long called “phoretic”, varroa does not just travel: it feeds. That is why “dispersal phase” is now the preferred term.
• New treatment avenues. Knowing which tissue is consumed opens the way to molecules that target the mite’s digestion, and helps explain why some substances circulating in the haemolymph work poorly.

The internal anatomy of varroa, described in relation to feeding and reproduction, has since been detailed in an atlas to which Samuel Ramsey contributed¹⁰. He also works on Tropilaelaps, the other brood mite threatening Europe: see the Tropilaelaps factsheet.

Recognising a colony with too many mites

Visible signs appear late. When you see them, the infestation is already heavy.

• Bees with crumpled or stunted wings and short abdomens, thrown out of the hive.
• Patchy, spotty brood with perforated cappings and dead pupae.
• Mites visible to the naked eye on bees, especially on the thorax.
• Many mites on uncapped drone pupae.
• Heavy mite fall on the sticky board.
• A population that dwindles in September-October although stores are present.

The term parasitic mite syndrome is sometimes used for this combination of abnormal brood, dead larvae of varied appearance and deformed bees. It can resemble foulbrood: if in doubt about the brood, do the matchstick test and, if needed, send a sample for testing.

Measuring infestation

Never treat blindly. Counting tells you when to treat and whether the treatment worked. Reference methods are described in the COLOSS BEEBOOK⁵.

Alcohol or detergent wash

This is the most reliable method.

1. Locate the queen to be sure you do not sample her.
2. Shake bees from a frame of open brood into a tub, because nurse bees carry more mites than foragers.
3. Take about 300 bees, roughly half a cup.
4. Put them in a jar with alcohol or soapy water and shake hard for one minute.
5. Pour through a mesh that holds back the bees and lets the mites through, then count the mites on a white background.
6. Divide the number of mites by the number of bees and multiply by 100: this gives mites per 100 bees.

The sampled bees die, but 300 bees are a tiny fraction of a strong colony.

Sugar roll

Same sample, but the bees are rolled in icing sugar in a jar with a mesh lid. The sugar dislodges the mites, which are shaken out onto a white background. The bees survive. The method is slightly less accurate, especially in humid weather.

Natural mite fall on a sticky board

A greased board slid under a mesh floor and checked after three to seven days gives an average daily mite drop. It does not disturb the colony and shows the trend over time. However, the link between natural fall and actual infestation varies widely with the amount of brood: it shows a trend, not a precise measure.

Sticky boards are very useful after a treatment, to see the mite drop, and in winter, to judge the remaining infestation.

Treatment thresholds

Thresholds vary with region, season and colony size. The guide values below, in mites per 100 adult bees, are based on reference guides⁶:

Two important points. First, the same figure is more worrying in spring than in summer, because the mite population has time to multiply. Second, an apiary surrounded by untreated colonies suffers reinfestation through drifting and robbing: counts can rise again quickly, even after a good treatment.

Non-chemical methods

They are almost never enough on their own, but they slow mite growth and make treatments more effective.

Drone brood trapping. In spring, place a drone comb, or a partly drawn frame, at the edge of the brood nest. Mites concentrate there. Remove and destroy the brood once capped, before the drones emerge: otherwise you have bred mites instead of trapping them. Repeated two or three times, this removes a significant share of the mite population.

Brood break. Cage or remove the queen for about 24 days, until all the brood has emerged. The colony is left without capped brood, so every mite is on the adult bees. An oxalic acid treatment applied at that point is very effective.

Total brood removal. Remove all capped brood combs, which can be used to make up a separate colony treated on its own. The original colony, now without capped brood, is treated straight away.

Splits and artificial swarms. They create a natural break in brood rearing and spread the mites across several units.

Breeding. Some lines show varroa-sensitive hygiene (VSH): workers detect and open cells where the mite is breeding. This trait is a major breeding goal, but no current line removes the need to monitor and treat.

Medicinal treatments

The legal framework

In France and the EU, only a veterinary medicine authorised for honey bees may be used⁸. Home-made preparations, mixtures and diverted pesticides are illegal. They are also dangerous for the bees, the beekeeper and the quality of the honey. Every treatment must be recorded in the apiary’s treatment register.

Main active substances

Product names and authorisations differ from country to country and change over time: always check your national register of veterinary medicines before buying.

Using a treatment well

• Read the label: dose, treatment duration, temperature range and compatibility with supers.
• Do not treat with supers on, unless the label explicitly allows it.
• Respect the treatment duration, neither shorter nor longer. A strip left in the hive for months releases too low a dose and selects resistant mites.
• Rotate product families from one year to the next.
• Treat all colonies in an apiary at the same time, or untreated colonies will reinfest the others.
• Check efficacy with a count a few weeks after the end of treatment.

Typical calendar in a temperate climate

1. Late winter and spring: baseline count. Drone frames in colonies that are building up.
2. May-June: count; a season-compatible treatment if the threshold is exceeded. Splits and queen rearing are opportunities to create brood breaks.
3. July-August, right after the last harvest: main treatment, without delay. This is the decisive period: winter bees start emerging in late August and must be protected from the start of their development.
4. September-October: control count to check efficacy; additional treatment if infestation is still high.
5. December-January, broodless colony: oxalic acid clean-up treatment, to start the next season with as few mites as possible.

This calendar shifts with climate and nectar flows. Where colonies never stop laying in winter, the strategy relies more on induced brood breaks.

The most common mistakes

• Treating too late, in September or October: winter bees are already parasitised.
• Relying on the absence of symptoms: by the time deformed wings appear, it is often too late for the season.
• Never counting, and so never knowing whether the treatment worked.
• Always using the same molecule, until it stops working.
• Leaving strips in beyond the prescribed duration.
• Treating colony by colony, on different dates, in the same apiary.
• Skipping the broodless winter treatment, which gives the best efficacy of the year.

Frequently asked questions

Can you keep bees without treating for varroa? In the vast majority of cases, no. A few honey bee populations live with varroa after years of harsh natural selection, at the cost of heavy losses. For a beekeeper, not treating means accepting high losses and exposing neighbouring apiaries to reinfestation.

Do treatments contaminate the honey? Used according to the label, with no supers on, authorised medicines do not cause residue problems in harvested honey. Unauthorised products, on the other hand, can leave residues in wax and honey.

Should I treat a newly bought colony or a swarm? A natural swarm has no capped brood when it settles: that is the ideal moment for an oxalic acid treatment. For a bought colony, count on arrival and treat according to the result, ideally at the same time as the rest of the apiary.

Why do my counts rise again after a good treatment? Usually because of reinfestation: robbing of collapsing colonies nearby, or bees drifting between hives. It is common at the end of the season.

References

1. Rosenkranz P., Aumeier P., Ziegelmann B. (2010). Biology and control of Varroa destructor. Journal of Invertebrate Pathology 103: S96-S119. doi.org
2. Ramsey S.D. et al. (2019). Varroa destructor feeds primarily on honey bee fat body tissue and not hemolymph. PNAS 116(5): 1792-1801. doi.org
3. Wilfert L. et al. (2016). Deformed wing virus is a recent global epidemic in honeybees driven by Varroa mites. Science 351: 594-597. doi.org
4. Traynor K.S. et al. (2020). Varroa destructor: a complex parasite, crippling honey bees worldwide. Trends in Parasitology 36(7): 592-606. doi.org
5. Dietemann V. et al. (2013). Standard methods for varroa research. The COLOSS BEEBOOK, Vol. II. Journal of Apicultural Research 52(1). doi.org
6. Honey Bee Health Coalition. Tools for Varroa Management (regularly updated guide, honeybeehealthcoalition.org).
7. GDS France. Technical sheets on varroa medicines (Apivar, Apitraz, Apistan, Apilife Var, Varromed), in French. gdsfrance.org
8. DRAAF Occitanie (2022). Reminders on the use of veterinary medicines in beekeeping (in French). draaf.occitanie.agriculture.gouv.fr
9. USGS Bee Inventory and Monitoring Lab. Photographs of Varroa destructor (public domain). usgs.gov
10. Sonenshine D.E., Posada-Florez F., Laudier D., Gulbronson C., Ramsey S., Cook S.C. (2022). Historical atlas of the internal anatomy of female Varroa destructor (Mesostigmata: Varroidae) mites. Annals of the Entomological Society of America 115(2): 163-193.
11. Ramsey S., Gulbronson C., Mowery J., Ochoa R., vanEngelsdorp D., Bauchan G. (2018). Multi-microscopy study of the feeding site and host tissue consumed by Varroa destructor. Microscopy and Microanalysis 24(S1): 1258-1259.
12. USDA Agricultural Research Service (2019). ARS microscopy research helps unravel the workings of a major honey bee pest. ars.usda.gov
13. Entomology Today (2019). An inside look at how the Varroa mite diet was discovered (images USDA-ARS ECMU). entomologytoday.org

Updated : October 8, 2026
Written by : Apisanitas editorial team
Reviewed by : Review in progress