UNDERSTAND BEES. PROTECT LIFE.An independent encyclopedia
apisanitas.HONEY BEE HEALTH
Parasites

Tracheal mite (acarapisosis)

Acarapis woodi

Severity : Moderate Late winter and springReview in progress
Topic symbol

Key points

  • A microscopic mite that lives and breeds inside the thoracic tracheae of adult bees.
  • Only very young bees are colonised: infestation builds up mainly in winter and early spring.
  • No reliable symptom: only examining the tracheae under a microscope gives an answer.
  • In France it has not been a regulated disease since 2006; other countries still control trade in bees.
  • Susceptibility depends on genetic origin: breeding remains the first means of control.

Acarapisosis, or tracheal mite infestation, is caused by Acarapis woodi, a microscopic mite that lives inside the thoracic tracheae of adult bees. It feeds and breeds there out of sight, and no symptom allows it to be identified with certainty2.

Long regarded as a major disease, it is now feared far less than varroa. Yet it is still present in most beekeeping regions of the world and can contribute to unexplained winter losses.

A disease born of a mystery: the “Isle of Wight”

Between 1904 and 1919, massive colony losses were recorded on the Isle of Wight, then across much of the United Kingdom. People spoke of “Isle of Wight disease”4. In 1921, the zoologist John Rennie and his colleagues at the University of Aberdeen described a mite found in the tracheae of sick bees and named it Tarsonemus woodi5. It was renamed Acarapis woodi the same year. The mite was then held responsible for the epidemic, and the disease became known as acarine disease.

This explanation was challenged in 1964 by Leslie Bailey of Rothamsted. He concluded that the mite did not explain “Isle of Wight disease”6. The mite is a real parasite, but its role in those losses was overestimated.

As early as 1922, the United States restricted imports of live bees to keep the mite out4. It was nevertheless detected there in 1984, in Texas, and in Canada the following year4.

The agent and its life cycle

Acarapis woodi is a mite of the family Tarsonemidae. The adult female measures less than 0.2 mm: it is invisible to the naked eye1.

Acarapis woodi mites inside a honey bee trachea, seen under high magnification
Tracheal mites (Acarapis woodi) inside a honey bee's trachea, magnified about 140 times. Photo: Lila de Guzman, USDA ARS, public domain.

A very short window of susceptibility

The mite only colonises very young bees. Mated females seek out workers, drones or queens generally less than four days old2, 4. Beyond that age, the mite finds it much harder to get in; a commonly cited explanation is the hardening of the hairs guarding the tracheal opening.

The cycle inside the trachea

The female enters through the first thoracic spiracle and settles in the large tracheae of the prothorax. She pierces the tracheal wall with her mouthparts and feeds on haemolymph1. She lays several eggs there, around ten according to sources2. Development takes about 11 to 12 days for a male and 14 to 15 days for a female; mating takes place in the trachea4.

Young mated females leave the trachea once their host is more than 12 days old, peaking between 15 and 25 days, mostly at night4. One generation takes about two weeks.

Why winter favours it

During the season, bees live only a short time. In winter, they live for several months, packed in the cluster: the mites have time to multiply and pass from bee to bee. This is why the heaviest infestations are generally seen in late winter and early spring2.

Do not confuse with other Acarapis

Two other species in the genus live on the outside of the bee’s body, without entering the tracheae4:

Species Location Importance
Acarapis woodi Inside the prothoracic tracheae Pathogenic parasite
Acarapis externus On the neck, between head and thorax Considered of little harm
Acarapis dorsalis In the dorsal groove of the thorax Considered of little harm

The three species look very similar; their position on the bee is the first distinguishing feature. Methods that grind up whole bees, such as some molecular tests (PCR), can pick up external Acarapis and give a false positive. The WOAH manual recommends confirming any positive result by microscopy2.

Effects on the bee and the colony

The mite gradually blocks the tracheae and damages their walls, which become stained brown then black. Heavily parasitised bees live, according to reviews, for a shorter time1.

At colony level, the effects depend on the level of infestation. A heavy infestation in winter can disrupt the cluster’s temperature regulation and lead to bees dying of chilling2. The colony then dwindles, sometimes to the point of dying in spring, often in combination with other stresses such as varroa, nosema or insufficient stores.

Recognising it: misleading signs

The WOAH manual is clear: there are no reliable clinical signs of acarapisosis2. Signs sometimes described are:

  • Bees crawling in front of the hive, unable to fly, climbing blades of grass.
  • Wings spread and misaligned, known as “K-wing”.
  • Signs of dysentery.

None of them is specific: they are also seen with chronic bee paralysis, nosema or poisoning.

Photo needed — Crawling bees at the foot of a hive in late winter, several of them with "K-wings".

Diagnosis by dissection, step by step

Only examining the tracheae gives an answer. Reference methods are described by WOAH and in the COLOSS BEEBOOK2, 4.

Sampling

  • When? Preferably in early spring or late autumn, when mite populations are highest2.
  • Which bees? Preferably those crawling near the entrance; otherwise, bees taken from brood combs2, 4.
  • How many? About 50 bees per colony are enough to detect an infestation of around 1 to 2%2.
  • Storage: in 70% alcohol or in the freezer, which makes dissection easier4.

Individual dissection

  1. Lay the bee on its back and hold it with a pin. Remove the head, the front legs, then the “collar” formed by the first thoracic ring: the two large prothoracic tracheae come into view.
  2. Examine the tracheae under a stereo microscope. A healthy trachea is clear, creamy white or translucent.
  3. An infested trachea shows brown to black spots or patches. Mites, eggs and immature stages can be seen.
  4. If in doubt, remove the trachea and examine it under a compound microscope, at about 100 times.

The thoracic disc method

Laboratories cut a thin slice of the thorax, in front of the second pair of legs, then dissolve the muscles in a heated potassium hydroxide solution. The tracheae are then stained with methylene blue and examined under a stereo microscope2.

Photo needed — Dissected bee thorax under a stereo microscope: a clear healthy trachea next to a brown-stained infested one.

Molecular methods (PCR) also exist, using larger samples2.

Regulatory status

In France, acarapisosis has not been a notifiable contagious disease (maladie réputée contagieuse) since decree no. 2006-178 of 17 February 20069.

In the European Union, it is not among the bee diseases listed under the Animal Health Law, which covers in particular American foulbrood, the small hive beetle, Tropilaelaps and varroa.

Elsewhere, rules vary. In Switzerland, acarapisosis is an epizootic disease under surveillance, subject to mandatory notification10. The mite is considered absent from Australia11. Internationally, the WOAH Terrestrial Code has a chapter on Acarapis woodi: an importing country may require a certificate stating that bees come from a free zone or from apiaries tested free of the mite8.

Prevention and control

Genetics, the first lever

Susceptibility varies widely with the bees’ origin; historically, this observation lies behind the Buckfast strain. Brother Adam (Karl Kehrle), a monk at Buckfast Abbey in Devon, saw the Abbey apiary lose 29 of its 45 colonies at the time of “Isle of Wight disease”; the survivors were of Italian and Carniolan origin. He then set out, through crossbreeding, to produce a bee resistant to acarine disease and a good honey producer13.

Later trials confirmed that resistance is heritable. In a US field trial published in 1995, Buckfast bees imported from the United Kingdom were among the stocks classed as resistant7. Other stocks, including the so-called “Russian” bee, are also described as resistant4. One of the mechanisms identified is autogrooming: young resistant bees more often try to dislodge the mite with their legs before it enters the trachea4.

Colony management

  • Overwinter strong colonies, with sufficient stores and varroa under control3.
  • Avoid bringing in colonies or queens of unknown origin without health guarantees10.

Treatments

  • In the United States, menthol is registered against tracheal mites, for example as Mite-A-Thol, for overwintering hives and outside the honey flow. Its efficacy depends on temperature: it works poorly in cold weather12, 4. Some formic acid products there also list tracheal mites on their label.
  • Sugar and vegetable shortening patties, placed on the top bars, are described as a way to limit infestation: they are thought to hinder the mite’s passage to young bees4.
  • In France and Europe, to our knowledge, no medicine is authorised specifically against acarapisosis. Some varroa treatments based on formic acid or thymol have a documented effect on Acarapis1. Only use medicines authorised in your country, following the label, and record every treatment in the apiary register.

In practice: what beekeepers actually do

Tracheal mites are not part of routine work in European apiaries. Beekeepers think of them in a few specific situations, and then a lab test settles the question.

When to suspect it

  • Unexplained losses in late winter, even though varroa was well managed and stores were sufficient.
  • Crawling bees at the foot of hives in late winter or spring, with no sign of poisoning.
  • A colony that stalls in spring while the others in the apiary build up.
  • Exporting queens or colonies: some countries require a prior test.

How to get bees tested

  • Collect 50 bees per suspect colony, preferably crawling bees near the entrance.
  • Kill and store them in the freezer or in 70% alcohol, in a labelled container.
  • Contact the laboratory first, or your local bee health body or vet: not all laboratories offer this test, and each has its own shipping instructions.
  • Also ask for nosema and chronic paralysis testing if the picture is unclear.

Equipment for home dissection

A stereo microscope (20 to 40 times), fine forceps, a scalpel or micro-scissors, a cork board and pins, and 70% alcohol.

Checklist

  1. Rule out varroa, starvation and poisoning first.
  2. Sample fresh bees, not old dead ones.
  3. Have any positive PCR result confirmed by microscopy.
  4. If acarapisosis is confirmed, consider requeening with a less susceptible strain.

Most common mistakes

  • Diagnosing tracheal mites from “K-wing” alone: this sign is not specific.
  • Confusing Acarapis woodi with external Acarapis, which do little harm, in a test that grinds whole bees.
  • Using a product registered only abroad, such as menthol in the United States.
  • Forgetting varroa: in most winter losses, it remains the first cause to check.

Frequently asked questions

Is acarapisosis still present in France? The mite is present in most beekeeping regions of the world. In France, with no regulatory surveillance since 2006, how common it is remains poorly known.

Are varroa treatments enough? Some treatments based on formic acid or thymol also act on Acarapis. Their effect is not guaranteed; always follow the label.

Do tracheal-mite-resistant bees exist? Yes. Resistance is heritable and has been shown in several strains, partly through better autogrooming by young bees. No strain, however, removes the need for testing after unexplained losses.

References

  1. Sammataro D., Gerson U., Needham G. (2000). Parasitic mites of honey bees: life history, implications, and impact. Annual Review of Entomology 45: 519-548. doi.org
  2. WOAH. Manual of Diagnostic Tests and Vaccines for Terrestrial Animals, chapter 3.2.1: Acarapisosis of honey bees (version adopted in 2022). woah.org
  3. FAO, IZSLT, Apimondia, CAAS (2021). Good beekeeping practices for sustainable apiculture. FAO Animal Production and Health Guidelines No. 25.
  4. Sammataro D., de Guzman L., George S., Ochoa R., Otis G. (2013). Standard methods for tracheal mite research. In: The COLOSS BEEBOOK, Vol. II. Journal of Apicultural Research 52(4). doi.org
  5. Rennie J. (1921). Isle of Wight disease in hive bees: acarine disease: the organism associated with the disease, Tarsonemus woodi, n. sp. Transactions of the Royal Society of Edinburgh 52(4). doi.org
  6. Bailey L. (1964). The "Isle of Wight disease": the origin and significance of the myth. Bee World 45(1): 32-37. doi.org
  7. Danka R.G., Villa J.D., Rinderer T.E., Delatte G.T. (1995). Field test of resistance to Acarapis woodi (Acari: Tarsonemidae) and of colony production by four stocks of honey bees (Hymenoptera: Apidae). Journal of Economic Entomology 88(3): 584-591.
  8. WOAH. Terrestrial Animal Health Code, chapter 9.1: Infestation of honey bees with Acarapis woodi (2024 edition). woah.org
  9. French Ministry of Agriculture, DGAL. Service note DGAL/SDSPA/N2007-8051: apiculture health report (acarapisosis, varroosis and European foulbrood no longer notifiable contagious diseases under decree 2006-178 of 17 February 2006). info.agriculture.gouv.fr
  10. Federal Food Safety and Veterinary Office (FSVO, Switzerland). Acariose des abeilles (acarapisosis of bees). blv.admin.ch
  11. NSW Department of Primary Industries and Regional Development (Australia). Tracheal mite. dpird.nsw.gov.au
  12. US EPA. Mite-A-Thol (menthol) product label, EPA Reg. No. 61671-1. www3.epa.gov
  13. Buckfast Abbey. The Buckfast bee (history of the Abbey apiary and Brother Adam). buckfast.org.uk
  14. USDA Agricultural Research Service, Image Gallery. Photo K5069-23: tracheal mites inside a honey bee's trachea (Lila de Guzman, public domain). ars.usda.gov

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