Tracheal Mite (Acarapis woodi)

The tracheal mite (Acarapis woodi) is a mite, invisible to the naked eye, that lives in the breathing tubes (tracheae) in the thorax of the adult bee. Its symptoms are not specific; a definitive diagnosis can only be made by opening the thorax of the bees and examining them under a microscope. Today its importance has declined in many regions, but it can contribute to winter losses in non-resistant lines.

What is it and how does it live?

The female mite is about 0.1–0.2 mm long. It can only enter through the first breathing opening (spiracle) on the thorax of young bees a few days old; as the bee ages, the hairs around this opening stiffen and make entry more difficult. The mite lays its eggs inside the trachea, the young develop there and feed on the bee's haemolymph.

Over time the tracheal wall is damaged, darkens and becomes blocked. Less oxygen reaches the bee's flight muscles and its lifespan is shortened. Mature females leave the trachea and move over the bee's hairs to a new, young bee.

How does it spread?

• From bee to bee within the hive, especially through close contact between young bees
• Between colonies through drifting, robbing and swarms
• Buying infested colonies, package bees and queens; moving hives

How to recognise it

There is no symptom specific to the tracheal mite that can be diagnosed by eye. The following picture also arises from other causes:

• Bees crawling in front of the hive, unable to fly, climbing up grass stems
• "K-wing": the two wings on one side separating and standing like the letter K
• Unexplained population decline in late winter and early spring; the colony dying out while there is still honey in the hive

Definitive diagnosis: tracheal examination

The diagnosis is made in the laboratory or by an experienced person who knows the technique. Roughly, the procedure is as follows:

1. A sample is collected from bees crawling in front of the hive or taken at the entrance (usually about 30 bees; ask the laboratory in advance how many it wants). The sample is sent in alcohol or frozen.
2. The bee's head and first pair of legs are removed, and the front ring of the thorax (the collar) is lifted off.
3. The main tracheae thus exposed are examined under a magnifying glass; if necessary the trachea is removed and viewed under the microscope.

A healthy trachea is white or pale cream, translucent and ringed. An infested trachea looks brown-spotted, dark, sometimes completely blackened.

Its importance today

The mite was first linked to heavy colony losses in England in the 1920s; later research showed that this mite was not the sole cause of those losses. When it spread to North America in the 1980s it caused significant losses.

Today its economic importance has declined in many regions. The main reasons are the resistance that has developed over time in bee populations and the fact that some treatments applied against varroa also suppress this mite. It can still be a problem, however, in non-resistant lines and in regions with long winters.

What to do

1. Get a diagnosis instead of guessing. When you see crawling bees with K-wing, send a sample to the laboratory for both tracheal mite and nosema.
2. Keep the colony strong. A well-fed colony with varroa under control tolerates the effects of the mite better.
3. Requeen from a resistant line. Resistance to the tracheal mite is heritable; in problem colonies it is the most lasting solution.
4. Do not buy colonies, package bees or queens from unknown sources.
5. Leave the decision on medicines to your veterinarian. Some products authorised for varroa (such as formic acid and some essential-oil-based products) are known to suppress the tracheal mite as well. Whether a treatment specific to the tracheal mite is needed, and its authorisation status in your country, should be decided on the basis of the laboratory result.