Tracheal Mites: The Overlooked Respiratory Parasite of Honey Bees
How Acarapis woodi infests bee breathing tubes, why it is easy to miss, and how resistant stock and colony strength keep it from becoming a winter loss problem.
A parasite that lives inside the bee
Tracheal mites (Acarapis woodi) differ from Varroa in a fundamental way: rather than living externally on the bee's body, they spend their entire life cycle inside the bee's tracheae, the tubes that carry air through the thorax. Female mites enter through the first pair of spiracles on young adult bees, usually within the first few days of life while the tracheal walls are still soft enough to penetrate, and then feed on the bee's hemolymph directly through the tracheal wall, lay eggs inside the tube, and complete several generations within a single host.
Because the whole infestation is internal, there is no external mite to spot on an inspection the way a Varroa mite might be seen on a bee's back. Diagnosis in the field therefore relies on indirect signs and, for confirmation, dissection and microscopy of the tracheae, which is why tracheal mite infestations are more often under-recognised than Varroa, especially by newer beekeepers who have not been shown what to look for.
Reading the indirect signs
The clearest field indicator is what beekeepers often call K-wing: affected bees hold their wings at an odd, disjointed angle rather than folded neatly over the back, a result of damage the mites cause to the flight muscles and surrounding tissue as they feed and multiply. Seeing a scattering of bees with this wing posture crawling near the entrance, unable to fly properly, is a reasonable prompt to investigate further, though it is not unique to tracheal mites and should not be treated as definitive on its own.
Disoriented, crawling bees unable to right themselves or fly in a straight line, particularly in late winter and early spring, are a second useful clue, since respiratory compromise from a heavy mite load reduces a bee's oxygen supply and impairs coordination. The pattern that most reliably points to tracheal mites, though, is an otherwise unexplained, severe winter loss: a colony that looked strong in autumn but is found dead or drastically reduced by late winter, with a die-off less obviously tied to starvation or Varroa, warrants tracheal mite dissection as part of working out what happened.
Why winter is the vulnerable season
Tracheal mite damage is cumulative and disproportionately dangerous in winter because a clustered colony depends on individual bees' ability to generate heat and maintain circulation within the cluster; bees whose respiratory capacity is compromised struggle to contribute to (or survive) the physical demands of clustering in cold weather. A moderate mite load that a colony might tolerate reasonably well during an active foraging season can translate into much higher mortality once the colony is confined and stressed by cold, which is why winter losses are the symptom most closely associated with this parasite historically.
This seasonal pattern also explains why tracheal mite problems can appear to come out of nowhere: a colony assessed as strong in September, with no obvious signs at the time, can be found severely weakened or dead in February, because the mite's impact only becomes fully apparent once the colony's physiological margin for error narrows over winter.
Management: resistant stock over routine treatment
Unlike Varroa, tracheal mites are managed today mostly through bee genetics rather than routine chemical treatment. Certain honey bee lines, most notably those selectively bred for tracheal mite resistance, groom mites off more effectively or present physical or behavioural traits that reduce successful mite entry, and sourcing queens from resistant stock is now the primary long-term control strategy for most beekeepers who have had problems with this parasite.
Menthol-based treatments were historically used and remain registered in some contexts, working through fumigation that affects mites within the tracheae, but many beekeepers today rely on maintaining strong, well-fed colonies and resistant genetics rather than routine chemical intervention, reserving treatment for confirmed, problematic infestations rather than applying it prophylactically. Keeping colonies strong going into winter, with adequate stores and minimal other stressors, reduces the compounding effect that tracheal mites have alongside cold and reduces overall winter loss risk regardless of the exact mite level present.
Frequently Asked Questions
How can I confirm tracheal mites without a microscope?
Field signs like K-wing posture and disoriented crawling bees are suggestive but not conclusive on their own. Confirmed diagnosis requires dissecting the thorax and examining the tracheae under magnification, which many bee inspection services or local associations can help arrange or demonstrate.
Are tracheal mites as serious a threat as Varroa today?
Generally no; in most well-managed apiaries with reasonably resistant stock, tracheal mites cause far less damage than Varroa does, and many beekeepers go years without a confirmed problem. They remain worth knowing about because they can compound winter losses that might otherwise be blamed solely on weather or stores.
Does treating for Varroa also help with tracheal mites?
Not directly, since the mites live in different locations on or in the bee and most Varroa treatments do not reach mites sequestered inside the tracheae. Managing the two pests is a largely separate task, though a generally strong, well-nourished colony copes better with both.
Can tracheal mites spread between colonies in an apiary?
Yes, primarily through drifting and robbing behaviour, where bees from an infested colony enter or are accepted into a neighbouring one, carrying mites with them. Keeping colonies well-spaced and minimising robbing opportunities during dearths reduces this spread.