How Tracheal Mites Affect Honey Bees and How to Treat Them

Tracheal mites are microscopic parasites, scientifically named Acarapis woodi, that live and reproduce inside the breathing tubes of honey bees. They feed on bee hemolymph (the insect equivalent of blood) by piercing the tracheal wall, gradually weakening individual bees and, when infestation levels climb high enough, threatening the survival of entire colonies. Though overshadowed in recent decades by the more headline-grabbing Varroa destructor, tracheal mites remain a significant concern for beekeepers and have a surprisingly dramatic history tied to one of the earliest recorded collapses of managed honey bee populations.

Where They Live and What They Look Like

Tracheal mites are tiny, even by mite standards. Adult females measure roughly 120 to 190 micrometers long, small enough that dozens can pack into a single tracheal tube. They belong to the family Tarsonemidae, a group of soft-bodied mites, and spend nearly their entire lives inside the prothoracic tracheae, the largest pair of breathing tubes located just behind a bee’s head. Low-temperature scanning electron microscopy has revealed features that help explain how these mites navigate such cramped quarters. Each leg can independently twist its segments, with the ambulacrum (the foot-like tip) rotating at least 180 degrees during movement, a surprisingly sophisticated locomotion system for such a small creature.¹

Inside the trachea, the mites do not appear to use their tarsal claws for gripping the tube walls. Larvae have swollen pads on their feet that essentially disable the claws, allowing them to move freely through the tracheal system. Adults instead use their body and leg setae, fine hair-like structures, as tactile sensors to gauge the space around them. Most of these setae point backward, which researchers believe helps the mite judge the diameter of the tube it occupies.²

How Tracheal Mites Spread

A mated female mite is the dispersal stage. She leaves the tracheal tube of her current host, crawls onto the bee’s thoracic hairs, and waits for physical contact with another bee to transfer. This transfer happens readily inside a crowded hive, where bees press against one another constantly. The tarsal claws that go unused inside the trachea are essential during this exposed phase, helping the mite cling to the host’s body hairs as it moves between bees.³

Crucially, female mites show a strong preference for young bees. When given a choice between extracts from bees less than one day old and extracts from five-day-old bees, mites consistently moved toward the younger host’s chemical signature. This preference is driven by cuticular hydrocarbons, the waxy compounds on a bee’s outer surface. Young bees have a different hydrocarbon profile than older bees, and it is specifically the hydrocarbon fraction, not other chemical components, that attracts the mites.⁴ This means newly emerged bees are the most vulnerable. As a bee ages and its cuticular chemistry shifts, it becomes less attractive to dispersing mites, though it can still harbor an existing infestation acquired when it was young.

Interestingly, old-bee extract still attracted mites more than a blank control, suggesting that older bees are not repellent so much as less appealing compared to fresh hosts.⁵ In practical terms, this means a colony with a high proportion of young bees, such as one that is rapidly growing, gives female mites plenty of preferred hosts to colonize.

What Tracheal Mites Do to a Colony

A single mite inside one bee’s trachea is not a death sentence for the colony. The damage is cumulative and insidious. As mites reproduce inside the breathing tubes, they physically block airflow and weaken the tracheal walls, which darkens the normally pale, translucent tubes to a brown or black color. Piercing the tracheal lining to feed on hemolymph further compromises the bee’s respiratory efficiency and flight muscle function.

The real threat emerges in winter. Honey bee colonies survive cold months by forming a tight cluster and vibrating their flight muscles to generate heat. Modeling work has shown that a colony heavily infested with tracheal mites can lose its ability to thermoregulate during late winter and early spring. A cascade of problems unfolds: reduced brood production leads to a shrinking bee population, the winter cluster becomes loose and inefficient, and individual bees with compromised flight muscles generate less heat. The mite population, meanwhile, keeps growing. The result can be colony collapse, not from a single dramatic event but from a slow erosion of the colony’s ability to hold itself together.⁶

Colonies that die from tracheal mites tend to do so in that late-winter window, when bee numbers are at their annual low and metabolic demands for heat are high. Spring and summer infestations are less immediately threatening because the colony has a growing workforce and less need for thermoregulation, but unchecked summer mite populations set the stage for winter losses.

The Isle of Wight Connection

Tracheal mites carry a notable historical burden. They were associated with a devastating condition in honey bee colonies across Britain and Ireland in the early 1900s, a crisis that came to be called the “Isle of Wight disease.”⁷ Massive colony losses alarmed beekeepers and scientists alike, and Acarapis woodi was formally described and named as the suspected culprit.

One provocative hypothesis links the outbreak to changes in beekeeping practice. The Isle of Wight disease coincided with a period when beekeepers were increasing brood-cell size from about 5.0 mm to about 5.5 mm in width, and some researchers wondered whether this physical change in the comb could have somehow triggered or worsened the mite problem.⁸ A controlled inoculation experiment testing this idea found that brood-cell size did not influence susceptibility to tracheal mite infestation, so the timing appears to have been coincidental.

A more compelling management-related explanation involves swarming, or rather, the suppression of it. Research has shown that swarming, the natural process by which a colony divides, reduces tracheal mite density within a colony. When a swarm leaves and brood rearing is temporarily interrupted, the mite population takes a hit. Managed hives, where beekeepers actively prevent swarming to maintain honey production, do not get this natural reset. Researchers have proposed that modern hive management, by allowing bee populations to grow without the periodic disruption of swarming, inadvertently created conditions where tracheal mite populations could build to damaging levels. This may explain why the mite seemed to appear “suddenly” as a serious threat during an era of intensifying apiculture.⁹

How Beekeepers Detect Tracheal Mites

One of the frustrating things about tracheal mites is that you cannot see them without a microscope. Unlike Varroa mites, which are visible on bees and in brood cells, tracheal mites are hidden inside the body. A heavily infested colony may show signs like bees crawling on the ground unable to fly, disjointed wings held at odd angles (“K-wing”), and unexplained winter die-offs, but none of these symptoms are unique to tracheal mites.

The standard diagnostic method involves dissecting the thorax of sample bees, opening up the prothoracic tracheae, and examining them under a microscope. Healthy tracheae appear clear and pale. Infested tracheae show dark discoloration, and the mites themselves, along with eggs and other life stages, can be seen inside the tubes.¹⁰ This dissection method is reliable but labor-intensive, which means many beekeepers do not routinely test for tracheal mites. A sample of 20 to 50 bees from a colony is typically recommended for a reasonable estimate of infestation levels. Because the process requires some skill and a decent microscope, many beekeepers rely on diagnostic labs or extension services to perform the analysis.

Treatment With Menthol and Formic Acid

The two most widely used treatments are menthol and formic acid, and they work in quite different ways.

Menthol is applied as crystals, foam strips, or mixed into vegetable shortening paste and placed inside the hive. It sublimes (turns from solid to gas) at hive temperatures and the vapors penetrate the bees’ tracheal system, killing mites on contact. In a controlled trial, menthol treatments applied in various forms, including foam strips and shortening-menthol paste at doses of 30 and 60 grams, reduced the proportion of infested bees to less than one percent by late summer. Untreated control colonies, by contrast, saw infestation climb to around 25 percent over the same period.¹¹ Menthol is most effective when ambient temperatures are warm enough for good sublimation, roughly above 15°C (60°F), which limits its usefulness in cooler climates during the seasons when tracheal mite damage is worst.

Formic acid takes a different approach. It is a volatile organic acid that penetrates capped brood cells and, in principle, reaches mites inside tracheae as well. Indoor winter fumigation with formic acid has been shown to kill a high percentage of the mites themselves. However, in one study it did not significantly reduce the proportion of bees with infested tracheae over the experimental period.¹² That distinction matters: killing mites inside a bee does not instantly clear the tracheal damage already done, and if enough surviving mites remain to reinfest young bees, the colony-level prevalence may not drop as quickly as you would hope. Formic acid also requires careful handling because it can harm bees and irritate human skin and lungs at high concentrations.

Essential Oils and Other Organic Approaches

Beyond menthol, beekeepers and researchers have explored a range of plant-derived essential oils as potential treatments. Oils from thyme, clove, lemongrass, cinnamon, grapefruit, rosemary, and marigold, among others, have shown lethal effects on various bee parasites including mites. Specific compounds isolated from these plants, such as thymol, carvacrol, eugenol, and citral, are thought to be the active agents.¹³

Thymol is probably the best-known of these and is commercially available in formulations designed for hive use, primarily marketed for Varroa control but with some ancillary activity against tracheal mites. The appeal of essential oils is that they leave minimal residues in honey and wax compared to synthetic acaricides. The drawback is that efficacy against tracheal mites specifically is less well-documented than efficacy against Varroa, and dosing can be tricky since too much oil vapor can stress or kill bees.

Vegetable shortening patties, sometimes mixed with sugar or essential oils, are a longstanding folk treatment in North American beekeeping. The theory is that the grease disrupts the mite’s ability to detect young bees by altering the cuticular hydrocarbon profile on bees that contact the patty. Given what we know about how mites use cuticular hydrocarbons to find preferred hosts, this is at least plausible, though controlled studies on grease patties alone show mixed results.

Natural Resistance and Bee Genetics

Some honey bee populations show markedly higher resistance to tracheal mites than others, and this has been a significant area of breeding work. Buckfast bees, developed by Brother Adam at Buckfast Abbey in England partly in response to the Isle of Wight disease, were specifically selected for tracheal mite resistance. More recently, lines bred from feral survivor colonies in various regions have also shown lower infestation rates.

One mechanism behind resistance is autogrooming, where individual bees use their legs to brush mites off their own bodies before the mites can enter the tracheae. Grooming behavior has been documented as a defense mechanism against tracheal mites in several studies.¹⁴ Bees that groom more actively physically remove dispersing female mites during the vulnerable transfer phase, before the mites can enter a tracheal opening. This behavioral trait appears to have a genetic basis and can be selected for in breeding programs.

Natural control of tracheal mite populations also depends heavily on environmental conditions. Colonies with good foraging opportunities and limited competition from other colonies tend to keep tracheal mite levels in check.¹⁵ Well-fed colonies can replace weakened bees more quickly and maintain the population dynamics that prevent mites from gaining the upper hand. Conversely, colonies under nutritional stress, with poor forage or heavy competition, are more vulnerable to tracheal mite buildup.

Why Tracheal Mites Get Less Attention Today

If you started beekeeping in the last 15 years, you might barely have heard of tracheal mites. Varroa mites dominate the conversation about bee parasites, and for understandable reasons: Varroa is more universally damaging, more visible, and more directly linked to virus transmission. But tracheal mites have not disappeared. Surveys still detect them in apiaries worldwide. A broad screening of honey bee colonies in China, for instance, found tracheal mite infestation in at least one colony of Apis mellifera ligustica, though Varroa and another mite, Tropilaelaps mercedesae, were far more prevalent in the sampled colonies.¹⁶

Several factors likely account for the relative decline in tracheal mite problems in some regions. The spread of Varroa treatments, particularly formic acid and thymol-based products, provides some incidental control of tracheal mites. Breeding efforts have incorporated tracheal mite resistance into many commercially available queen lines. And beekeepers who lost their most susceptible colonies to tracheal mites in the 1980s and 1990s were effectively selecting for more resistant stock, even without deliberate breeding programs.

That said, tracheal mites remain a real problem in certain areas, particularly where beekeepers use untreated or under-managed colonies, in colder climates where winter stress amplifies mite damage, and in regions with limited access to resistant bee stock. The difficulty of diagnosis means that tracheal mites may be contributing to colony losses that get attributed to other causes.

The Role of Swarming and Colony Management

The relationship between management practices and tracheal mite pressure is worth understanding for practical reasons. As noted in the historical context, swarming naturally disrupts mite reproduction. When a colony swarms, the departure of a large number of adult bees (carrying their mites with them) and the temporary halt in brood rearing both reduce the mite population in the remaining colony. Experimental evidence has confirmed that mite density drops in swarm-like conditions where brood rearing is interrupted, even when overall prevalence (the percentage of bees carrying at least some mites) does not change as sharply.¹⁷

For beekeepers trying to manage tracheal mites without chemical treatment, this suggests that allowing some degree of natural colony division, or at least incorporating brood breaks through techniques like requeening, may help keep mite populations from spiraling. This is not a standalone solution, but it works with the biology rather than against it. Combined with resistant bee genetics and good nutrition from quality forage, these management-level approaches can make chemical treatments less frequently necessary.

Other Acarapis Mites on Honey Bees

Acarapis woodi is not the only member of its genus found on honey bees. Two external species, Acarapis externus and Acarapis dorsalis, live on the outer surface of bees rather than inside the tracheae. A. externus is found around the neck region and wing bases, while A. dorsalis occupies the dorsal groove of the abdomen. These external species are considered far less harmful than the tracheal mite because they do not obstruct the respiratory system. However, they look very similar to A. woodi under the microscope and can cause confusion during diagnosis. Correctly distinguishing external Acarapis from the tracheal species requires careful examination of where on the bee the mites were found and, in ambiguous cases, attention to subtle morphological differences in leg and body structure.

The existence of these external relatives also raises questions about the evolutionary path that led A. woodi to an internal tracheal lifestyle. Moving from the body surface into the breathing tubes represents a significant ecological shift. The morphological adaptations seen in A. woodi, the flexible rotating feet, the sensory setae for gauging tube diameter, the enlarged larval foot pads, all point to a parasite that has been fine-tuning its fit inside bee tracheae for a long time. Understanding how the three species diverged could eventually help researchers predict whether other external mites might evolve similar tracheal-invading strategies.