Tracheal mites may no longer be a serious concern for North American beekeepers, but they are causing havoc elsewhere.
It isn’t often that problems go away in beekeeping. More typically, new challenges pile on top of the old in a stack that might make someone contemplate leaving the business. Tracheal mites (Acarapis woodi) are a rare exception, at least for the western honey bee, but not all bees are so lucky.
The prevalence of tracheal mites has dropped so precipitously in western honey bees that many new beekeepers don’t know what they are, nor the symptoms of infestation. These mites are microscopic nemeses — colonizing, scarring, and copulating in the bee’s trachea, or internal breathing tubes. In some ways, they are similar to varroa, but instead of hiding away inside brood cells to complete their life cycle, they spend almost all of their time living inside the bees themselves.
These miniature mites arrived in the U.S. in 1984, with the first detection reportedly in Weslaco, Texas.1 Shortly after, they were found in Canada. In a pattern that sounds all too familiar, the mites are now found on all continents except Australia, but unlike varroa, which arrived a few years later, they have not become the bane of beekeeping.
Why not? In a cross-Canada survey conducted in 2016, tracheal mites were identified in only 3.8% of apiaries — far less than varroa, which occurred in 84% of apiaries.2 Researchers at the USDA honey bee lab in Beltsville, Maryland, report a steep decline of tracheal mite detections, from around 40% of submitted samples in 1986 to about 2% in 2013.1 The widely accepted rationales for this drop are that the bees have developed innate resistance, and that since treatments for varroa also kill tracheal mites, the infestations of both parasites have been concurrently controlled.
Peril in Japan
Not every beekeeper has the luxury of writing off tracheal mites as a non-issue, though. In 2010, tracheal mites jumped hosts, but not in the direction you might think. Native Japanese honey bees, Apis cerana japonica (a subspecies of the eastern honey bee) acquired tracheal mites from A. mellifera, and they have been wreaking havoc ever since.
“This mite parasite is a major cause of winter bee mortality and it is the beekeeper’s greatest fear,” says Taro Maeda, a researcher at Japan’s Institute for Agrobiological Sciences. “Many A. mellifera are thought to have acquired resistance to this mite. A. cerana, on the other hand, was susceptible.” Maeda explains that in Japan, where both species are kept in close proximity, there are ample opportunities for contact where the mite can jump from one species to the other.
Maeda and Yoshiko Sakamoto, a researcher at Japan’s National Institute for Environmental Studies, reported in 2020 that 40% of A. cerana japonica colonies on Japan’s main islands are infested with tracheal mites, and within those colonies, 50% of bees were parasitized.3 This is well above the treatment threshold defined by North American researchers as 10% infestation. In an earlier paper, the same researchers demonstrated that tracheal mite infestation “drastically increased” overwintering mortality
of Japanese honey bees, and that about 20 mites were living in each bee’s trachea.4
Why would tracheal mites be more damaging to A. cerana japonica than A. mellifera? The difference appears to be due to variation in the bees’ grooming abilities. Whereas A. cerana is generally more effective than A. mellifera at grooming off varroa, surprisingly, this skill appears not to translate to the tiny tracheal mite. And this is a problem because, as Maeda points out, currently “no treatment is registered for tracheal mites in Japan” for either bee species.
Off with their legs!
The idea that grooming is important for resistance began with a rather morbid experiment conducted in the 1990s.5 When researchers amputated the legs of young, mite-free bees, the bees were more likely to become infested. Furthermore, if a leg was amputated from only one side, trachea on the opposite side of the bee still became occupied. The same effect was observed when the researchers amputated just a distal section of their legs, as well as when legs were not amputated but restrained, leading to the conclusion that self-grooming is an important resistance trait.
When Maeda, Sakamoto, and colleagues compared grooming abilities of A. cerana japonica to A. mellifera, they found that the Japanese bee was only half as likely to initiate grooming in response to a tracheal mite.6 Yet, other research shows that A. cerana is superior at grooming and damaging varroa.7
“The grooming behavior of A. cerana is efficient for varroa but it is not suitable for tracheal mites,” Maeda says, but acknowledges that it isn’t clear if other behaviors might be involved. Right now, it seems as though A. mellifera may simply be better at detecting the particular tickle of the tracheal mite, whereas A. cerana may be better at removing varroa because the large mite has no blind-spot in which to hide on the smaller bee’s body.
The propensity to initiate grooming is especially important for tracheal mites because the bees have only a short opportunity to scrape the mites off their bodies. Two to three weeks after infesting a bee, new female mites will emerge from the trachea to find a new host bee to invade. Like pole-vaulting burglars, the mites tend to emerge and reinfest at night, when bees are in closer proximity to one another. The mites must move quickly — after just a few hours outside of their host, they will die — and so the bee must move quickly too.
But, even though A. mellifera is quick to scratch an itch, we shouldn’t forget about these little mites entirely. First, not all varroacides kill tracheal mites, and we should consider this when designing our treatment plans. Apivar, the preferred synthetic miticide for many beekeepers, does not kill them.8 And efficacy of Apiguard, a thymol-based product, against tracheal mites is questionable — although some vendors claim that it works, in 1999, researchers demonstrated that Apiguard is not effective.9 There is anecdotal evidence that oxalic acid vapor kills tracheal mites, but this has not been empirically demonstrated, and a drizzle application is unlikely to reach mites inside the trachea.
The only registered treatments against tracheal mites in North America are formic acid, which tends to be used early in the season, and menthol, which tends not to be used at all. I, for one, have not even considered tracheal mite control when I am deciding on my miticide rotation schedule, and I am uncomfortable with the idea of succeeding without intention.
Moreover, saying that I have been “succeeding” at controlling tracheal mites is far too generous. After all, I have never checked. Thinking back, I have observed symptoms of what I assumed was some kind of pesticide toxicity or virus in the past, with hundreds of lethargic bees crawling on the ground outside the hive’s entrance. Other times, I have seen poopy top entrances, but checked for nosema and found nothing of concern.
Incidence of tracheal mites may be low, but it is not zero. Because I did not know the symptoms of tracheal mites at the time, I never thought to ….
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