Photo by Shelley Hoover

Originally published in American Bee Journal

Though previously famous for being one of the last places in the world untouched by varroa, Australia is quickly losing hope of retaining that title. As of Nov 24, 2022, over a hundred infestations have been detected in the state of New South Wales, including one site four hundred kilometers (about 250 miles) away from the suspected port of entry.

This is devastating news for Australian beekeepers. But varroa is not the only pest invading mainland Australia. A less threatening, and perhaps more bizarre hitchhiker, the Braula fly, recently made the leap from Tasmania to continental Australia. And we know astoundingly little about its impact on honey bees.

These flies do not have wings, nor halteres, the structures behind the wings that help insects maneuver in the air. They are flies without flight, and taxonomists have argued over how it should be classified for hundreds of years, since its first acknowledgement in 1740 and formal description in 1818. Braula are small, brown, and could be easily mistaken for mites to the untrained eye.

Though widely believed to be harmless to honey bees, this point is debatable. Shelley Hoover, a research associate at the University of Lethbridge, Alberta, and former president of the Canadian Association of Professional Apiculturists, is embarking on a research project to help settle this question.

“There’s very little actual information about Braula,” Hoover says. “Right now, we’re just trying to find the best way to measure infestation levels. We tried an alcohol wash, but they have these crazy feet that are like Velcro, and they don’t come off.” Those “crazy feet” look like combs with dozens of teeth, perfectly adapted to grab on to the width of a bee’s hair.

It’s impossible to study the impact of Braula on colonies without an accurate method to detect and quantify them, so that’s the first groundwork that Hoover is laying. Sticky boards take about a week to show appreciable “fly drop,” and the accumulation of a week’s worth of hive debris makes counting Braula a challenge. Right now, the most accurate, if tedious method of quantification, Hoover says, is to inspect individual bees after being washed in ethanol.

Beyond measuring infestations, there are endless questions to investigate. “Even the basic life cycle isn’t really known,” she says, pointing to conflicting statements in the literature about something as simple as where the fly pupates. “We are currently trying to breed Braula to do more experimental work in future years.”

The assumption of harmlessness is likely to blame for the lack of research, but, as Hoover points out, not everyone shares that opinion. “The theory that Braula is not a problem is just an assumption that’s been perpetuated,” she says. “In some places, beekeepers do think it’s a problem. I’ve talked to beekeepers in Alberta who say that their yards with persistent infestations produce less honey.”

This bee is attempting to groom off a Braula fly with her midleg. Photo: Shelley Hoover.

Braula” actually refers to five different species of wingless flies, B. coeca, B. kohli, B. orientalis, B. pretoriensis, and B. schmitzi. The most commonly discussed species is B. coeca, though it is not clear if this is correct, or a case of species misidentification. After all, Varroa destructor was misidentified as V. jacobsoni for decades, and here we have a similar case of several closely related species inhabiting hives of different types of bees.

To the best of our knowledge, B. coeca is found on all honey bee subspecies, whereas the other species are somewhat restricted, with B. schmitzi living with Italian bees and B. kohli and B. pretoriensis preferring Carniolan bees and subspecies in the Middle East. Another closely related genus, Megabraula (which, as you might expect, is larger than Braula), has two species, both of which live in association with the giant Himalayan honey bee, and are even more mysterious.

Despite being known to science for over two hundred years, there is a paucity of data on the impact of Braula on honey bee health or colony outcomes, and existing information is almost entirely anecdotal. This is despite the fly now being found nearly everywhere in the world, except for New Zealand and probably some other island nations.

What we do know is that, like bandits, Braula steal food from bee larvae’s gelatinous swimming pools as well as straight out of the mouths of adult bees. This may be a nuisance for the bees, but some beekeepers consider Braula to be a pest because its larvae tunnel through the wax cappings on honeycomb, reducing the value of cut comb. However, the peculiar behavior of Braula toward queens is probably where the bigger threat to colony health lies.

Braula have opposite hitchhiking preferences to varroa: They prefer queens the most. And adult Braula can dogpile on queens, sometimes forming immense congregations. One report from Germany in 1858 details a beekeeper plucking an astonishing 187 Braula from a single queen. I have personally found thirteen Braula on one of my Tasmanian queens, and it’s hard to imagine that having this many piggy-backers would not have a serious impact on the queen’s productivity, let alone 187.

This is a topic largely ignored by scientists, but Hoover thinks it’s important to address. “If you see twenty Braula on a queen, presumably she is getting less food. I don’t know why we would assume there is no impact, at least on the queen.”

Furthermore, Braula could be acting as a vector for pathogens. A recent paper by researchers in Ecuador and Argentina suggest that the fly could indeed be transmitting acute bee paralysis virus. If true, this may be one more way that queens might become infected, despite being largely spared from varroa.

In addition to adding more useless weight for the queen to carry around, Braula have an uncanny ability to sense when a bee’s jaw muscles engage as it accepts food during trophallaxis, at which time the fly scurries to the bee’s mouth and laps up some of the food for itself. In an almost comical description by Jamie Ellis and colleagues, Braula are also said to stimulate bees to regurgitate food by tickling their mouth just right, “stroking the upper edge of a bee’s labrum until the bee extends its tongue.”

All those extra mouths to feed mean less nutrition for the queen, and likely also fewer eggs laid, although this has not been formally investigated. To make matters worse, this attraction to the queen seems to intensify in the fall, possibly because brood rearing dwindles and there’s less food to steal from larvae. And if the queen becomes compromised in the fall or winter, the colony will certainly die.

According to Jósef Lósy, a Hungarian researcher who studied Braula anatomy and behavior in the early 1900s, “Their number by the end of November becomes so great that the queen is in danger and she becomes weakened … in late fall she perishes.” In an almost Shakespearean tone, Lósy goes so far as to state that Braula is “a burden and torture of the first order” for queens.

So why, then, is Braula considered by some researchers to be a commensal species (one that neither benefits nor harms the host), and not a parasite? There is simply not yet enough information to tell how the nature of its relationship to honey bees should be categorized. Living on the bee and stealing its food seems no less parasitic than worms living in a mammal’s intestine. Recently, and perhaps most correctly, some researchers have referred to Braula as a “kleptoparasite.”

Why potential negative consequences of Braula infestations have not been formally investigated is unclear. While Lósy’s take might be embellishment, sources since then have also expressed concern, or at least uncertainty, over whether Braula infestations are truly benign, contrary to the somewhat relaxed attitude under which it entered the United States.

It is difficult to say for sure, but Braula likely entered the U.S. in the early 1920s through importation of queens from Italy to Maryland, although it is possible that the fly arrived even earlier. Beekeepers did not seem to pay much attention to it because at the time, there was an incorrect belief that Braula could not persist through the winter. We now know that’s false: Braula can overwinter on adult bees, even in Alberta, where winters are exceptionally harsh.

According to a 1925 report from the USDA, commercial beekeepers with Braula infestations noted that the fly did not seem to damage the condition of strong, healthy colonies, which probably added to the nonchalant attitude. However, in the same report, the authors note that the presence of Braula might cause a colony to go queenless, and new queens are subsequently harassed.

“The insects collect in considerable numbers on the young queen and within a few months, she may have the appearance and behavior of an old queen,” the authors write. They suggest that while it may be true that Braula do not considerably harm the productivity of a strong colony, colonies that are already weak may be more vulnerable. “The question arises, therefore, whether the weakness of colonies containing Braula is the cause or result of the infestation.”

The arrival of varroa to the U.S. several decades later may have reinforced the lack of concern over Braula, first because varroa is a much bigger threat, and second because some of the early synthetic miticides also kill Braula. As beekeepers pulled out the coumaphos and fluvalinate against varroa, Braula was caught in the crossfire.

However, today, these miticides have fallen out of favor for amitraz, which is not effective against Braula. Not all miticides have been tested against the fly, but according to Hoover, beekeepers in Alberta are now seeing it persist in yards where they are actively managing for varroa, and on bees that were not imported from Tasmania.

While Tasmania might still be enjoying freedom from varroa, Braula infestations can be exceptionally high, with sometimes alarming numbers showing up in package bee imports to Canada. This might be in part due to a lack of historical mite treatments that could have knocked back the population, and perhaps more likely, the mild winters in Tasmania which could enable not only adult survival, but continued reproduction.

“I think the number one thing we need to find out is if Braula is impacting the health or productivity of colonies and queens,” Hoover says. “Maybe it is a problem and maybe it isn’t, but we need actual data to make a decision.”