These clingy flies might not be as harmless as they seem
As varroa pops up in ever more Australian apiaries, it is becoming increasingly likely that this time, the parasite will not be contained. 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 July 6, 2022, the mite has been detected at 24 sites 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 from which mainland Australia has enjoyed some freedom. A less threatening, and perhaps more bizarre hitchhiker, the Braula fly, has not yet made the leap to continental Australia, despite being abundant in Tasmania and many other parts of the world. 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 this insect should be classified for hundreds of years, since its first acknowledgement in 1740 and formal description in 1818.1 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 very much 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.2
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 a challenge. Right now, the most accurate, if tedious method of quantification, Hoover says, is to inspect individual bees after washing them 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.”
So, what do we know?
“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 (limited) 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.3
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 being found nearly everywhere in the world, except for mainland Australia and New Zealand, where it is a reportable pest.
What we do know is that, like little 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.4 I have personally found 13 Braula on one of my Tasmanian queens, and it’s hard to imagine that having this many piggybackers 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 suggests that the fly could indeed be transmitting acute bee paralysis virus.5 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.3 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.”6
All those extra mouths to feed mean less nutrition for the queen, and likely also fewer eggs laid, although this has not been ….