New research is settling the debate over whether drone congregation areas really exist
A common honey bee party fact is that drones fly to congregation areas to find a queen. But this nugget of knowledge is debated among scientists, since most of the data that led researchers to this conclusion used lures to attract the drones. Drones, therefore, may have ostensibly formed congregation areas even in the absence of an existing gathering, and for some scientists, the very existence of drone congregation areas is still an unsettled question.
But new research from Queen Mary University of London, England, is providing some answers. Dr. Joe Woodgate and his colleagues used a technique called harmonic radar tracking to analyze the flight patterns of individual drones during mating flights. This let them watch where the drones ended up, without using a lure to bias the bees’ natural behavior.1
“Many beekeepers and scientists have long suspected that drones gather in large numbers in places that remain stable year after year, but it’s incredibly difficult to study such behavior when the drones are so small and fly so high above us,” says Woodgate. “The vast majority of evidence for congregation areas comes from people using caged queens or queen pheromone lures, which they raise into the air on long poles or dangle from balloons.”
Woodgate explains that since drones will return to a location at which they have smelled a queen, scientists might even have unwittingly created the congregation areas they were studying. To avoid this problem, he and his colleagues glued small radar transponders, which resemble sewing pins, to the drones’ backs, enabling the researchers to track the drones’ flights and see if congregation areas really exist.
How to track a bee
Regular radar technology works a lot like sonar, but uses beams of radio waves instead of sound. However, honey bees are too small, and their environment too complex, for this technique to enable tracking of individual bees. With harmonic radar, though, the radio wave doesn’t exactly bounce: The special transponder attached to the bee absorbs the signal, then emits another one at a higher frequency (a harmonic signal), making it easier to detect in an otherwise noisy landscape.
When a drone equipped with a transponder exits its hive, the researchers can detect the harmonic signal using a large radar receiver parked out in the field. The receiver station has what look like large satellite dishes attached to a rotating turret, which pick up the drone’s signal as it flies around, exploring the outside world. Dr. James Makinson, one of the study’s coauthors, tweets that he “used to live in the back of a radar van,” to help make sure they didn’t lose the drone’s signal.
The researchers’ diligence paid off, and they made some surprising discoveries. “The first thing we noticed was that drones seemed to switch between two distinctive modes of flight,” says Woodgate. “They get around from place to place with fast, efficient, straight line flights, but occasionally make an abrupt switch to a very different behavior in which they make tight loops, staying within a small area of space and with a lot of speeding up and slowing down.”
Even drones from different hives make these small, loopy flight patterns at the same places, which appear to be the ever-elusive congregation areas. Whether these are actually mating hot-spots is yet to be proven, but they have similar properties to other species’ mating swarms.
As if tethered by elastic bands, drones venturing farther from the core of these tight congregations would usually reverse course and accelerate back toward the center. “This creates the apparent effect of a physical force, binding the drones to the congregation and allowing a swarm of bees to remain stable over long periods, even though each individual drone only stayed for a few minutes,” Woodgate explains. “This pattern is shared by mating swarms of male midges and other insects.”
However, approximately one in five drones will change its flight path completely from this boomerang pattern, and instead make a beeline to a neighboring congregation. A single drone appears to shop around for virgin queens at different congregation areas, should his initial gallivants be unsuccessful.
My way is the flyway
Despite being interested in the topic for decades, scientists have not yet been able to define consistent attributes that make a good congregation area. As Drs. Cyprian Zmarlicki and Roger Morse describe in 1963,2 with what sounds like a hint of frustration, “It has so far been almost impossible to define a drone congregation area physically.”
But Woodgate and his team found that drones from different hives appear to navigate the landscape, including to and from congregation areas, using aerial highways, or “flyways,” which are defined by the terrain — a finding which agrees with previous research.3 A location’s potential as a congregation area, therefore, probably depends as much on its specific landscape as its convenience, like whether it’s on a flyway.
The researchers infer that flyways help define congregations because drones were able to efficiently find congregation areas despite being apparently naïve to their location. Radar tracking showed that the drones took short orientation flights, averaging at about 13 minutes and flying around 100 m from their hive — just long enough to recognize their home, but not broad enough to discover new landscape features or congregation areas.
“On subsequent flights, drones were able to go straight to a congregation area without showing any evidence of extensive searching,” Woodgate says, “so whatever they use to guide them must be something they can observe from pretty much anywhere.” Moreover, drones congregated in the same location year after year, so the congregations weren’t discovered by following or learning from their brothers. The drones probably find the party, no invitation needed, by following the lay of the land.
These data explain why, as Drs. Hans and Friedrich Ruttner describe in the 1970s, drones from colonies newly relocated to unfamiliar terrain quickly find and participate in the established congregation areas.4 At the time, the authors postulated that the drones likely found the congregations using optic clues, and now the data more firmly back that claim up. Pheromonal cues from other drones may also play a role, but this is yet to be investigated in detail.
Woodgate and his colleagues say that visual reconstruction of the drone’s perspective, using path, trajectory, and altitude data from the radar tracks, will further clarify the visual cues that drones might use to find the congregation. “We are using virtual reality to recreate what a drone sees as it approaches a congregation area with the aim of discovering exactly what they are using to guide them,” he says.
Dr. Keith Delaplane, professor and director of the University of Georgia honey bee program, and who was not involved in the research, says that Woodgate and his colleagues skillfully use technology to resolve behaviors down to the individual drone. “Although this study informs emergent processes that sustain the drone congregations, we still don’t know how they achieve permanence year after year,” he adds. “I am confident that this too will be shown to be outcomes of landscape features and emergent properties of the drones themselves.”
How big are congregations, really?
These findings are so fundamental to honey bee reproduction, it is hard to believe the data hadn’t been gathered already. It certainly wasn’t for lack of trying: Scientists have been interested in honey bee mating flights since at least the 1960s. Early investigations into congregation areas involved tying helium balloons to queens, floating them in the air, and recording which locations attracted drones and which didn’t.2
From these early studies, the data suggested that congregations were huge — up to ….