Radar Love

Queen bees and drones exclusively mate in “drone congregation areas”, right? Although crucial to the survival of our livestock, as beekeepers we actually know very little about the mating locations of honey bees and, believe it or not, what we do know has been aided by research using radar technology originally designed to prevent car crashes…

By Dave Black

Science, especially ‘bee’ science, often has a fortuitous and many-fathered ancestry. In the summer months of 1974 an article, published and discussed in the British magazine ‘Wireless World’, inspired an idea that would begin to fill in some of the gaps in our knowledge about honey bees, but was initially proposed as a life-saving safety measure.[i] It described technology developed by a group of Radio Corporation of America (RCA) Lab engineers from Princeton that was designed to prevent rear-end collisions on highways, at the time a third of all vehicle accidents[ii]. It was a radar technology that may have its descendants in your car today.

A honeybee with a harmonic radar transponder fitted to track flight activity. The transponders weigh less than the bee’s usual pollen or nectar load.  Photo: S Wolf.

The problem with using a reflected signal from radar is that you need a clear space to operate in; easy if you are a ship or an aeroplane, not so easy if you are among a ‘clutter’ of reflecting objects on the ground. The RCA team’s solution was what we now call ‘harmonic radar’, which detected the presence of a returned harmonic of the broadcast signal rather than the reflected signal itself. By fitting the object you are interested in with a device that only had to radiate a harmonic of the signal it received, in coming years tiny, unpowered, harmonic radar devices would be small and portable enough to be used to detect avalanche victims, hornets[iii], and even track beetles.[iv] You just had to be able to ‘hit’ them with the beam from a radar transmitter.

Prevailing Wisdom

One of the things about bees that we’ve never properly resolved, despite the assumptions we make, is where queens mate. The wisdom that prevails is that mating only occurs in a persistent, defined space we call a Drone Congregation Area (DCA) and perhaps it does, but a recent paper in Apidologie that reviewed over 200 science papers on the topic reminds us that, actually, this isn’t something we know, it’s something we believe.[v] Studies using harmonic radar to follow drones and queens seemed to be the best response to answering many of the open questions about this idea.

The best ‘evidence’ we have for queen mating in DCAs is the argument that it ‘ought’ to be true; it just makes sense. We hear drones that aggregate, or congregate’ in certain areas, and some queens have been seen to mate there, although not always.[vi] By congregating, drones make themselves easy for a queen to find and, like schooling fish, reduce their individual exposure to predation. By making finding an appropriate mate easy the behaviour limits the risks inherent in a queen’s solitary mating flight.

Open Questions

The discovery of the queen pheromone (9-ODA[vii]) that would attract drones in flight was used to survey landscapes for DCAs, and nearly all the research showing the existence of DCAs and queen mating relied on lofting 9-ODA lures or queens using poles, kites, or balloons to attract drones. The fact that the ‘bait’ itself might also stimulate the formation of a DCA called for a method that could independently study the formation of these areas. The early studies used conventional radar, but had to operate in relatively featureless landscapes where DCAs do not readily form. There has been evidence of bees happily occupying spaces like semi-desert or marshland where no DCA has ever been found, which casts doubt on the ‘essential’ nature of DCAs.[viii]

‘Heat maps’, a graphical display of drone density in flyways and DCAs using radar data. The left column is tracking of more direct flights, the right column maps when flights become “convoluted” and thus might indicate what are deemed “drone congregation areas”. Reference: Woodgate /et al/, 2021, iScience 24, 102499. https://doi.org/10.1016/j.isci.2021.102499

There are also explanations required about how drones (and queens) find a DCA. There can be no permanent intergenerational ‘knowledge’ of these spaces passed on, they must each be discovered by every individual drone rather than being learned from each other. Their orientation flights typically do not take them far enough from their hive to discover congregations, yet after a few local orientation flights fixing their home bearing they magically fly off to one of several DCAs.

It’s difficult to see how drones might recognise a DCA by its common features, not just because we haven’t been able to discern what these are, but also because each shared location would look/feel/smell different for the many thousands of drones that must visit. For example, a particular horizon or feature location will appear differently for drones that approach from the east, compared to the ones originating from a hive in the west.

A Little Knowledge...

There have been very few radar-traced honey bee studies,[ix] [x] perhaps we are too obsessed with parasites and disease, but it looks like we should be thinking of drone congregations as an emergent feature of behaviour, rather than some physical phenomenon. Joseph Woodgate from Queen Mary College in London has been tracking the routes of bumble bees,[xi] [xii] but in 2021 applied the experience to drones, and unsuccessfully, to queens.[xiii]

The radars have documented drones moving through the landscape along ‘flyways’; corridors constrained by the physical geography of the site that funnel all the drones from the different local hives onto the same path. These are more or less fixed, year on year, but may shift, perhaps because of unusual wind conditions for example, and they may be the result of all drones using the same set of navigational strategies rather than any ‘knowledge’.

Earlier studies could only ‘see’ clouds of drones. Harmonic radar revealed the track of individual drones on sections of straight flight, each interrupted by sudden episodes of much more ‘convoluted’ flight. The later behaviour was described in terms of a ‘swarming’ flight, where bees move away from a point, then accelerated back to that point with a speed that was proportional to the distance they had travelled. The mating swarms of midges and mosquitoes are another example where this is seen. This is typical of lots of ‘swarming’ animals, the sudden bouts of acceleration are part of what keeps a swarm together.  The drones would circulate through several DCAs in a single flight using the flyways, stopping for a minute or two to perform these convoluted flights and then move on. The places where the convoluted flights of hundreds of drones happen to coincide are an artefact emerging from the drone flyways, and these are what we are calling DCAs.

The Woodgate team’s effort to track virgin queens was unfortunately not very successful – even harmonic radar has limitations (and is also why, incidentally, it wasn’t very good at tracking hornets). Over three years, out of 94 tagged, there were 26 traces of queens flying, all but three too short for mating flights. Only two returned with mating signs but the longer flights could not be completely tracked. One was not likely to have been close to a (known) DCA and was thought to have mated near a hive; the other flew off towards a DCA but dislodged its tag. It wasn’t clear whether queens also use the same flyways, but it seems a stretch to claim queens only mate in DCAs.

Where queens actually mate is something we could do with knowing more about, partly because we could improve the commercial utility of our queens, but it also has important implications for conservation, where the reproductive isolation of bee strains matters. Microelectronics and battery technologies are now sufficiently developed to enable tiny radio transmitters, small enough to be carried by an insect the size of a bee, and producing a signal powerful enough to be detected kilometres away. These seem to be the next logical step. Beekeepers may not be reading Wireless World any more, but maybe there will be some redundant radio tracking gear available from Auckland soon, and we can find out more...

Dave Black is a commercial-beekeeper-turned-hobbyist, now retired. He is a regular science writer providing commentary on “what the books don't tell you”, via his Substack Beyond Bee Books, to which you can subscribe here.

References

[i]Radar for cars, clutter-free, J. Shefer, R. J. Klensch, G.Kaplan & H. C. Johnson, Wireless World,  p117 May, p199 June (1974)

[ii]J. Shefer;R. J. Klensch, Harmonic radar helps autos avoid collisions, IEEE Spectrum (1973) Vol: 10, Iss: 5

[iii]Lioy, S., Laurino, D., Maggiora, R., Milanesio, D., Saccani, M., Mazzoglio, P.J., Manino, A., Porporato, M., 2021. Tracking the invasive hornet Vespa velutina in complex environments by means of a harmonic radar. Sci Rep 11, 12143. https://doi.org/10.1038/s41598-021-91541-4

[iv]Mascanzoni, D., Wallin, H., 1986. The harmonic radar: a new method of tracing insects in the field. Ecological Entomology 11, 387–390. https://doi.org/10.1111/j.1365-2311.1986.tb00317.x

[v]Otis, G.W., 2026. Where do honey bees (Apis mellifera) mate? Apidologie 57, 3. https://doi.org/10.1007/s13592-025-01237-1

[vi]Doolittle, G.M., 1889. Scientific queen-rearing as practically applied. Chicago. https://doi.org/10.5962/t.173039

[vii]9ODA - 9-oxodecenoic acid

[viii]Butler, C.G., Fairey, E.M., 1964. Pheromones of the Honeybee: Biological Studies of the Mandibular Gland Secretion of the Queen. Journal of Apicultural Research 3, 65–76. https://doi.org/10.1080/00218839.1964.11100085

[ix]Loper, G.M., Wolf, W.W., Taylor Jr., O.R., 1987. Detection and monitoring of honeybee drone congrgation areas by radar. Apidologie 18, 163–172. https://doi.org/10.1051/apido:19870206

[x]Loper, G.M., Wolf, W.W., Taylor, O.R., 1992. Honey Bee Drone Flyways and Congregation Areas: Radar Observations. Journal of the Kansas Entomological Society 65, 223–230.

[xi]Woodgate, J.L., Makinson, J.C., Lim, K.S., Reynolds, A.M., Chittka, L., 2016. Life-Long Radar Tracking of Bumblebees. PLoS ONE 11, e0160333. https://doi.org/10.1371/journal.pone.0160333

[xii]Woodgate, J.L., Makinson, J.C., Lim, K.S., Reynolds, A.M., Chittka, L., 2017. Continuous Radar Tracking Illustrates the Development of Multi-destination Routes of Bumblebees. Sci Rep 7, 17323. https://doi.org/10.1038/s41598-017-17553-1

[xiii]Woodgate, J.L., Makinson, J.C., Rossi, N., Lim, K.S., Reynolds, A.M., Rawlings, C.J., Chittka, L., 2021. Harmonic radar tracking reveals that honeybee drones navigate between multiple aerial leks. iScience 24, 102499. https://doi.org/10.1016/j.isci.2021.102499