Swarm Movement: A Mystery Explained

[1992 Wenner, A.M. Swarm movement: A mystery explained. Am. Bee J. 132 (1):27-31.]

Department of Biological Sciences,
University of California, Santa Barbara,
Santa Barbara, California 93106,
(805) 893-2838

People have been fascinated by honey bees for thousands of years. What few realize today is that early accounts of bee biology were often written by very perceptive people. By gleaning information from nearly forgotten books and essays and blending that information into current thought, one can fit pieces of a puzzle together and form a coherent whole. So is it with the mystery of how a swarm manages to keep together as it travels in a straight line through the air on its way to a new home.

Surely the movement of a bee swarm through the air ranks as one of the most astonishing events one can witness; honey bees by the thousands circle about quite harmlessly. One can walk through the swirling mass without fear of being attacked, even though the event is as awesome as Maeterlinck described in his classic 1901 book, The Life of the Bee [1920:73]:

“The man who never before has beheld the swarm of a populous hive must regard this riotous, bewildering spectacle with some apprehension and diffidence. He will be almost afraid to draw near; he will wonder, can these be the earnest, the peace-loving, hard-working bees whose movements he has hitherto followed?”

Specialized books and articles on beekeeping contain much information on swarming, including events leading to swarming, symptoms, causes and prevention of swarming, afterswarms, the swarming season, and hiving the swarm. Surely every beekeeper has seen queen cells on the brood combs, and most can distinguish clearly between supersedure and swarm cells.

On the other hand, perhaps few beekeepers know much about swarm movements through the air or recognize the telltale and distinctive sounds that are produced during the last few days before the swarm issues from its parent colony. Virgil gave us the first mention of that sound, a phenomenon described more fully by Columella about 50 A.D [1954:461]:

“[A beekeeper] will be able to find out beforehand their decision to escape by putting his ear to each of the hives in the evening; for about three days before they intend to break out, an uproar and buzzing arises like that of an army setting out on the march. From this, as Virgil very truly say’s,

You can foreknow the purpose of the herd: The martial roar of loiterers, and a voice is heard whose notes The broken sound of trumpets imitates.”

A description matching Virgil’s and Columella’s has persisted these past two thousand years. Charles Butler’s 1609 account, in fact, is remarkably similar to that of Columella. Wildman (1768:66) added a new twist: “For three or four nights before a swarm sallies forth, there is in the hive a peculiar humming noise, of which authors give very different descriptions, probably owing to the strength of imagination in each.”

The influence of “imagination” is now being reduced somewhat by the use of electronic analysis and experimentation. Spangler, et al. (1990) characterized a “buzz-run” produced by workers while clustered, a sound that may even trigger swarm departure (Spangler, 1991).

Much more striking than the subtle sounds emanating from the colony is that sound known as queen piping, a sound likely referred to in the last line of the above verse by Virgil (“The broken sound of trumpets imitates”). By the time of Butler, queen piping was known to occur most often in advance of the departure of after-swarms, as follows (1609: C.5, 23):

“…the next [queen], when she perceives a competent number to be fledged and ready, begins the music in a begging tune, as if she did pray her queen-mother to let them go…then [you should] look for a swarm…”

Butler even provided a musical score for queen piping, that of both the laying queen’s “tooting” and the virgin’s queen’s “quacking” (Wenner, 1962, 1964). Wildman (1768:67) quoted a musical description similar to that of Butler, as written by Worlidge (Mystery of Husbandry, Ch. 9, #3):

“The signs of after-swarms are more certain; when the prime swarm is gone, about the eighth or tenth evening after, when another brood is ready and again has over-filled the hive, the next queen begins to tune in her treble voice, a mournful and begging note; then in a day or two shall you hear the old queen in her base note reply, and as it were consent. In the morning before they swarm, they come down near the [hive base], and there they call somewhat louder. At the very time of swarming they descent to the [base], where answering one another in more earnest manner, with thicker and shriller notes, the multitude come forth in great haste…”

Apparently, it is mostly the young bees that leave with the old queen in the swarm (e.g., Ribbands, 1953:263), but a few days prior to departure older foragers have not been idle. During those last days, as some have contended, scouts have been “sent out” from the hive to traverse the countryside in search of suitable quarters; Buzzard stated that hypothesis succinctly in 1946 (p. 78):

“Perhaps one of the most fascinating facts connected with. . . swarming is the dispatch of several hundred, perhaps thousands, of scouts in many directions to reconnoitre sites with a view to selecting a new home. It is, I believe, generally agreed among experts that these scouts are usually dispatched from the parent hive before the issue of the swarm itself.”

A quite different possibility is that “scout bees” are merely regular foragers who may already know much of the suitable territory and switch from foraging to scouting just prior to swarming. Lindauer concluded just that when he wrote (1961:57):

“Some. . .of the old, marked bees came back occasionally to [my] feeding table, but no longer as forager bees; they sipped only briefly at the sugar water, but they did not fly back immediately to the hive. Rather they began working in the neighborhood in a strange way: they sought nearby for dark holes and cracks, crawled into mouse holes in the ground and into deep cracks in the bark of trees, and finally inspected [my] two empty nesting boxes. There was no doubt about it: these former forager bees had become house-hunting bees.” (emphasis Lindauer’s)

The outpouring from the hive at the time of swarm emission catches one by surprise, except for attentive beekeepers who may have listened for the distinctive sounds that precede swarming. The emerging bees seem to flow out like water, but up as well as down, as they emerge from the entrance. The air is soon filled with the sound of those who have taken flight; the pitch is easily recognized as higher than that of a similar number of bees in normal flight.

Nearly always the swarm soon settles nearby on a suitable bush, tree, or other object, as described by Buzzard (1946:78):

“As a rule a swarm after issuing will settle within twenty yards of the parent hive or nest.” Before advent of the moveable frame hive, beekeepers could divide colonies only with difficulty; instead, they increased the number of their colonies by gathering these newly emerged swarms.

Any time a beekeeper gets a request to remove a newly settled swarm and goes to that site, inspection of the nearby area should reveal the location of the parent colony if such is not known already. A property owner might appreciate knowing the whereabouts of that colony and want the source of the swarm removed as well; likely a new one will emit from the parent colony a year later and pose the same problem.

Lindauer (e.g., 1951, 1953, 1955, 1957, 1961) studied the behavior of settled swarms more intensively than anyone had done earlier, with a major focus on waggle dances performed on the surface of swarm clusters. His ability to “read” the information in waggle dances and determine where swarms would move led him to hypothesize that scouts were furnishing direction and distance information to others in the swarm cluster. That conclusion, of course, complimented von Frisch’s earlier dance language hypothesis.

Despite Lindauer’s conclusion that dances on the surface of a swarm cluster “inform” bees in that cluster about an eventual destination, several facts do not fit his interpretation. That is, there remains some mystery as to how the swarm actually orients and keeps together as it moves through the air on its way to a new destination found by scout bees. Seeley (1958:145) labelled the mechanism of African bee swarm movement “an almost total mystery.” Gould and Gould (1988:67) expressed that uncertainty another way:

“Much remains to be discovered about which bees elect themselves as scouts, how the movement of the swarm is triggered after a consensus is achieved, and how the swarm, composed for the most part of bees that have not attended a cavity dance, is herded to the goal, which may be more than 2 kilometers away.”

As an aside, a most curious and persistent notion is that one can bring a swarm down from the air by beating on pans and throwing dust into the swarm, as described early on by Columella and later expanded upon by Wildman (1768:69):

“Whenever the bees of a swarm fly too high, they are made to descend lower, and disposed to settle, by throwing among them handfuls of sand or dust; probably the bees mistake this for rain. It is usual at the same time to beat on a kettle or frying-pan; perhaps from its being observed that the noise of thunder prompts such bees as are in the fields to return home.”

Later experts have considered such measures ineffective, as expressed in Root’s ABC and XYZ of Bee Culture (Root, et al., 1947:608):

“When a swarm issues it is not necessary to ring bells or beat tin pans as was formerly done in order to induce the bees to settle. So far as can be determined, such a procedure has no effect whatever upon the swarming bees.”

However, one should not dismiss too readily any claim that has been with us for centuries, as this one has. A “ringing of bells and beating upon pans” can produce quite different sounds, depending on the size and composition of those bells and pans. In the past a proper selection of a pan or a bell (brass in those days, perhaps) may have led to success repeatedly and demonstrably, thus perpetuating that curious notion.

A lesser known fact is that swarms in motion may become disoriented after having travelled only part of the way to their ultimate destination. Lindauer, in his studies of waggle dances on swarm clusters, soon encountered one of these incidences as he followed a moving swarm after it had left its original cluster site. He labelled the temporary layover “a fresh surprise” and described the interruption of flight as follows (1951: in translation):

“The dancers in this swarm had certainly indicated the direction of Schillerstrasse, but had indicated the distance to be not 500 meters, but 800 meters. The swarm therefore, clearly made a temporary halt. . .the previously marked bees began once again to dance, indicating the original direction, but no longer as being 800 meters, but 350 meters away.”

Any sudden change in wind direction can interfere with the scouts’ ability to keep the swarm under their direction (e.g., Lindauer, 1953:385); the odor trail produced by the scouts then becomes disrupted (see below).

We encountered that same phenomenon last May on Santa Cruz Island while hunting “wild” (feral) bee colonies (Wenner, 1989a). As we drove down a steep grade at 10 in the morning, we found a swarm clustered on a very small bush alongside the road. Dancers could be seen on its surface, with their direction orientation pointing away from the site of one of our already located feral colonies. When we came back up the grade two hours later the swarm was gone, just as Lindauer had described.

A month later I observed exactly the same event in my backyard in Santa Barbara at the same time of day. A swarm travelling across the yard suddenly became disoriented and settled into a rose arbor. Two hours later the swarm once again arose and proceeded in its original direction.

All of the above inevitably leads back to the question: “How does a swarm manage to travel in a straight line through the air when individual bees fly in circles?”

Although Lindauer’s find that scout bees danced on the surface of swarm clusters fit in nicely with the von Frisch “dance language” recruitment hypothesis, the notion that bees can “transcribe” direction and distance information while flying in circles would be viewed by most of us as a rather impossible feat (even for us with our computers!).

It is at times like this that one should not lose sight of the fact that observation is oftentimes more reliable than theory (Wenner, 1989b). Three years ago I had the good fortune of having a swarm move into a swarm hive (e.g., Schmidt et al., 1989) in my yard while I was at home all day. This rather rare event permitted me to observe the entire sequence leading up to the swarm movement itself, just as Lindauer had done earlier with artificial swarms.

At nine in the morning, a few scouts were inspecting the swarm hive in their characteristic manner (Figure 1). Each would excitedly go in through the opening and buzz while inside, presumably measuring the dimensions of the interior (e.g., Seeley, 1985:71). Occasionally one would come back to the entrance and circle the opening, again perhaps measuring its diameter.

Figure 1. Scout bees inspecting a swarm hive. They apparently "measure" the size of the internal cavity; also, an individual can sometimes be observed to run around the edge of the opening.

By noon, many bees were involved, with some of them most excitedly executing the waggle dance on the outside surface of the swarm hive (and what good would that do?). Traffic between the parent colony and swarm hive increased more rapidly after that time; that increased flight activity permitted me to identify the aerial pathway that had been established between the parent colony and the swarm hive, as surmised by Allen Latham decades ago (in Root, et al., 1947:606):

“There is little doubt in my mind that there is a line of flying scouts from swarm to tree which the swarm follows when it leaves.”

Figure 2. A swarm has just settled on the small end of the swarm hive, around the opening. Moments later they began to flow into the hive.

By four in the afternoon, hundreds of “scouts” were flying back and forth between colony and swarm hive. At the same time the swarm flew in from the direction travelled by the scouts and settled on the swarm hive (Figure 2), evidently led by these hundreds of bees that already knew the landmarks along the way. The whole behavioral sequence was just as Lindauer had described – as summarized by von Frisch (1967:276):

“Apparently it is also the scout bees, familiar with the route, that by their lively guidance further assure the finding of the goal: while the swarm cloud is proceeding gradually along one sees a few hundred bees shooting ahead through the crowd in the direction toward the nesting place, then flying slowly back at the margin of the swarm cloud, again pushing forward rapidly, and so on, until the goal is reached (Lindauer 1955:319).”

The mystery thus is partially solved; swarms are lead through the air by scouts who rely on landmarks. As Michener wrote (1974: 133), “It is the only place in Apis biology. . .where guidance of this sort is evident.” However, guidance is not all; scent exuded from the Nasanov gland is also important while swarms settle, as first recognized by Sladen in 1901 (Chap. 21 in Ribbands, 1953), and by scouts as they travel between swarm cluster and the new site (Lindauer, 1951:513). In his book on social insects, Wilson (1971:245) termed the Nasanov substance a “true assembly pheromone.”

As all beekeepers know, if one shakes all the bees off a frame onto the ground in front of a hive, disorientation is soon followed by a streaming of the bees toward the hive entrance. At the same time, many of the bees that have become oriented expose their scent gland (Figure 3) and exude a sweet odor (one component of which is geraniol). Those bees still disoriented apparently act on that stimulus and likewise begin to move toward the hive entrance.

Figure 3. A scout bee exposing its Nasanov gland while fanning near the swarm entrance. Scout bees also apparently expose this gland while they are leading the swarm through the air.

In the 1920s von Frisch conducted some experiments on Nasanov glands; his results led him to believe that the Nasanov gland exudate attracted other bees to food sources. However, foragers apparently never expose that gland while visiting flowers under normal conditions. For that and other reasons, one can conclude that the scent produced is not an “attractant” in the usual sense (see Excursus NG in Wenner and Wells, 1990).

On the other hand, when a swarm is settling, a very high percentage of the settling bees expose that gland. V.G. Milum described that process as follows (in Root, et al., 1947:572):

“The scent gland is. . .used by the bees when swarming, the odor enabling them to keep together and, as the cluster starts to form, the bees on the edge of the cluster expose the scent gland while fanning vigorously, throwing the scent back of them to the other bees. Also when the swarm enters the hive, the scent gland is visible as a white spot near the back tip of the abdomen, as the fanning bees line up in front of the hive entrance.”

That exudate is thus more accurately labelled a “settling” or “orienting” pheromone than an “attracting” pheromone as proposed earlier by von Frisch. (Beekeepers can recognize the importance of that distinction quite easily, since bees from non-swarming colonies pay no heed to the odor produced by settling swarms.)

As one might suspect, even here the ancients pre-empted us. In about 45 A.D., Columella advocated use of a settling pheromone (albeit not with that title) for capturing swarms. He wrote (1954:459):

“There are some people who during the early spring collect wild parsley and, in the words of [Virgil], ‘bruised balm and wax-flower’s lowly greenery,’ and other similar herbs in which [bees] take delight, and rub the hives thoroughly with them, so that the scent and juice stick to them; then, after cleaning them, they sprinkle them with a little honey and place them here and there in the woods not far from the springs and, when they are full of swarms, they carry them back home.”

Columella’s advice is significant and almost prophetic, in that the very plants mentioned contain chemical components remarkably similar to those exuded by the Nasanov gland (Burgett, 1980). People use lemon grass (Cymbopogon citratus) for the same purpose in Brazil (Cristina Sandoval, personal communication); crushed leaves from the lemon-scented gum tree (Eucalyptus citriodora) should also work (Wenner & Wells, 1990:312).

Swarms tend to move only when weather conditions are favorable; otherwise winds could lead to disorientation and temporary halts part way to the destination. European honey bees also do not normally move very far from their parent colonies. The average distance is about one-half mile (800m), but the scale is logarithmic and some move a much greater distance (Wenner, Meade, and Friesen, in press).

Where wind directions remain constant over a period of time, it also seems likely that swarms would generally move upwind, in the same direction that they normally forage under those conditions (see Friesen, 1973). However, no one seems to have gathered data on this important point.

I suspect that swarm movements by “African” bees are similar to those of European bees in principle (i.e., the mechanism of movement) but differ in some important respects (e.g., Ratnieks, Otis, Winston, 1991). European honey bee swarms tend to move a relatively short distance, move only once, and then settle into their new location and stay there indefinitely. By contrast, African bee swarms often move great distances, may “bivouac” in an interim location for a few days before moving on again, may even enter a colony already occupied by European bees, and may well abscond after only a few months. It is no wonder their spread has been so rapid.

Beekeepers can gather useful information by establishing a swarm hive (e.g., Schmidt, Thoenes, and Hurley, 1989) in a location convenient for day-to-day observation. It should be done just prior to swarming season; notes could then be taken on all that transpires between the time of first scout inspection and eventual occupation. Information gathered could include number of bees seen at the swarm hive entrance at each interval of time (e.g., once each hour), wind direction, direction of dancing relative to sun direction, and direction of the scouts aerial pathway, once it became well established. Surprises will surely be in store for those who participate.

Finally, a bit of intrigue and a new perception. It is the experienced foragers who become scouts. One can suspect that swarming is a process by which older occupants expel the old queen and set her adrift with the young of their overpopulated hive (“cast them to the wind”), perhaps themselves even returning to their parent colony once the swarm has settled into its new location (see Butler, 1949:116).

However, all is not as bad (or as cruel) as it may seem; benefits acrue to both populations. The older bees at the prior location still have their proven foraging grounds and a new queen that can lay for 3-4 more years. The younger bees have a proven queen and quite likely can range further upwind in the same proven foraging grounds.

While many like to discard the old “superorganism” concept of Maeterlinck and Wheeler (see Excursus MM in Wenner and Wells, 1990), that term still applies well in cases such as the above. Another example of the applicability of the superorganism concept is that a re-analysis of results on how far bees travel collectively from their colonies (Wenner, Meade, and Friesen, in press) reveals that a colony as a unit seems to range a given distance, on average, when environmental conditions remain fairly constant. However, that topic will have to be the subject of a future article.

Adrian M. Wenner
Dept. of Biol. Sciences
Univ. of Calif., Santa Barbara
Santa Barbara, California 93106
(805) 893-2838


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