Why Are Honey Bees Such Great Pollinators?
Author: Heather Broccard-Bell, Ph.D., Honey Bee Health Researcher
The Western Honey Bee is a Pollinator Pro
Happy Pollinator Week! As I am sure you are well aware, there are a whole lot of different kinds of pollinators out there: from the familiar bees to other insects, like flies and beetles, to birds, and even bats! Each is important, and each has a role to play. Within this diverse group of animals, however, the Western Honey Bee, Apis mellifera, stands out – especially when it comes to the pollination of food crops, like fruit and nut trees. Interestingly, the Western Honey Bee (hereafter referred to as the honey bee, even though there are actually several other species of honey bee*) is somewhat of a “perfect storm” of factors that, combined, make it a powerhouse pollinator.
Honey Bees are Hearty Little Creatures
First, honey bees are widespread. Like really widespread. Although there is some disagreement, they are thought to be native to Asia, with expansion into Africa and Europe about 1 million years ago[2,3]. We humans have been keeping honey bees for their delicious honey for thousands of years—which means that we’ve also been taking them with us as we moved around the planet. And thanks to their particular way of life, honey bees don’t just tolerate a huge range of climates, they positively thrive in them. For example, where I am in Ontario, Canada, feral honey bee colonies are perfectly capable of overwintering all on their own, without the help of anyone, thank-you very much. Like it or not, honey bees are now well established on every continent, except Antarctica**.
They Aren’t Picky Eaters
Second among the reasons for the honey bee’s outsized influence on pollination is a combination of two factors: Not only is it a “super-generalist” forager (meaning that it isn’t picky about what it eats), it’s also a very efficient pollinator (honey bees are excellent at transferring pollen between plants). In the natural world, there are many examples of plants and pollinators that are so precisely adapted to one-another that each species literally cannot survive without the other***. The honey bee is decidedly not one of these species. Although honey bees certainly do have preferences, they will ultimately gather nectar and pollen (“forage”) on a wide range of flowering plants, especially if there is nothing else available.
Super Seeking Breeds Success
A third reason honey bees are so important as pollinators, and the focus of this post (as well as a lot of my past research) is that, compared to other animals, honey bees are really, really good at foraging. Now, it’s absolutely true that humans have selectively bred domestic bees specifically for this trait to maximize honey production. However, what’s going on with honey bees is much more than just selective breeding. Potentially the biggest reason for the global success of honey bees is that they are naturally exceptional communicators.
Communication is Key
You are probably familiar with honey bee communication in one form or another. A lot of us know that the queen releases “queen pheromone”, which is actually a cocktail of chemicals that has varied effects depending on the context, such as suppressing the development of worker ovaries, and promoting calmness during swarming. Almost everyone is familiar with the “alarm pheromone”—that banana-smelling odour (or household cleaner scent, depending on who you ask) that greets the nostrils when a beekeeper has been a touch too indelicate in her handling of the colony. Alarm pheromone, released especially when a bee is crushed or stings, signals to nearby workers that the colony is under attack, and a defensive response is then mounted.
Not all communication in the colony is odiferous. Sometimes, it involves taste. For instance, returning foragers may offer up a sample of the contents of their honey stomach to bees in the hive in an act called “trophallaxis”. Trophallaxis lets the recipient do things like assess the quality of the nectar being brought into the hive.
See Figure 1: Honey bees engaging in trophallaxis (photo credit: H. Broccard-Bell).
The Dance Language
Yet another form of honey bee communication happens through vibrations. The most famous of which is the waggle dance. While most beekeepers are aware of this dance language, it can be tricky to catch. Unless you have an observation hive, or have been lucky enough to watch a swarm hanging from a tree branch for some time (honey bees also use waggle dancing when they are house hunting), you may have never seen it yourself. Many people have noted this unique movement throughout the ages, but it wasn’t until the twentieth century that German scientist, Karl von Frisch, finally figured out its meaning.
A Map Made of Dance Moves
The waggle dance is performed by foragers who have recently returned to the colony from a successful foraging trip. Its purpose? To tell other members of the colony how to find the site they’ve just visited. Dancers are keenly observed by other bees in the colony called “dance followers”, and the more rounds these bees “watch”, the more precisely they are able to navigate. Each round of the waggle dance consists of two parts: a “waggle run” and a “return”. During the waggle run, the dancer walks forward in a straight line, swinging her rear end back and forth rapidly or “waggling”. At the end of the waggle run, the dancer stops her butt-swinging, turns, and circles back to her approximate starting position. Each time she ends a waggle run, she circles back in the opposite direction to the time before, roughly tracing out a path in the shape of a “figure eight”.
I mentioned earlier that the dance followers watch the dance, but in fact, this is incorrect. They listen to it—just not anything like the way people hear****. When it comes to waggle dancing for foraging purposes, it is performed inside the colony, a normally dark place where vision is of little use. Unbeknownst to the casual human observer, the waggle portion of the dance actually makes a sound! The sound is reminiscent of a helicopter, although you need a special omnidirectional microphone and some spare time to be able to hear it.
Giving Good Directions
Anyone who has ever tried to find a new location knows, a good set of directions must answer at least two key questions: How far away is it? And in what direction? Fortunately for honey bees, the waggle dance has these necessary bits. The length of the waggle run, in terms of how long it lasts, tells the followers how far away the foraging site is. One second of waggle run is roughly equivalent to 1 km of distance. Distance is relatively straightforward and easy to understand, but things get a little trickier when we consider how the waggle dance indicates direction.
Regardless of whether they are feral or managed, honey bees operate on vertical surfaces inside their nests. Thus, waggle dancing is naturally performed on a vertical surface (think of a fly on a wall). Their directions are based on two similarities that honey bees share with other animals: the ability to sense gravity and using the sun to navigate. Yet unlike any other animal that we know of, honey bees combine these skills to communicate. Using gravity as their reference inside the colony, bees give instructions that “point” their followers in the desired direction to fly when leaving the colony.
Remember that waggle run forward by the waggle dancer? The specific direction she walks in, relative to straight up, tells the dance followers which way to fly when they leave the colony. If she were to walk straight up, this means, “fly directly toward the sun”, and if she walks straight down, it means, “fly directly away from the sun.” Using this system, waggle dancers can indicate any direction—provided that the sun isn’t directly overhead.
See Figure 2: Diagram of the waggle dance used by honey bees to share directions with other foragers.
Bees Share Location Reviews for Five-Star Feeding
One last piece of information contained in waggle dances, albeit indirectly, is how good the foraging site is. Like many humans, each worker bee is a special little snowflake with her own unique personality (seriously, there is a whole scientific literature on this[10,11]). Some bees will visit a flower, fly back to the hive, and do a waggle dance for several minutes, leading a number of foragers from the colony to visit that same spot. Another bee may visit the exact same flower, at exactly the same time and not dance for it at all, directing no additional foragers to the location.
Individual bees will also decide to dance more or less for a site. Performing a large number of waggle dance rounds allows more dance followers to follow her instructions to visit that site. Whether or not a bee performs a waggle dance and for how long depends on how highly she rates the quality of the spot she found. This “star rating” is dependent on two factors: 1) genetics, which determine sensitivity to things like the sugar concentration of nectar, and 2) the state the forager is in at that moment (i.e., her “mood”), which depends on things like the availability of food in the environment, and her recent foraging experiences. The better the foraging site is overall, the more bees will dance for it, and the more bees that are “recruited” to that location allow the colony to gather the most resources from the best sites.
So, to sum it up (TLDR), the waggle dance tells followers the distance and direction they need to travel in to find a good food source, and indirectly indicates the quality of the resource. Unlike any other insect that we know of, honey bees seem to be able to “talk” about something (the location of a source of food) that isn’t right in front of them. This complex communication allows them to gather food in a much more efficient way than any of their relatives. For example, bumble bees also live in colonies, but their colonies are much smaller. A lot of the reason is that they do not communicate the location of resources the way honey bees do, so every individual leaving a bumble bee colony is, at best, relying on her own memory, and at worst, having to search for food randomly, with no information to go on at all[13,14].
See Figure 3: A forager performs a waggle dance while several dance followers observe (Photo Credit: H. Broccard-Bell).
But Wait—There’s More!
Actually, there is a lot more when it comes to honey bee communication. People build whole careers around studying it because there is so much to know! For this article, however, I am going stick to telling you about two more vibrational signals used by honey bees.
The tremble dance looks a little bit like a waggle dance to the uninitiated; however, rather than the periodic waggle – return – waggle – return, it’s kind of all waggle, all the time. Unlike the waggle dance, the tremble dance doesn’t seem to have any obvious “directionality”—although we understand the tremble dance and its functions to a much lesser degree, so it is possible scientists have missed this aspect so far. The purpose of the tremble dance, which is often performed by nectar foragers shortly after they return to the hive, seems to be to shift what everyone else is doing in the colony. Normally, nectar foragers get help unloading their nectar by younger in-hive bees. When a lot of nectar is being gathered, however, there may be no in-hive bees available to help with the offloading. Tremble dancing appears to cause a shift in the jobs being performed by the in-hive bees in a way that causes them to prioritize helping with the nectar offloading. In other words, tremble dancing allows the colony to adjust its behaviour to meet its own needs at that moment in time.
And this brings us to our final action: the stop signal. If you watch enough waggle dancers, you’ll see a move that looks like one of the dance followers is head-butting the waggle dancer. This causes the waggle dancer to briefly pause, giving this move its name. With the aid of that special omnidirectional microphone I mentioned before, you can hear that the head-butt is accompanied by a little “eek” sound. The vibration produced by this sound is actually what makes the waggle dancer stop. While the waggle dancer may start up again, a stop signal has the effect of making it more likely that she will not continue dancing and spreading her message.
But why stop this waggle dancing fun? There are at least two reasons. First, if a forager visits a site and encounters danger there (maybe a spider or some other bee predator), she will return to the colony, locate waggle dancers advertising that site, and tell them to STOP. Interestingly, she doesn’t find the dancers by decoding the waggle dances, which would take too long. Instead, the returning forager takes a short-cut: she smells the waggle dancers to see who smells like the site she was just at, and targets them. These stop signals relay the message to pause location sharing, so fewer bees are sent to the now dangerous site. And the same thing happens if a forager goes to a spot that has too many other visitors at it.
See Figure 4: Diagram of waggle dance and stop signal: A. waggle dance is performed, which leads to B. dance followers following the directions to find the good patch of flowers. If foragers encounter danger or overcrowding at the patch, they will C. return to the colony, find waggle dancers advertising that location, and tell them to stop! Note that if the forager DOES NOT have a bad time at the flower patch, she will come back to the colony, not tell anyone to stop dancing, and may even perform a waggle dance of her own. (Photos: H. Broccard-Bell).
Communication Creates a Super Pollinator
We have discussed three vibrational signals that honey bees use to help the colony gather food for itself in the most efficient way possible. Waggle dancing allows foragers to tell other foragers how to get to the best sites, tremble dancers cause in-hive bees to prioritize nectar storage when a lot of nectar is being brought in, and stop signals cause waggle dancers to stop advertising sites that have become dangerous or crowded.
I hope this shares a sense of just how amazingly well-adapted honey bees are to foraging—which, of course, brings us back to pollination! Honey bees are successful because their communication system allows them to forage so efficiently that they can stockpile vast quantities of food, which, in turn, allows their colony sizes to be very large. Large numbers of bees foraging on an ever-shifting landscape of foraging sites translates into a lot of pollen movement, which further translates into a LOT of pollination. Honey bees are super pollinators because they are super-communicators!
*Apis mellifera is one of between 7 and 12 species (depending on which authority you want to go with) in the genus Apis, all of which are colony-living honey bees. Most species except Apis mellifera are found only in Africa and Asia, but many of them are kept, at least to a limited extent, in managed colonies for their honey.
**It is important to note that some researchers now suspect that honey bees can outcompete native species for food sources, and recent evidence suggests that some honey bee diseases may “spill-over” into native species[20,21]. Although the impact of honey bees on other pollinators is not always positive, we can minimize these impacts through best management practices, such as treating for pathogens like Varroa. For the reasons discussed in this article, even if we wanted them to, honey bees are not going anywhere any time soon!
***Probably the most famous example of host-pollinator co-evolution is a species of orchid, now known as Darwin’s orchid, and its pollinator, Morgan’s Sphinx Moth. When Darwin first examined the newly-discovered orchid, he surmised that there must exist a moth species with a proboscis long enough to pollinate it. In fact, this turned out to be the case – although the moth itself was not discovered until well after Darwin’s death.
****I will be covering how bees hear the world—as well as see, taste, and smell it—in an upcoming post!
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