While strolling through the Arnold Arboretum during the summer, visitors may see bees flying from flower to flower. Some bees are pushing their heads deep into flowers and collecting nectar, others are more interested in collecting pollen from the flowers’ anthers. Bees that collect pollen are not collecting most of it for themselves; they are taking it back to their colony to feed to the larvae. Pollen provides a protein and mineral source for the developing brood.
In flowering plants, pollination is the process of moving pollen from the anthers to the stigma. As bees collect pollen and nectar, they inadvertently transfer pollen from the anthers of one flower to the stigma of another flower, either on the same plant or different plants. For plants that are self-incompatible (they cannot reproduce without cross pollination from another plant), this transfer service is essential. For both self-incompatible and self-fertile species, the transfer of pollen between plants allows for genetic variation in the plants’ offspring, preventing plant populations from becoming inbred. Without bees, many species would make neither fruit nor seed.
Give It Up for the Bees
Plants have developed a variety of ways to “give” their pollen to bees. Some have longitudinally dehiscent anthers; these split open down the sides when the pollen is ready, making it easily accessible. Longitudinally dehiscent anthers have benefits and drawbacks. One benefit is that plants with these anthers can be pollinated by many insects, birds, or even humans. (In Sichuan, China, the decline of pollinators has led to pollination of apple and pear orchards by humans armed with vials of pollen and small brushes.) These plants are generalists when it comes to attracting pollinators, but this could also be seen as a drawback. What if an animal rushes by and knocks all the pollen off the flowers? What if an insect visits different species and never actually transfers pollen between conspecifics (members of the same species)? Changing the shape of the anther can help solve both of these potential problems.
Multiple lineages of plants have evolved anthers that are tube-shaped. Instead of splitting down the sides, these anthers simply open tiny pores when pollen is ready to be released. These anthers are known as poricidally dehiscent, or poricidal.
Poricidal anthers help keep pollen from being knocked off the flower, and they prevent many pollinators from reaching the pollen. About eight percent of flowering plants (some 20,000 species) have poricidal anthers (Buchmann 1983). Because the pollen is in a tube, animals cannot easily shake it free. Three common ways to access this pollen are biting through the outside of the anther; squeezing pollen out by treating the anther like a tube of toothpaste; or vibrating the anther to eject pollen out the hole. Bumblebees (Bombus spp.), some of the most common pollinators in the Arnold Arboretum, employ the technique of vibrating pollen out of the anthers. This technique is known as buzz pollination, and every summer many flowers “get buzzed” at the Arboretum.
Some of our favorite fruits and vegetables are efficiently fertilized by buzz pollination—these include blueberries, tomatoes, eggplants, and cranberries. Honeybees (Apis mellifera), the most common pollinator in the United States, cannot buzz pollinate (the reasons for this are not clear). This means that we must rely on bumblebees and other buzz-pollinating native pollinators to fertilize these crops. For example, greenhouse tomato growers often place colonies of bumblebees in their greenhouses to pollinate the tomatoes.
In flight, bumblebees flap their wings at about 190 cycles per second, or hertz (Hz); this vibration sounds like the F-sharp below middle C on a piano. If an average eye blink lasts for about 300 milliseconds, that means that bees flap their wings over 50 times while we blink. Bumblebees use their flight muscles for another purpose as well: creating the vibrations needed for buzz pollination. After a bumblebee lands on a flower and decides to try buzz pollination, she folds her wings into their resting position over the abdomen. While the wings are decoupled from flight muscles, she contracts the muscles that normally power wing strokes. Bees’ flight muscles are not directly attached to the wings, but instead to parts of the thorax. One group of muscles attaches to the top and bottom of the thorax. As these dorsoventral muscles contract, the thorax deforms. The sides of the thorax are pushed outward as the top and bottom of the thorax are pulled together. Another group of muscles attaches to the front and back of the thorax. These longitudinal muscles contract and the dorsoventral muscles relax. This deforms the thorax differently—now the top and bottom of the thorax get pushed outwards as the front and back of the thorax get pulled together. This whole cycle happens with every wing stroke when the wings are engaged, but while the wings are decoupled, the thorax experiences this cyclic deformation while the wings stay relatively stationary. The thorax usually deforms at a higher frequency during buzz pollination than during flight. Although the bee is not moving a large distance with each deformation of the thorax, the accelerations are huge! One species of bumblebee has been found to buzz with accelerations nearly 20 times the acceleration due to gravity (De Luca and Vallejo-Marín 2013). This produces forces high enough to expel pollen out of the anthers, where the bee can then gather it easily.
Characterization of Buzz Pollination at the Arboretum
During the summer of 2013, I spent over a month at the Arnold Arboretum, characterizing the buzzing behavior of bumblebees. I usually arrived early in the morning and made a beeline for the Leventritt Shrub and Vine Garden, where I could easily access flowers that were abuzz with pollinators.
I used a microphone to record the sounds the bumblebees made while flying, buzz pollinating, and just buzzing in irritation. Bumblebees’ typical behaviors made data collection easy. For one, bumblebees were unfazed by my presence—I could hold a microphone just a few centimeters from their bodies while they they were collecting pollen and they still were not scared away. Another behavior that made data collection easy was that bumblebees very habitually forage on the same flowers (Heinrich 1976). If I did scare a bee away, I could rely on it to come back soon. To get wingbeat frequency, I followed the bee from flower to flower, recording a few segments of flying and buzzing. Last, I captured the bee in a net, and jostled the net around. This caused the bee to irritation buzz, which I recorded from the outside of the net. I used a computer program to calculate the Fast Fourier Transform (an algorithm) on segments of these recordings to get the wingbeat, buzz pollination, and irritation buzz frequencies. During buzz pollination and irritation buzzing, the bumblebees’ wings stayed folded over the abdomen. After recording over 350 individual bumblebees while they were foraging, I found an average buzz pollination frequency of about 270 Hz. The pitch of this vibration frequency is equivalent to a C-sharp above middle C on the piano. Through this research I was able to answer some basic questions about buzz pollination, but my observations also led to more questions.
Answers about buzz pollination:
Q. Can bumblebees change the frequency at which they buzz?
A. Yes. Without extending their wings, individual bumblebees can increase their buzzing frequency by at least 90 Hz.
Q. Do bumblebees buzz pollinate at different frequencies on different plants?
A. Probably. My data show different buzz pollination frequencies on different plants, but it’s possible that the differences arose because the plants flowered at different times of the year and samples were done in uncontrolled humidity. Future experiments in a more controlled setting will compare flowering plants at the same time of year and at the same humidity, which will provide more definitive results.
Q. What other conditions affect vibration frequency during buzz pollination?
A. Humidity and time of year. Out of all the conditions (including bumblebee size, temperature, and time of day), these were the two conditions that had the greatest effect. Bumblebees tended to buzz at higher frequencies in high-humidity conditions and at the beginning of the summer.
New questions about buzz pollination:
1. Are all bumblebee species equally good at buzzing?
I observed many individuals of the common eastern bumblebee (Bombus impatiens) collecting pollen by buzzing. However, I observed a number of two-spotted bumblebees (Bombus bimaculatus) collecting pollen without buzzing. This was happening on St. John’s wort (Hypericum ‘Hidcote’), which was being buzz pollinated by other bees. More controlled experiments on different bumblebee species will provide more information on this topic.
2. Why is the Arboretum dominated by a single bumblebee species?
I found at least five bumblebee species pollinating. However, out of nearly 400 bumblebees, I found 375 individuals of the common eastern bumblebee; that is about 95%. It would be interesting to survey bumblebee species at other habitats (i.e., city vs. rural) to see if common eastern bumblebees are also dominant in those locations.
3. Why do buzz frequencies change?
Could it be that different flowers require different frequencies to get maximum pollen release? Is humidity affecting the bumblebee or the flower more?
4. Why do bumblebees buzz on plants with longitudinally dehiscent anthers?
Before I started collecting data, I thought that bumblebees shouldn’t buzz on longitudinally dehiscent anthers since the pollen is readily accessible. But I found multiple instances of buzz pollination occurring on St. John’s wort (Hypericum ‘Hidcote’) and Carefree Beauty rose (Rosa ‘Bucbi’), and I even recorded some buzzing on Chinese stewartia (Stewartia sinensis). All three of those plants have longitudinally dehiscent anthers. Stephen Buchmann (1985) published similar observations and hypothesized that buzzing may increase effectiveness at collecting pollen on longitudinally dehiscent anthers, especially when the flower has a “shaving brush” structure (contains numerous stamens with long filaments). One suggestion for why bumblebees use buzz pollination on this type of flower is that it allows them to get pollen from many anthers at one time. With these flowers, bees gather many anthers together and hold them close to their bodies while they buzz. Though this doesn’t require buzz pollination, buzzing could result in faster pollen collection than collecting from one anther at a time.
Answering questions about bumblebee pollination can help humans effectively manage plant populations, including our food supply. Spending 30 days in the Arboretum helped me answer a few pollination questions, but there are still lots of unanswered questions. The next time you walk through the Arboretum (or your own yard) try to identify some of the most common pollinators. By looking closely at the shape of flower anthers and how they dehisce (open), you can make a guess about what type of bee pollinates the plant. And the next time you’re eating blueberries or combing through a recipe book to find something to do with all your tomatoes, you can thank the bees.
Buchmann, S. L. 1985. Bees use vibration to aid pollen collection from non-poricidal flowers. Journal of the Kansas Entomological Society 58: 517–525.
Buchmann, S. L. 1983. Buzz pollination in angiosperms. pp. 73–133 In: Handbook of Experimental Pollination Biology, C. E. Jones and R. J. Little, eds. New York: Van Nostrand Reinhold.
De Luca, P. A. and M. Vallejo-Marín. 2013 . What’s the “buzz” about? The ecology and evolutionary significance of buzz-pollination. Current Opinion in Plant Biology 16: 429–435.
Heinrich, B. 1976. The foraging specializations of individual bumblebees. Ecological Monographs 46: 105–128.
Callin Switzer is a graduate student in Stacey Combes’s research lab at Harvard University
From “free” to “friend”…
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