I snapped this photo in my backyard while enjoying the last bit of the summer holidays – just look at how busy this little critter has been, flying from flower to flower, collecting pollen. I’m fascinated by the role that physics plays in pollination, and not just the unlikely aerodynamics of bumblebee flight. Electricity plays an important part too, with electric forces pulling pollen from the insect to the plant, as well as from the plant to the insect. The same way your hair stands on end when a charged balloon is nearby, so too are the pollen particles drawn away from the stigma as the insect comes closer.
When flying around, insects such as bees become charged through friction in a process called the triboelectric effect, like when you shuffle in your slippers across the carpet and discharge when you touch a metal door handle. Different materials have different tendencies to become charged through this process – some become negatively charged, some become positively charged. Some easily pick up and hold a lot of charge, others don’t.
Way back in 1962, an experiment was conducted to assess the tendency of bumblebees to accumulate charge, concluding that they were one of the most easily charged materials tested. They also found that bumblebees always become positively charged through friction, building up charges of +30 to +50 pC during flight. These tiny charges have been measured experimentally by training insects to fly into Faraday cups, the charge-measuring devices used by nuclear and particle physicists in accelerator beams – bee counting rather than beam counting, if you will.
While the insects become slightly positively charged in the air, the plants on the ground possess small amounts of negative charge. As the charged pollinator gets closer, that positive charge pulls more negative charge up from the ground into the plant where it accumulates at the tips in significant amounts. There is enough opposite charge on the two objects to pull pollen particles off the surface, with forces greater than their adhesion and greater than the force of gravity too. There is a lovely illustration of this process in a recent paper by Dominic Clarke and collaborators, showing the mathematical model they used to study the phenomenon:
The image on the left shows the accumulation of charge on the pointy parts of the two surfaces involved and the image on the right shows the resulting paths taken by pollen grains moving between the two objects. This model doesn’t include wind and humidity effects, but it’s a pretty compelling study showing the importance of electrostatics to the overall exchange of pollen between plants, facilitated by our charged friends, the bees. And, electricity doesn’t just play a passive role here, lazily moving pollen from one place to another as an unsuspecting bee buzzes around. Not only can bees sense the electric fields around flowers, but they can even learn to use them to find nectar.
A study reported in Science in 2013 demonstrated that bumblebees could be trained to preferentially fly to artificial flowers containing a sucrose reward versus identical artificial flowers containing a bitter solution made with quinine (like in tonic water), just by adding an electric field. The sucrose flowers were indistinguishable from the quinine ones except for the presence of this electric field and the bees chose correctly 81% (+/- 3%) of the time after training. When the electric field was then removed, the bees did no better than random guessing, hitting the sugar jackpot only 54% (+/- 4%) of the time.
The insects in this 2013 study could even distinguish between flowers with different electric field geometries, such as fields around flowers with longer stigma or flowers in which the petals are more open. A subsequent study by this group looked at the effect of applied electric fields on the tiny hairs on bumblebees, showing that these fields cause the hairs to vibrate and thereby identifying the physiological mechanism for electric field detection. The image here shows a closeup of the hairs examined and the measured velocity of vibrations that resulted from different frequencies of applied electric fields.
I think it’s pretty cool that electrostatics is the driving force behind the transfer of pollen between insects and plants. But I think it’s even cooler that bees aren’t just along for the ride and free nectar! Bees can detect electric fields, learn to find nectar based on these fields, and may even communicate to their hive-mates through an electrified waggle dance upon their return from foraging. Now that’s something to buzz about!
Image Citations
Opening image – insect on yellow flower – image taken by the author
Cartoon image of someone getting shocked when touching a doorknob – Shutterstock.com ID 2080565146
Faraday cup used in particle accelerators – Angelpeream – Own work, Public Domain, https://commons.wikimedia.org/w/index.php?curid=3537763
Particle accelerator – Shutterstock.com ID 1287557641
Mathematical model to understand the process of electrostatic pollen transfer – Figure 7 from The bee, the flower, and the electric field: electric ecology and aerial electroreception. Dominic Clarke, Erica Morley, and Daniel Robert, 2017 J Comp Physiol A 203:737-748 doi: 10.1007/s00359-017-1176-6, open source
Close up of bee to show the hairs being studied and the resulting data collected of vibrational speed as a function of frequency of applied electric field – Figure 1 from The bee, the flower, and the electric field: electric ecology and aerial electroreception. Dominic Clarke, Erica Morley, and Daniel Robert, 2017 J Comp Physiol A 203:737-748 doi: 10.1007/s00359-017-1176-6, open source
Gif of honey bee waggle dance: https://thumbs.gfycat.com/AggravatingScholarlyAmericancicada-size_restricted.gif
References
Electrostatic charges on insects due to contact with different substrates. DK Edwards, 1962 Can J Zool 40:579-584 doi:10.1139/z62-051
The role of electrostatic forces in pollination. Y Vaknin, S Gan-Mor, A Bechar, B. Ronen, and D Eisikowitch, 2000 Plant Syst Evol 222: 133-142
Detection and learning of floral electric fields by bumblebees. Dominic Clarke, Heather Whitney, Gregory Sutton, and Daniel Robert, 2013, Science 340: 66-69 doi:10.1126/science.1230883
Mechanosensory hairs in bumblebees (Bombus terrestris) detect weak electric fields. GP Sutton, D Clarke, EL Morley, D Robert, 2016, Proc Natl Acad Sci doi: 10.1073/pnas.1601624113 The bee, the flower, and the electric field: electric ecology and aerial electroreception. Dominic Clarke, Erica Morley, and Daniel Robert, 2017 J Comp Physiol A 203:737-748 doi: 10.1007/s00359-017-1176-6
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