Can Bees Really Sense the "Electricity" of Flowers?
― Flying bees are positive, flowers are negative. Bees read the electric field between them with their body hairs
When choosing a flower, bees don't rely on colour and scent alone. A flying bee's body carries a slight positive charge. Experiments have shown that bumblebees sense the weak electric field between themselves and a flower with their body hairs, and use it to spot flowers that another bee visited a moment ago. It is a sense we will never experience.
In a spring flower bed, a bumblebee moves from flower to flower. Look closely, and you will see it sometimes fly straight past a flower without landing.
The flower looks the same. Its colour and shape haven't changed. Yet the bee seems to have decided, "Not this one."
In fact, another bee may have drunk its nectar only moments before. What the bee picked up is thought to be a change in the "electricity" left around the flower.
Only two reasons
A flying bee rubs against the air as it beats its wings and becomes positively charged. A flower is connected to the ground by its stem, so it sits on the negative side compared with the air around it.
An electric field forms between positive and negative. The fine hairs on a bee's body are pulled slightly by this field and bend a little. Nerves are thought to pick up that bending.
In other words, just by approaching a flower, a bee receives a kind of "electric touch". Let's go through it step by step.
Why is there electricity between a bee and a flower?
On a clear day, the air already has a weak electrical slope. The higher the air above the ground, the more positive it is, and near the ground the difference is said to be about 100 volts for every metre of height. This slope is called the "atmospheric electric field".
A flower is connected to the ground by its stem and roots. So its electrical level is the same as the ground's, which is on the negative side compared with the surrounding air.
A flying bee's body, meanwhile, carries a positive charge. Electricity builds up as the bee rubs against the air with its wings and touches flowers and other bees. The amount it can hold is reported to be mostly a few tens of picocoulombs. That is hundreds of times smaller still than the static you feel from a doorknob in winter.
Even a tiny charge matters when the bee and the flower are close. The field a charge creates becomes four times stronger when the distance is halved. Use the slider in Figure 1 to bring the bee closer to the flower. At 1 cm, the field made by the bee's charge is tens of times stronger than the ordinary field in the air.
What does the bee do with that electricity?
When a bee lands on a flower, some of its positive charge moves to the flower. This changes the electrical state around the flower, and the change is reported to last for a minute or two after the bee has left.
In 2013, a British research team tested this. They put sugar water in artificial flowers that carried an electric charge, and a bitter liquid in uncharged ones, and let bumblebees choose. The flowers looked exactly alike. Even so, the bees learned to pick the charged ones. When the charge was switched off, their success rate fell to no better than guessing.
So where do they sense it? A 2016 study showed that fine hairs on the body are pulled by the field and move, and that nerves at their base respond. Because the hairs are light and thin, they are thought to move easily even in a weak field.
The flower benefits too. Pollen on the negative side is drawn toward the positive bee, so it sticks to the body even before contact. Electricity is thought to help both the flower and the bee.
People notice an electric field only when it is strong enough to make the hair on their arm stand up. Yet some living things are masters of sensing electricity. Sharks and rays pick up the faint electricity given off by the muscles of their prey. Spiders sense the electric field of the sky and rise into the air on their silk. Bees are members of this group, reading the "untouchable feel" of a flower.
Summary
A flying bee is positive, and a flower connected to the ground is on the negative side. The electric field between them grows sharply stronger as they get closer, and the bee senses it with its body hairs. The change in electricity is also thought to be a clue for recognising a flower that has just been drunk from.
Flowers speak not only through colour and scent, but through "electricity".
Bees hear it with their hairs.
For creatures that use the sky's electric field to fly, see "How can spiders fly without wings?"; for fish that hunt with electricity, see "How do sharks find prey hidden in the sand?". The way static electricity pulls objects together is explained in more detail in "Why does a rubbed balloon stick to the wall?".
- On a day when the air is dry, rub an inflated balloon well against your hair or clothes.
- Bring the balloon to within a few cm of the fine hairs on your arm. Without touching, you should feel the hairs lift softly toward the balloon.
- Tear a tissue into thin strips and let them hang, then bring the balloon near in the same way. Compare how much stronger the pull becomes when you halve the distance.
A balloon carries far more charge than a bee, so even human hairs can feel it. A bee reads a charge thousands of times smaller, with much finer hairs.
For those who want to know more ― terms, formulas and links to textbooksEach section is labelled by level, from junior high science to university courses
- Junior highCovered in junior high school science
- High schoolCovered in high school physics
- High school+Advanced high school material, or treated as a textbook sidebar
- UniversityUniversity-level courses not covered in high school (electromagnetism, sensory physiology)
- ResearchNot taught as settled fact even at university; researchers are still investigating it
Junior highTerms: this phenomenon has a name
- Static electricity (frictional electricity): when objects rub together, some charge moves across, leaving one positive and the other negative.
- Electric field: the "region where electric force acts" that a charged object creates in the space around it. Its strength is measured in volts per metre (V/m).
- Atmospheric electric field: the upward electrical slope that is naturally present in clear-day air. Near the ground it is said to be about 100 volts per metre.
Junior highHigh schoolChecking with a formula: how strong a field does the bee's charge make at the flower?
Treat the bee as a tiny point of charge, and find the field strength E at a flower a distance r away. A real bee has a complicated shape, so this is a rough estimate.
| In symbols | E ＝ k × Q ÷ r² |
| In words | Field strength ＝ constant × amount of charge ÷ (distance × distance) |
| Where it comes from | It comes from Coulomb's law, which says the force between charges is inversely proportional to the square of the distance. Picture the lines of force from a charge spreading out and thinning over a sphere of radius r, and you get a form inversely proportional to the sphere's surface area. |
| E | Field strength. Unit: V/m (volts per metre) |
| k | Constant in Coulomb's law. About 9×10⁹ (unit: N・m²/C²) |
| Q | Amount of charge on the bee. Unit: C (coulombs) |
| r | Distance from bee to flower. Unit: m (metres) |
| Bee's charge Q (taken as 30 picocoulombs, from the commonly reported range) | 0.03×10⁻⁹ C |
| Constant k | 9×10⁹ |
| Multiply k and Q, and 10⁹ and 10⁻⁹ cancel out | Left with 9 and 0.03 |
| Bee–flower distance r | 1 cm (0.01 m) |
| Atmospheric field on a clear day | About 100 V/m |
| k × Q | 9 × 0.03 ＝ 0.27 |
| Distance squared r² | 0.01 × 0.01 ＝ 0.0001 |
| Field at the flower E | 0.27 ÷ 0.0001 ＝ 2700 V/m |
| Compared with the usual atmospheric field | 2700 ÷ 100 ＝ 27 times |
| At a distance of 3 cm, the distance squared is | 0.03 × 0.03 ＝ 0.0009 |
| The field E then | 0.27 ÷ 0.0009 ＝ 300 V/m |
If the distance becomes one third, the field becomes nine times stronger. Even a bee's tiny charge, once within 1 cm, makes a field tens of times stronger than the ordinary air. A change big enough to move the bee's hairs appears suddenly just before the flower.
High schoolHigh school+Electric potential, and why the field is stronger at a flower's tip
High schoolA flower is connected to the ground by a conducting stem, so its electric potential (electrical "height") is almost the same as the ground's, 0 volts. The air at 1 m height, on the other hand, has a potential of about 100 volts. The flower acts as "a part of the ground lifted up", and so sits on the negative side of the surrounding air.
High school+At the tip of a pointed conductor, the lines of force crowd together and the field gets stronger. By the same logic as a lightning rod, the field is thought to be stronger at the tips of petals and stamens than around them. When a bee lands and passes charge to the flower, electrostatic induction changes the distribution of the flower's potential, and that change lingers for a short while.
UniversityPoint-charge field, Gauss's law, mechanosensory hairs
The formula in the main text is the point-charge field that follows from Coulomb's law. Applying Gauss's law to the spherically symmetric case gives the same form. Real bees and flowers are not points, so the accurate approach is to solve for the electrostatic field around bounded conductors numerically. On the bee's side, the field exerts force on the charge it induces in the body hairs, bending them slightly. Mechanosensory neurons at the base of each hair are thought to detect that bending. Some also suggest that Johnston's organ, at the base of the antenna, is involved.
📖 Derivations and further reading: Coulomb's law (Wikipedia, Japanese) ／ Gauss's law (Wikipedia, Japanese)
ResearchWhat is still unclear
- Do other insects also sense flower electricity? Examples of responses to electric fields are starting to be reported in flower-visiting bees other than bumblebees, and in hoverflies. How widespread the sense is remains under investigation.
- How do they combine colour, scent and electricity? Experiments can make bees choose by electricity alone, but how much weight the electrical cue carries in the real outdoors is still not well understood.
- What about weather and human activity? The atmospheric field changes greatly with the weather. Whether bees choose flowers differently on cloudy or rainy days, or near power lines, has not yet been answered.
So the content of this article is also "an explanation of what is known so far". Research on flower electricity only got properly under way in the 2010s, and future discoveries may change the picture.
Links to the textbooks (by level)
| Level | Subject / unit | Where in this article |
|---|---|---|
| Junior high | Science, Year 2: "Static electricity and current" | How a bee becomes positively charged by beating its wings |
| High school | Physics: "Electric field and potential" | The point-charge field formula; a flower's potential equals the ground's |
| High school+ | Physics (advanced): "Electrostatic induction, pointed conductors" | The field growing stronger at a flower's tip |
| University | Electromagnetism, sensory physiology | Gauss's law, mechanoreception by body hairs |
| Research | Sensory ecology, pollination ecology | How much electrical cues are used outdoors |
| ― | Link to daily life | The same principle as arm hairs standing up near a balloon |
- Clarke, D. et al. (2013) Detection and Learning of Floral Electric Fields by Bumblebees. Science 340: 66–69
- Sutton, G. P. et al. (2016) Mechanosensory hairs in bumblebees (Bombus terrestris) detect weak electric fields. Proceedings of the National Academy of Sciences 113: 7261–7265
- Vaknin, Y. et al. (2000) The role of electrostatic forces in pollination. Plant Systematics and Evolution 222: 133–142
- Atmospheric electricity (大気電気学), Wikipedia in Japanese
- Morley, E. L. & Robert, D. (2018) Electric Fields Elicit Ballooning in Spiders. Current Biology 28: 2324–2330
* This article is a popular-science explanation for general readers. The figures given are rough guides to help you understand the mechanism. A bee's charge and the strength of the field vary greatly with species, weather and how they are measured. If you watch bees up close, keep your distance so you don't get stung.