Plant Cuttings

This image of a colorized scanning electron microscope image of pollen grains from a variety of common plants, by Dartmouth Electron Microscope Facility, Dartmouth College, is in the public domain.

Pollination – “the transfer of pollen from an anther of a plant to the stigma of a plant”* – is the first step of many in the long journey that can result in fertilisation and production of the next generation of plants. It is such an important part of the life story of flowering plants that many strategies have evolved to ensure that pollen is moved correctly from the male organ [the anther] that makes the pollen to the female organ [the stigma] that receives it.

Generally, there are two main ways to transfer pollen – abiotic and biotic. Abiotic pollen transport is brought about by environmental factors, water [hydrophily] and wind [anemophily]. Biotic factors – i.e. living organisms – that can promote pollination include birds [ornithophily] (Steven Johnson, 2022), bats [chiropterophily] (Constance Tremlett et al., 2020), bees [apiphily] (Keng-Lou James Hung et al., 2018), beetles [cantharophily (Cerruti R Hooks & Anahí Espíndola)] (Rachel Krasner), and butterflies [lepidopterophily] (Beyte Barrios et al., 2016).

The numerous interactions that lead to plant-animal pollen transfer have been studied for many years, and generally require the animal to make physical contact with the plant whose pollen is to be transferred elsewhere** But, an intriguing addition to the growing catalogue of non-contact pollination events*** has been reported by Sam England & Daniel Robert (2024), and involves butterflies and moths, and static electricity [Ed. – yes, the phenomenon that can give you an electric shock].

Although a role of ‘electrostatics’ and pollination has previously been discussed in connection with hummingbird- and bee-pollinated flowers (e.g. Corbet et al., 1982; Vaknin et al., 2000; 2001; Badger et al., 2015; Clarke et al., 2017), little was known about lepidoptera (butterflies and moths) (Will Sullivan). Adding to that knowledge gap was one of the reasons England & Robert (2024) undertook their study. In essence, what England & Robert (2024)**** show is that “butterflies and moths build up static electricity***** as they fly, and these charges could allow the insects to collect pollen without touching flowers” (Will Sullivan).

Quite telling in that quote from Will Sullivan is the two words “could allow” because England & Robert (2024)’s study doesn’t actually show pollen being transferred from a flower to a butterfly or a moth. Rather, their study only allows them to infer such pollen transfer after measuring the electrical charges involved and modelling the phenomenon with a computer program to show what might be possible. Usefully, Catherine Offord‘s scicomm article about the work features a video clip of “a simulation of a positively charged peacock butterfly 6 millimeters [sic.] from a flower, showing pollen grains being attracted across the gap”******.

However, until such a pollen transfer has been observed in nature, it is a moot point as to whether such a contactless pollination route with butterflies or moths actually occurs. The model suggests it should be so, but that’s no substitute for direct observation in the environment. Also, and the article appears silent on this matter, if it’s a case of attraction that might pull pollen from a flower to a butterfly, surely it is that same phenomenon that will keep the pollen in place upon the insect. How does pollen then get transferred from the insect to a flower? Does the electrical charge on the pollen become reversed during the insect’s flight such that the pollen might then be repulsed from the insect and attracted towards the appropriately charged surface of the stigma? In other words, unless and until insect-located pollen is deposited upon the stigma (or female cone should lepidoptera and gymnosperms* be involved), pollination – the transfer of pollen from male to female floral parts (regardless of the vector employed to move the pollen around) – hasn’t taken place. Does this reverse transfer of pollen, from insect to plant, also happen?

So, more questions to answer, but, certainly promising work that causes us to acknowledge that floral biology may have its ‘shocking’ aspects. It is noteworthy that representatives of both day-flying (Kyle Schiber) butterflies, and – predominantly – night-active moths were studied which suggests that this pollen transfer mechanism may operate for flowers during daylight hours and at night.

In addition to the insights into any role of static electricity and lepidopteran pollen transport, England & Robert (2024) also make various inferences regarding differences in electrical charges between species which may have ecological and evolutionary relevance relating to their natural environments. However, at this point a note of caution is probably warranted courtesy of Víctor Ortega Jiménez‘s words, cited by Will Sullivan: “The 11 species studied in the paper are too few to draw conclusions about ecological differences, since there are more than 100,000 butterfly and moth species in the world”. Whilst suitable caution is required, it is also the case in science that you’ve got to start somewhere, which is what England & Robert (2024) have done. And having made inferences about a few species one can then make hypotheses that can be tested in, and for, the other >99,989 species of butterflies and moths.

* Or, more completely, embracing the notion that both angiosperms and gymnosperms produce pollen, “Pollination simply defined is the transfer of pollen from an anther to a stigma or – in gymnosperms – from a male cone to a female cone” (Daniel Murphy).

** A notable exception to that general ‘do touch the plants’ rule is buzz-pollination (Alice Sun; Mario Vallejo-Marín, 2019), which doesn’t require animal-plant contact. Interestingly, there are suggestions that release of pollen during buzz-pollination in some plant species may involve triboelectricity (Sarah Corbet & Shuang-Quan Huang, 2014; Yaftich Vaknin et al., 2000), which has some similarities to the static electricity pollination connection considered in the post above.

*** Electrostatics has previously been proposed to play a part in non-plant-contact pollination services provided by a number of animals, “For example, the electric fields surrounding plants and their charged pollinators are sufficient for contactless pollen transfer, wherein pollen is electrostatically attracted onto the surface of nearby insects (Corbet et al., 1982; Vaknin et al., 2000; Clarke et al., 2017) and hummingbirds (Badger et al., 2015)” (England & Robert, 2020), “and vice versa” [Ed. – by which is inferred to mean the transfer of pollen from animals to plants].

Electrostatic interactions have also been invoked in anemophily, non-animal mediated transfer of pollen by wind and air currents (George Bowker & Hugh Crenshaw, 2007a; 2007b).

And, by way of  demonstrating that it’s not just beneficial things like pollen that has a static electricity dimension, we should here mention that this electrical phenomenon also seems to have a role in passive attraction of ticks from vegetation to their hosts (Sam England et al., 2023).

**** For scicomm articles on England & Robert (2024)’s work, see Laura Baisas, Catherine Offord, and Will Sullivan.

***** And what causes static electricity has now been solved. Although the phenomenon has been known for over 2,600 years, it has apparently defied a scientific explanation until this year. Read more, in a scicomm article by Andrew Paul, and/or in the research paper by Karl Olson & Laurence Marks (2024). For more on static electricity, see Ken Stewart, here, here, and here.

****** For a series of illustrations showing indicative bidirectional pollen transfer in an electric field, demonstrated between daffodil (Narcissus sp.), and “the electric field produced by a triboelectrically charged acrylic rod”, and vice versa (Eoghan Ryan), see Fig. 6 in Clarke et al. (2017).

REFERENCES

Marc Badger et al., 2015. Electrostatic charge on flying hummingbirds and its potential role in pollination. PLoS ONE 10(9): e0138003; doi:10.1371/journal.pone.0138003

Beyte Barrios et al., 2016. Butterflies visit more frequently, but bees are better pollinators: the importance of mouthpart dimensions in effective pollen removal and deposition. AoB PLANTS 8: plw001; https://doi.org/10.1093/aobpla/plw001

George E Bowker & Hugh C Crenshaw, 2007a. Electrostatic forces in wind-pollination – Part 1: Measurement of the electrostatic charge on pollen. Atmospheric Environment 41(8): 1587-1595; https://doi.org/10.1016/j.atmosenv.2006.10.047

George E Bowker & Hugh C Crenshaw, 2007b. Electrostatic forces in wind-pollination – Part 2: Simulations of pollen capture. Atmospheric Environment 41(8): 1596-1603; https://doi.org/10.1016/j.atmosenv.2006.10.048

Dominic Clarke et al., 2017. The bee, the flower, and the electric field: electric ecology and aerial electroreception. J Comp Physiol A 203: 737–748; https://doi.org/10.1007/s00359-017-1176-6

Sarah Corbet et al., 1982. Are electrostatic forces involved in pollen transfer? Plant, Cell & Environment 5(2): 125-129; https://doi.org/10.1111/1365-3040.ep11571488

Sarah A Corbet & Shuang-Quan Huang, 2014. Buzz pollination in eight bumblebee-pollinated Pedicularis species: does it involve vibration-induced triboelectric charging of pollen grains? Annals of Botany 114(8): 1665–1674; https://doi.org/10.1093/aob/mcu195

Sam J. England & Daniel Robert, 2020. The ecology of electricity and electroreception. Biological Reviews 97(1): 383-413; https://doi.org/10.1111/brv.12804

Sam England et al., 2023. Static electricity passively attracts ticks onto hosts. Current Biology 33(14): 3041-3047.e4; https://doi.org/10.1016/j.cub.2023.06.021

Sam J England & Daniel Robert, 2024. Electrostatic pollination by butterflies and moths. JR Soc Interface 21(216): 2120240156; http://doi.org/10.1098/rsif.2024.0156

Keng-Lou James Hung et al., 2018. The worldwide importance of honey bees as pollinators in natural habitats. Proc R Soc B 285: 20172140; http://dx.doi.org/10.1098/rspb.2017.2140

Steven D Johnson, 2022. Bird pollination. Current Biology 32: R1059–R1060; doi: 10.1016/j.cub.2022.06.081

Karl P Olson & Laurence D Marks, 2024. What Puts the “Tribo” in Triboelectricity? Nano Lett 24(39): 12299–12306; https://doi.org/10.1021/acs.nanolett.4c03656

Constance J Tremlett et al., 2020. Pollination by bats enhances both quality and yield of a major cash crop in Mexico. Journal of Applied Ecology 57(3): 450-459; https://doi.org/10.1111/1365-2664.13545

Yiftach Vaknin et al., 2000. Are flowers morphologically adapted to take advantage of electrostatic forces in pollination? Plant Syst Evol 222:133-142; https://doi.org/10.1007/BF00984099

Yiftach Vaknin et al., 2001. Are flowers morphologically adapted to take advantage of electrostatic forces in pollination? New Phytologist 152(2): 301-306; https://doi.org/10.1046/j.0028-646X.2001.00263.x

Mario Vallejo-Marín, 2019. Buzz pollination: studying bee vibrations on flowers. New Phytologist 224(3): 1068-1074; https://doi.org/10.1111/nph.15666