Beyond Bee Books: Electrifying pollination

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Dave Blacks blog prefix
Of the many ways plants attract pollinators to their blooms the production of scent might be the most complex. Just as the formation of a flower or the secretion of nectar impose a resource drain on the plant the manufacture and release of volatile odours must be costly. Some plants seem to exercise little to no control over this, tobacco and petunia release their fragrance to a nocturnal rhythm; snapdragons smell in the daytime, hopefully when their pollinators are most likely to be present. These rhythms don't care if it's day or night. Clover flowers too emit scent in a rhythmic manner but it is controlled by the light and dark intervals rather than by an intrinsic clock. Release begins 7–12 h after a light is switched on and the quantity increases as the temperature warms. Light influences fragrance emission significantly, the most intense emission happening at high light levels. Other plants are a little more sensitive to the presence of their visitors. One of the Evening Primroses was shown to 'listen' to the specific sound of approaching pollinators and increase the sugar concentration of its nectar. It didn't have to be real, the researchers could trigger the same response with a recording of the right frequency. It took less than three minutes.

Clara Montgomery from the School of Biological Sciences in Bristol wondered whether is was possible pollinators could go further and provide direct evidence of their presence to a plant. Other studies had previously shown that plants will respond to being eaten by producing bitter compounds to discourage the practice, even warning adjacent plants by releasing volatile organic compounds in to the air. She wondered whether an electric charge carried by a pollinator could alter a plants behaviour.

Since the 1960s it was apparent that insects carried a small electrical charge as a result of friction with the air or surfaces that they travelled over. In 1975 Eric Erickson (who came up with the small-cell varroa notion) was able to measure a voltage on bees. In 1991 Colin, Richards, and Chauzy showed the magnitude of the charge in various circumstances for honeybees using a sensor originally designed to measure the charge (in coulombs) on raindrops. Almost always positively charged (93%), bees had a static charge usually around 150pC but between -400pC to 600pC, more than enough to attract pollen grains. Larger bumblebees can sustain a higher charge, worth measuring in nano-coulombs (nC). While its better studied in bees for reasons we all understand it's not unique to bees and other pollinators are charged too.

In what I think looks a clever and well controlled experiment published recently Montgomery and her colleagues used bumblebees from Koppert acclimatised to foraging in a perspex 'arena' on petunias and snapdragons. They were able to measure the electrical charge on the bees, measure the 'headspace' volatile chemicals above the flowers, and, in the case of the petunias) show an increase in the amount of these in response to the bees' visit or to stimulation by an electostatically charged rod. They also managed to measure the bee's antenneal response to work out which of the volatile chemicals the bees were responding to. I'll let them explain what they found:

The results presented here show for the first time that repeated visits by B. terrestris augment the emission of pollinator-attractive volatiles in P. integrifolia in a laboratory environment. Many plants modify their volatile emissions in response to biotic stresses such as predation, as well as environmental factors such as light and temperature, but we show for the first time here that plants may use cues provided by their pollinators to modulate their emissions of attractive scent... Therefore, in addition to increasing pollinator attraction and achieving greater pollen dispersal, direct detection of pollinators may reduce the risk of attracting folivores and herbivores by benzaldehyde. In effect, the direct detection of pollinators, using electric charge sensitivity or other cues such as pollinator specific vibrations, could offer more reliable prediction of pollinator phenology than more correlational parameters such as temperature or luminosity, which are strongly affected by weather... Attracting strongly charged pollinators has an additional reproductive benefit to the plant: charged pollinators create an electric field between plant and pollinator, which encourages the bidirectional transfer of pollen through the air due to Coulomb force... Electric charge holds adaptive value for pollinators by increasing pollen attraction and adhesion and allowing sensing of electrostatic cues. As flight has been shown to contribute to charge generation in insects, flying pollinators may have a greater electric charge than less aerial and agile herbivores. We therefore propose here that, as pollinators are found to be consistently electrically charged, the detection and use of charge as an indicator of pollinator abundance has adaptive value for entomophilous plants.

Regulation of Circadian Methyl Benzoate Emission in Diurnally and Nocturnally Emitting Plants, Natalia Kolosova, Nina Gorenstein, Christine M. Kish, Natalia Dudareva. (2001) The Plant Cell, Volume 13, Issue 10, pp 2333–2347, Validate User

Jakobsen, H.B., Olsen, C.E. Influence of climatic factors on emission of flower volatiles in situ. Planta 192, 365–371 (1994).

Marine Veits et al, Flowers respond to pollinator sound within minutes by increasing nectar sugar concentration. (2019) Ecology Letters, Volume22, Issue9 1483-149. Error - Cookies Turned Off.

Clara Montgomery et al, Bumblebee electric charge stimulates floral volatile emissions in Petunia integrifolia but not in Antirrhinum majus. (2021) Naturwissenschaften, The Science of Nature, 108:44.

E. H. Erickson, Surface Electric Potentials on Worker Honeybees Leaving and Entering the Hive, Journal of Apicultural Research (1975)
Volume 14 - Issue 3-4, pp141-147.