Newsflash: Plants are pollinated by insects

Image information: This image of an Andrena bee gathering pollen from the stamens of a rose by Debivort is licensed under the Creative Commons Attribution-Share Alike 3.0 Unported license.

Yes, plants are pollinated by insects*. And, surely, that’s something that everybody who reads posts about plant biology on the Botany One web log will know? I’d certainly hope so. In which case, they may well ask, is that announcement really news? No [as already explained]. And yes: That headline is not a statement of fact about present-day flowering plants, but is intended to alert the reader that the nature of the organism that pollinated the very first flowers on the planet has been announced.

Research published in 2023 by Ruby Stephens et al. concludes that the first pollinators of angiosperms (flowering plants) were “most likely insects” (Ruby Stephens et al.) [as a science-based study, and without the ability to go back to those long-distant times and observe what was actually doing the pollinating, that is the best conclusion one can hope for]. The team do speculate about the particular insect that might have been involved: “The first flower pollinators must have been quite small, too, to poke around in these flowers. The most likely culprits are some kind of small fly or beetle, maybe even a midge, or some extinct types of insects that have long disappeared” (Ruby Stephens et al.).

Although the conclusion, that angiosperms were “ancestrally insect pollinated”, is arguably the main take-home message from that study, it considered broader questions about pollinators and looked at data on the pollination agents of 1160 plant species collated from the primary literature. Other key findings are that: insects have pollinated angiosperms for approx. 86% of angiosperm evolutionary history; vertebrate pollination evolved at least 39 times from an insect-pollinated ancestor (with at least 26 reversals); and wind pollination evolved at least 42 times (with few reversals to animal pollination). Overall, though, Stephens et al‘s work highlights the long history of interactions between insect pollinators and angiosperms, which is still vital to biodiversity today.

Continuing the theme of insects and plant-pollination, much is made about the importance of bees and butterflies in this regard (e.g. Susan Albert, here, here, here, here, here, here, and here). But, what about the roles played by other pollinating insects, how important are they? Adding to that conversation is an important study by Max Anderson et al.** that examines the role of nocturnal moths in plant pollination. This study was not as wide-ranging as Stephens et al‘s, and concerned – arguably – a single taxon, Rubus fruticosus (Stephen Harris), known as the bramble (Chris) or blackberry in Europe.

However, although taxonomically-constrained, and most importantly, Anderson et al‘s work underlines the point that any truths you may discover about biological phenomena are only relevant to the particular circumstances of the investigation [or, you only find what you’re looking for…]. For instance, if you only study pollination during daylight, you’ll almost inevitably conclude that day-flying insects – such as bees and butterflies – are very important pollinators. Or, as Fiona Mathews & Max Anderson put it: “Almost all scientific research on pollinators happens during the day, which means we know little about what happens at night. So we designed a study to compare the contribution of nocturnal and day-active pollinators”. As a result they showed that flower visitation was significantly higher during the day, by a range of taxa; nocturnal visitation was almost exclusively by moths. But, and crucially, pollen deposition rates of bramble [i.e. transfer of pollen between flowers, from anthers to stigmas] were significantly higher during the night compared with the day. Which leads to the conclusion that moths are more efficient pollinators of bramble compared with diurnal insects. And is a conclusion that’s only possible because of the way in which the study was carried out.

However, whilst that night-time activity is good news for the plant, the downside is that there may not be sufficient moths around to do the necessary, as Fiona Mathews cautions “Sadly, many moths are in serious decline in Britain, affecting not just pollination but also food supplies for many other species ranging from bats to birds”. But, if brambles can be left to flower – another proposal from the work of Anderson et al. – rather than cut down and removed as ‘weeds’ as they so often are in nature, the blooming brambles “can provide important food sources for moths, and we will be rewarded with a crop of blackberries. Everyone’s a winner!“.

But, which bramble species was investigated by Anderson et al.? Famously, and as in this study, bramble is often just identified as Rubus fruticosus*** ‘aggregate’. Use of the term ‘aggregate’ – usually abbreviated as ‘agg.’ as here – acknowledges the fact – as do the authors of the study – that there are hundreds of so-called microspecies of R. fruticosus (John Norton). It would be interesting – and rather useful – if the microspecies examined in this work was/were identified by a bramble specialist, a batologist. If more than one microspecies is identified, that information might give greater insight into which moths are associated with which particular microspecies [Ed. – and hopefully does not invalidate the original conclusions from the work].

Finally, let’s not forget that the insects aren’t only doing the plants a favour with this pollinating, they’re not winged botanical benefactors. They visit the flowers for a tangible reward – usually food, which may be nectar, something else produced by the plant such as oils (Beryl Simpson & John Neff; Anahi Espindola), or the pollen itself [or sexual gratification in the case of some insects that visit orchids and engage in pseudocopulation (Oakes Ames; Jerry Coyne)…]. From the insect’s perspective, pollination is just an unintended consequence of its foraging visit. Whatever the reason for these wanted invertebrate attentions, the plant’s ‘hope’ is that enough of the pollen isn’t consumed by the visitors and survives to be transferred to another flower to effect pollination, and – hopefully – fertilisation**** to perpetuate the species into the future.

* And not just insects. In a 2023 report, Ze-Yu Tong et al. estimate that 90% of all angiosperm species are animal-pollinated. If we accept the unsourced numbers provided by The National Wildlife Federation, “Over 100,000 invertebrates—including bees, butterflies, beetles, moths, wasps, and flies—and more than a thousand mammals, birds, reptiles, and amphibians take on the job of pollinating plants”.

** For completeness, it is worth mentioning that the work of Anderson et al. adds to that previously published by Richard Walton et al. and Callum Macgregor & Alison Scott-Brown on the importance of nocturnal pollination by moths.

*** Although this seems to be a nice piece of work, there’s no excuse for spelling the studied plant species’ specific epithet incorrectly in the title: Marvellous moths! pollen deposition rate of bramble (Rubus futicosus L. agg.) is greater at night than day. Whilst that error is perpetuated in the journal’s recommended citation of the article, it is spelt correctly – fruticosus – in the Abstract, and throughout the paper. Which reminds me of an occasion many years ago when xylem was spelt zylem in the title of a Plant Physiology paper listed in the journal’ issues table of contents – rather annoyingly I never made a copy of that to provide chapter-and-verse for this anecdote.

**** It’s always worth reminding whoever needs to be reminded that pollination is not the same as fertilisation. Getting the pollen to the female part of the flower is only part of the journey involved in angiosperm sex and production of the next generation.

READ THE ARTICLES

Ames, O. and Ames, B. (1937) “Pollination of orchids through pseudocopulation,” Botanical Museum Leaflets, Harvard University, 5(1), pp. 1–xix. Available at: http://www.jstor.org/stable/41762973.

Anderson, M., Rotheray, E.L. and Mathews, F. (2023) “Marvellous moths! pollen deposition rate of bramble (Rubus futicosus L. agg.) is greater at night than day,” PLoS One, 18(3), p. e0281810. Available at: https://doi.org/10.1371/journal.pone.0281810.

Macgregor, C.J. and Scott-Brown, A.S. (2020) “Nocturnal pollination: an overlooked ecosystem service vulnerable to environmental change,” Emerging Topics in Life Sciences, 4(1), pp. 19–32. Available at: https://doi.org/10.1042/etls20190134.

Simpson, B.B. and Neff, J.L. (1981) “Floral Rewards: Alternatives to Pollen and Nectar,” Annals of the Missouri Botanical Garden. Missouri Botanical Garden, 68(2), p. 301. Available at: https://doi.org/10.2307/2398800.

Stephens, R.E., Gallagher, R.V., Dun, L., Cornwell, W. and Sauquet, H. (2023) “Insect pollination for most of angiosperm evolutionary history,” New Phytologist. Available at: https://doi.org/10.1111/nph.18993.

Tong, Z.-Y., Wu, L.-Y., Feng, H.-H., Zhang, M., Armbruster, W.S., Renner, S.S. and Huang, S.-Q. (2023) “New calculations imply that 90% of flowering plant species are animal-pollinated,” National Science Review. Available at: https://doi.org/10.1093/nsr/nwad219.

Walton, R.E., Sayer, C.D., Bennion, H. and Axmacher, J.C. (2020) “Nocturnal pollinators strongly contribute to pollen transport of wild flowers in an agricultural landscape,” Biology Letters, 16(5), p. 20190877. Available at: https://doi.org/10.1098/rsbl.2019.0877.