Close Encounters

Some pollen hitches a lift on butterfly wings to travel between plants

There's not a lot on a butterfly, if you ignore the wings. That's why some plants keen on pollination target them.

Two interesting papers have come out recently that share a common author, and a common pollinator. Work by Steven Johnson and colleagues has been looking at butterflies as pollinators. It’s known that butterflies can act as pollinators, but there’s not a lot of opportunity to place pollen on legs or bodies of butterflies. They’re mainly wing. What these two papers look at are plants that have developed to use the wings on butterflies to carry pollen.

Hannah Butler and Steven Johnson report on butterfly-wing pollination in Scadoxus and some other South African plants in the Botanical Journal of the Linnean Society. The authors studied Scadoxus multiflorus, sometimes known as the ‘blood lily’, that grows in eastern South Africa. They examined to see if the plants were self-compatible and then also observed butterfly visits. 

Scadoxus multiflorus. Image: BetterWorldPhoto / Wikipedia

“Because much of the pollen on butterfly wings is lost when they are netted and handled, we used high-resolution (16.2 mega pixels) macrophotography of butterflies visiting flowers to assess the distribution of individual pollen grains on wings of butterflies visiting flowers of S. multiflorus subsp. katherinae, Brunsvigia marginata (Jacq.) W.T.Aiton (n = 5 photographs), Cyrtanthus elatus (Jacq.) Traub (n = 3 photographs) and Clivia miniata (Lindl.) Verschaff. (n = 6 photographs),” write Butler and Johnson.

“For each plant species we captured a sample of butterfly visitors and, using microscopy, confirmed that the pollen visible on their wings was from the species on which it was observed.”

Pollen distribution mapped as percentages on the ventral wing surfaces of butterfly visitors. Source Butler and Johnson 2020, Bot J Linn Soc.

Results showed the wings of the butterflies were well dusted with pollen. The authors were not surprised, and mention that the flowers, looking like brushes look like they’re specialised to use butterfly wings to pollinate plants. But it’s not just the flowers that are adapted for wing pollination. Butler and Johnson say the pollen also shows signs of adaptation. “The relatively flattened shape of the pollen grains may increase their adhesion to the corrugated surface created by the scales on the surface of the butterfly wing.”

Another recent paper by Ryan Daniels, Steven Johnson and Craig Peter finds evidence of butterfly-wing pollination in another plant, Gloriosa superba. G. superba, the Flame lily, is another plant with an elaborate shape. The flowers are divided into five male meranthia and one hermaphrodite meranthium, these being pollination units of a flower.

Gloriosa superba. Image: Canva.

“Based on previous reports of visitors to Gloriosa flowers and our preliminary field observations, we hypothesized that G. superba is pollinated by butterflies and that pollen is transferred on their wings. The style of G. superba often appears orientated towards open spaces in the vegetation. We hypothesized that this is a developmental adaptation that promotes cross-pollination by increasing the probability that butterflies will settle first on a hermaphrodite meranthium,” write Daniels and colleagues.

The team performed a number of experiments on self-compatibility and visitor observations to find out how butterflies were interacting with the flowers. They then correlated their findings with citizen science observations on LepiMAP and iNaturalist. This highlighted Eronia cleodora, the vine-leaf vagrant, as the likely key pollinator for the flower.

Eronia cleodora. Image: Purves M. / Wikipedia

“From field observation and collected specimens we observed pollen on the underside of the wings of the visiting butterflies, particularly the pierid E. cleodora. We further documented clear evidence of regular contact of their wings with the stigma and anthers when feeding and we found a positive correlation of pollen grains and wing scales on stigmas… This supports the likely role of butterflies in pollen transfer, as has been shown for other butterfly-pollinated species. Citizen science records… further indicated that pierid butterflies of similar sizes may be important throughout Africa as they have been frequently observed on flowers of G. superba,” write the authors.

Again, it’s not just the flower that has adapted to pollinate on the wing. Daniels and colleagues find evidence of adaptation in the pollen too. In particular, in the pollenkitt, the outermost hydrophobic lipid layer of the pollen grain. “Pollenkitt has numerous possible functions,” say the authors. “In entomophilous species there is increased viscosity of the pollenkitt and an increased presence of pollenkitt on the exine, while in anemophilous species pollenkitt is often between columellae instead of the exine. This is likely related to its role in adhesion to the pollinator. The adhesive force between neighbouring pollen grains in G. superba, possibly facilitated by the exine microstructures observed, may cause clumping, which could collectively contribute to adhesion to the wings.”

This stickiness means that when the butterflies are lured to the flower by the colour and promise of nectar, the pollen can attach to the wings as the insect beats them. The elaborate shape of the plant peppers the wings with pollen, that can travel to another flower. Getting a lift means that the pollen is much more likely to be able to outcross with another plant, and not be wasted in self pollination.

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