To pollinate, a rare orchid takes advantage of beetles' libido

April 10, 2021
A rare orchid has a very creative way of handling its pollination. (Callan Cohen)

A rare orchid has a very creative way of handling its pollination. (Callan Cohen)

An extremely uncommon African orchid has been found to control, in a peculiar way, how its fellow flowers are pollinated: by releasing an aphrodisiac-like chemical that ignites the lust of a male longhorn beetle, consequently tricking the insect into engaging in sexual activity.

Some orchids are already known to use a similar sort of sexual deception on wasps, bees and flies, but the study published March 25 in Current Biology is the first to observe the phenomenon in the rare species Disa forficaria, and with a beetle.

"It was a chance discovery," said study author Callan Cohen, a research associate at the University of Cape Town's FitzPatrick Institute of African Ornithology, in an interview with The Academic Times. "There's only been one [of these plants] found in the last almost 70 years — only one individual plant."

Steven D. Johnson, a postdoctoral researcher at the University of KwaZulu-Natal in South Africa and another author of the study, emphasized a second striking fact: This is the first time any beetle, in this case, Chorothyse hessei, Cerambycidae, has been seen to take part in the sexual mimicry mechanism, "despite there being hundreds of thousands of species — probably millions."

That mechanism, in this case, begins with scent of the orchid, which carries a compound to which the male longhorn beetle is sexually attracted. Once the insect catches the aroma, the flower's trap is set. 

While the beetle copulates with the flower, it gets smeared by the orchid's pollen. That pollen is transferred to various places as the little bug flies around, making the orchid's mission of pollination a success.

"I went to look at the plant and see a flower, then I noticed the beetle coming to it and then pollinating it in this way," Cohen recounted. "That orchid itself had been a bit of a mystery because of its strange shape; as to what might pollinate it, it was completely unknown."

The unique pinkish, waxy plant actually resembles an insect. 

"The petals are sort of long and hairy, almost like insect antennae. That's something not found in any of its relatives," Johnson said. "The fact that the flower looked like an insect, and the beetle was mating with it, meant I immediately thought that there was something interesting going on."

To confirm Cohen's anecdotal observation that the beetle appeared to be mating with the flower, the researchers sought to find concrete evidence to confirm the story: ejaculate on the petals of the orchid. 

"The critical thing we needed to know was whether there was real evidence of mating, and that's why we had this long and hard search for trying to see if there was sperm on the flower," Johnson explained.

In the past, ancestors of this orchid used another form of trickery called food deception to enhance pollination practices. In that case, flowers pretend to offer sustenance to an insect to lure them onto the petals. Sexual mimicry, in which the flower pretends to be a mating partner, is believed to be a derivative of that.

"The process of genetic variation basically stumbled on the chemical sex attractants of insects," Johnson said. "Those plants that, by chance, produced these were immediately attractive to male insects and had greater pollination than others."

According to both researchers, orchids' tendency to use these tactics could be because it's difficult for them to pollinate normally. They store their pollen in sacs that have a very sticky end, meaning the positioning of an insect for potential pollination is crucial.

"If you think of a daisy, pollen just goes everywhere," Cohen explained. "But orchids have this very particular system, and that can lead to … orchids exploiting different pollinators, doing different things and preventing wastage of pollen." 

"It does explain why any kind of new chemical recipe that results in attraction of just one kind of insect would be particularly favored in orchids," Johnson added. "It allows this very precise mechanism of pollen transfer."

He also suggests that the natural waxiness of the plant may contribute to its excretion of the sex pheromone-like chemical. 

Once the researchers isolated the chemical that allows the orchid to seduce the beetle, they continued to test it in the laboratory to understand the precise nature of the compound at hand. That included its orientation, or chirality. 

"This is another quite special aspect of the studies," Johnson said. "It's very rare that people sort of synthesize a whole range of related molecules to work out just how sort of functionally specialized the system is."

They found that no matter the dilution of the flower's extract, the compound was just as effective. That means potency would matter only for the distance that the chemical can reach rather than its capability of attracting the beetle.

"This compound, even if you diluted the extract of the flower a thousand or even a million times, it still excited the antenna," Johnson said.

Cohen suggested that the team's findings could be used to aid in conservation efforts surrounding the scarce orchid. The method would entail using beetles with the flower's pollen on their bodies as "search devices" for the orchid. He says that technique, which was loosely performed in this study, would be a world first. 

"Using the chemical, we can actually attract the beetles in a given area," Cohen explained. "In doing so, we discovered that some of them actually carried the pollen of other orchids of the species that hadn't yet been found. So we know that there are at least two other individual plants out there." 

The paper, "Sexual deception of a beetle pollinator through floral mimicry," published March 25 in Current Biology, was authored by Callan Cohen, University of Cape Town; William R. Liltved and Jonathan F. Colville, South African National Biodiversity Institute; Adam Shuttleworth, Benny Bytebier and Steven D. Johnson, University of KwaZulu-Natal; and Jerrit Weissflog and Aleš Svatoš, Max Planck Institute for Chemical Ecology.

Correction: A previous version of this story incorrectly stated Callan Cohen's title. The error has been corrected.

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