Sexual deception in orchids

Alfred, Lord Tennyson once said “In the spring a young man’s fancy lightly turns to thoughts of love”, but he could have said the same for insects too. Male insects will be following the scent of females, looking for a partner, but not every female is what she seems to be.

It might look like the orchid is getting some unwanted attention in the video below, but it’s actually the bee that’s the victim. The orchid has released complex scents to fool the bee into thinking it’s meeting a female. Why would an orchid do this? It’s because the orchid is hoping to reproduce, and this is a cunning ploy to share pollen with other orchids like itself.

We’re used to the idea that a flower provides food to insects to encourage them to carry pollen to other flowers. It works for many plants, but it isn’t always a good deal. Insects can pick up nectar from many plants, but pollen only works if it’s delivered to a plant of the right species. So a plant keeps having to produce sugars to feed insects in the hope that some of them with eventually find the right plant to feed on.

Some orchids don’t bother with all that effort. Instead of welcoming many insects, they’ve set their own messages to appeal to just one.

The orchids emit a perfume that smells like a pheromone from a female insect. This can attract males from quite a distance. When they’re close enough they can see the flowers. The orchids provide a landing pad that mimics a female. The combination of sight and smell pulls the males in. It works but it doesn’t work for every insect, and but that might work to the orchid’s advantage.

Everyone likes food, so a flower offering nectar can attract a lot of different kinds of insects. However to be attractive to thynnine wasps you really have to be a thynnine wasp yourself. The orchid Chiloglottis attracts only thynnine wasps, so it doesn’t get a huge number of visitors, but when a wasp lands it’ll be a thynnine wasp looking for love. When it’s disappointed it’ll fly off again looking for another partner and it’s not so likely to land on a random plant. It’ll either find a female wasp or another Chiloglottis. So while it might not get a lot of pollinators, the ones it does get are more likely to deliver the pollen to the right destination.

You might think this kind of thing would annoy an insect, and you’d be right. Scientists captured and marked wasps that visited sexually deceptive orchids to see how they reacted to the flowers. They found that after a short while, the wasps learned to avoid the area and looked for females elsewhere. However, they also found that the next day the wasps would be back again, showing short-term, but not long-term learning. Either their brains weren’t capable of learning, or else when they got a whiff of scent the next morning they stopped thinking with their brains.

It’s the appeal to a specific insect that has helped drive diversity among orchids. If you mutate to produce more than one scent, then you might attract a second pollinator, but if that pollinator hits another orchid with a similar mutation then they could produce a flower that appeals to the new insect and not the old pollinator. Once that change in scent happens, the pollinators will isolate DNA between orchids, by only visiting one sub-group. It seems that even a small shift in chemistry can be enough to create a barrier between species.

It means that specific orchid species can be limited by their pollinators. After all, they cannot be pollinated in places where their pollinators don’t live. So it seems sexual deception ties a plant very closely to the insect it exploits. Despite this, sexual deception must be a good idea as it has independently evolved many times.

It seems that orchids in the Mediterranean were able to sexually exploit insects around ten million years ago, with a combination of chemicals and developing more complicated petals, like the labellum, which is effectively the landing pad for an insect. Independently, in Australia, they’ve evolved to exploit quite a few insects. Most scientists found even fungus gnats were good enough pollinators for one orchid to exploit. Such is the success of deception in Australia that some scientists have recently proposed that the combination of isolation and climate makes the continent a ‘perfect storm’ for deception.

But while sexual deception for orchids might be a free ride, it’s not always easy. Making the complex chemicals to lure insects takes a lot of complicated chemistry. Recently scientists found that the visible spectrum wasn’t enough for Chiloglottis orchids. To make their perfume, they need UV-B.

UV-B is in the ultraviolet range of the electromagnetic spectrum. It’s the part of sunlight that gives you a rich tan or, if you skimp on the sunscreen, the red glow of sunburn. A lot of it is blocked by the ozone layer, but the fraction that comes through is essential for orchids to make chiloglottones, the chemicals that attract insects. However, UV-B light is part of sunlight. While we can walk into the shade, a plant cannot. Making the scent needs a lot of resources that the plant cannot afford to waste, so how can the plant avoid making the perfume until it is ready? The answer is in the colour of its petals.

The colour comes from how the petals reflect light. We see the light that gets reflected, so not all light is treated equally. When it comes to UV-B, the petals block it. Tests on wavelengths showed that the petals were particularly good at blocking the short-wavelengths of ultraviolet light. The labellum, the landing pad that makes the scent, only sees ultraviolet when the flower opens up. The plant doesn’t waste any energy making chemicals to attract an insect to a closed bud.

It’s easy to think of plants as simple and passive. However, they are not passive at all, they simply live on a different timescale. Evolutionary biologists love orchids because their diversity shows that plants are as capable of exploiting any niche as anything else alive. They are extraordinarily diverse. There are more species of orchid than there are of birds and mammals combined. Nor are they simple. They’ll use anything they can to get an advantage over their competitors, synthesising fantastically complex chemicals with ease that an industrial chemist can only envy. The sexually deceptive orchids have combined their traits to create an irresistible lure that ensures their reproductive success. The unfortunate insects on the other hand will have to make a bit more of an effort.

Featured image credit: “Bee Orchid – Ophrys apifera”, by Björn S. CC-BY-SA-2.0 via Flickr.