view gallery The dwarf hammer orchid are each pollinated by males of a single species of thynnine wasp. The male wasps – like this one above – are drawn to specific odours emitted by the orchids, which cleverly imitate the sex pheromones of receptive female wasps. Image Credit: Esther Beaton

Orchids: The coy seduction

  • BY Emma Young |
  • December 16, 2014

More so than in any other country, our orchids are deceitful temptresses, with powerful perfume and come-hither good looks.

A LONE WASP PICKS up on an exciting scent. Instantly he zigzags, following
the pheromone trail until he glimpses his target, 30cm away. Its allure is overwhelming. He flies straight at it and grasps it. But five other males have picked up the same scent, and they push and shove each other, competing to mate. At last, the first wasp gets a strong grip. He has the target right where he wants it, and he ejaculates.

Unfortunately for him, he hasn’t mated with a female wasp – but with a flower. Most orchids are pollinated in the traditional way; they produce attractive flowers to advertise the presence of nectar, and when insects visit to drink the offering, they brush up against the pollen and transfer it.

In another form of deception, about 30 per cent of orchids produce stunning flowers but then don’t go to the trouble of producing nectar, so visiting insects complete the pollination job without any reward. A few have taken the level of deception to an extreme, employing sophisticated sexual trickery – and Australian orchids are the queens of seduction.

Orchids cleverly deceiving wasps

THERE ARE 26,000 species worldwide in the orchid family. In Australia, about 1400 of them thrive in wet habitats around the margins of the landmass, with the biggest diversity in the south-west and south-east.

Europe holds one or two groups of orchids that imitate female insects to lure pollinators; South Africa is home to a single group that does the same. But in Australia, at least 250 species in some 10 genera have adopted the strategy, deceiving male insects – mostly wasps – into believing that they’ve found an elusive female. Hammer orchids, spider orchids, flying duck orchids and elbow orchids all use the same devious approach.

Most Australian orchids that hoodwink hapless males in this way are pollinated by a group of wasps known as thynnines. The female wasps are dumpy, flightless creatures that spend much of their adult lives underground, laying eggs on beetle larvae in the soil. The males are fast-flying and large, with a wingspan of up to 5cm. Many thynnine wasps are black, but others are spectacularly coloured, with combinations of black, yellow, red and orange markings.

When a female thynnine is ready to mate, she crawls out of the ground and releases a pheromone to attract males. “There aren’t many females around at any one time,” says Dr Ryan Phillips, a research scientist at the Botanic Gardens and Parks Authority in Perth, who studies sexual deception among West Australian orchids. “So when one does come up, the males descend upon her in this massive scramble of wrestling wasps... It’s just like that with the orchid.”

Orchids have evolved to release chemical copies of the females wasp’s pheromones – and it can be irresistible. Back in 1928, an Australian amateur naturalist, Edith Coleman, became the first to realise that scent was key when it came to attracting the males.

“She saw them flying in through her window and trying to copulate with the orchids on her kitchen table,” Ryan says. “She realised that scent was working over a long distance... When I’ve had orchid specimens in the car, I’ve had wasps fly in through the window when I was parked at the lights.”

Once an orchid has lured a pollinator close, the colour of the petals and ultraviolet spots can make the flower hard to resist. When a male lands on the flower, its shape ensures he grips it in the right position to make contact with the pollen.

Some orchids make absolutely certain the wasp does the job. The main lip of the flower of a hammer orchid is an insect-shaped blob hinged partway along. When the wasp grabs the flower, the momentum flips him upside down and whacks him into the pollen.

Most Australian orchids that mimic female insects have their own specific pollinator: as a general rule, it’s one species of wasp to one species of orchid. But there are exceptions.

One hammer orchid is unusual in that it shares a pollinator with a species of big, gaudy spider orchid. And there are the five different-looking Australian tongue orchids (the Cryptostylis genus) that all share a single pollinator species – a wasp called the orchid dupe. It is one of the few Australian sexually deceptive orchid groups not pollinated by a thynnine wasp.

When Dr Anne Gaskett of the University of Auckland, New Zealand, who has studied the group extensively in NSW, Victoria, WA and New Zealand, analysed the orchids from this genus, she discovered ultraviolet spots that were invisible to the human eye, but which resemble the UV reflected from the wings of female wasps.

“To our eyes, C. erecta has pink and white stripes and looks a bit like a satellite dish,” she says. “C. leptochila is burgundy with a plush, velvety surface with glistening black spots and looks like a narrow, curled tongue.”

To the wasps, the flowers would appear similar. And while the overall shapes of the tongue orchids are different, up close, they all have the small “grips” or “love handles” which mimic the shape of the female wasp.

Regular  AG contributor, photographer Esther Beaton, who has travelled with Ryan to orchid sites around the Margaret River region of WA and who has photographed Cryptostylis in the grounds of Macquarie University, Sydney, has long been fascinated by the pollinators.

“Each orchid is so complex and magical to look at,” she says. “Then the wasp flies in and you watch them grappling with the flowers. You bend down, and a new world opens up.”

Using her cameras to magnify this micro world, Esther recently made a new scientific discovery. In January, she went with two amateur orchid enthusiasts to an Aboriginal site called Bulgandry in the Brisbane Water National Park near Gosford, north of Sydney.

“I saw this orchid with half-a-dozen blooms along its stem, and I noticed little white specks on it,” she says. They were barely visible to the naked eye. But as she watched them through her camera, she saw they were moving. When she  later enlarged the pictures (opposite), she realised that she’d photographed tiny flies with what looked like twin yellow plumes on their heads.

She sent the pictures to Dr Dan Bickel, an entomologist at the Australian Museum in Sydney. Dan confirmed the plumes were pollen, being carefully carried on the heads of eye gnats, which belong to a family known as chloropid flies. The particular orchid, the fringed midge orchid, was known to attract flies. But now it seems that these tiny flies actually pollinate it.

Chloropid flies are known to pollinate certain orchids in South America and South Africa, and the species that attract them produce strong decay-like smells, such as rotting meat and smelly socks. The larvae of the flies develop in dying organic material, which is why they’re attracted to foul smells. Esther can attest to the stink of the fringed midge orchid.

“It has a slight citronella tang overlaying an odour of…well…toilet.”

The ecological interaction between chloropids and orchids, as revealed by Esther’s photographs, is exciting because it is so rarely documented, says Dr Terry Wheeler, director of the Lyman Entomological Museum at McGill University in Canada.

“This is also a great example of how photographers and other naturalists can provide valuable data to research scientists,” he says.

Ryan agrees. “This highlights how little is known about the pollination of even some of our most common orchids,” he says. “It also shows there are plenty of opportunities for discovery – not just by scientists but by interested amateur naturalists.”

 

The full story can be found in Australian Geographic #110.