Sexually deceived by orchids? Well I never!
A sexually deceptive
Photo by Anne Gaskett

It's all very well for women to use their seductive wiles to attract and snare men, but now itís the orchidís turn. Known for their beauty, smell and elegance, orchids are admired by humans and insects alike. But they also have a deviant side - they are sexually deceptive, using their charms and perfumes to lure and entice male insects to mate with them.

One particular Australian wasp is a little fellow that is so captivated and sexually stimulated by 'Orchid Fever' he ejaculates while pollinating the flower, leaving female wasps wanting but the orchid happy and pollinated.

According to research at Macquarie University in Sydney, Australian Tongue orchids lure male insects with counterfeit sex signals that mimic those produced by females. But it's the hapless male Orchid Dupe wasps (Lissopimpla excelsa) that can't resist mating with the orchid flowers and are lured into becoming pollen couriers.

Until recently, this trick was not thought to harm the insect Romeos, but Macquarie biologists Anne Gaskett, Claire Winnick, and Marie Herberstein, have discovered that male wasps visiting Australian Tongue orchids waste thousands of sperm on the flowers.

"If males waste all their sperm on orchids, what have they got to offer a real female?" asked Gaskett, a PhD student whose research paper on the study was recently published in American Naturalist (volume 171, June) and is the first conclusive evidence of insect ejaculation in response to floral stimuli. "These pollinator species could suffer considerable reproductive costs if orchids inhibit mating opportunities," she continued. "This is particularly true if the pollinating male wasps prefer having sex with the orchids instead of with a real female, or be unable to find real female mates due to the confusion created by false orchid signals."

The researchers also believed that the cost of sperm wastage could encourage the male wasps to avoid the orchids and in turn make them more aggressive in their seductive wiles to attract the wasps, creating an "arms race between pollinator learning and escalating orchid mimicry". Gaskett explained that most flowers attract and reward their pollinators with nectar but these sexually deceptive orchids lure their pollinators with counterfeit signals that mimic those of female insects.

"When a male insect is fooled into gripping or copulating with an orchid flower, the pollinia stick to his body which he then delivers to other orchid flowers during subsequent deceptive liaisons," she said.

Professor Anne Gaskett 
Photo by Julia Cooke

The reasons why male insects are attracted to the orchids has been well studied with major discoveries indicating that floral scents that mimic species-specific female-insect sex pheromones are the main attractant. However the reasons for the evolution and maintenance of the orchidís sexually deceptive pollination signals and processes needed further exploration.

To investigate this issue the team of Macquarie researchers performed a worldwide literature survey of about 200 of these sexually vulnerable insects on a variety of sexually deceptive orchid species and conducted research on the behaviour of 104 Orchid Dupe wasps that approached two species of Australian tongue orchids, Cryptostylis erecta and Cryptostylis leptochila.

Overall the researchers found that more than 90 per cent of these duped pollinators were from species with a haplodiploid mating system - which means that males from these species have only one set of chromosomes. Extraordinarily, females from haplodiploid species such as wasps, bees and ants can actually produce offspring without sperm from males.

"Even without mating these females can still reproduce, however all the offspring will be male," said Gaskett. "These consequent extra male wasps could be important pollinators for orchids, and as long as some normal sexual reproduction still occurs, the cost of orchid deception can be mitigated." Interestingly, Australian orchids were shown to be global leaders in sexual deception. While other orchid species only needed the pollinator to grip or be briefly entrapped for effective pollination, these species induced energetic copulation from the wasps.

"Our observations of the Australian wasps during their liaisons with the two Australian orchid species found that the wasps energetically copulated with the orchids resulting in around 74 per cent ejaculating large amounts of sperm," she said.

"We also found that most of the wasps copulated readily with the first orchid but were less likely to copulate with the second and third.

"Overall it was clear that the Australian orchid species was the more sexually deceptive and despite the extreme demands they place on their pollinators, are incredibly successful by having the highest pollination rate ever discovered in a sexually deceptive orchid," Gaskett concluded.

A story provided by ANSTO's Velocity Magazine - science in motion and Macquarie University. This article is under copyright; permission must be sought from ANSTO to reproduce it.