The narrow-leaved helleborine orchid, also known as Sword-leaved Helleborine (Cephalanthera longifolia), produces almost no nectar or pollen, and yet it attracts bees to pollinate it.
How does it do it? Athayde Tonhasca explains.
The answer is by trickery: a small section of the flower is coloured yellow-orange. A passing bee from the genera Halictus or Lasioglossum (these are small wild bees), mistaking the yellow splotch for pollen, zeroes in. The orchid’s pollinium (a mass of pollen grains that are transferred as a single unit) sticks to the hairs on the bee’s back and it is then transported to another orchid, thus pollinating it.
Orchids are one of the largest families in the plant kingdom, with over 28,000 species worldwide; that’s more than all bird and mammal species combined.
But unlike common flowers, many orchids attract pollinators without offering any pollen or nectar rewards. They rely instead on ruses: some orchids produce flowers that look like female insects, usually bees or wasps. Unsuspecting males are attracted and attempt to mate with them. In doing so, they accidentally attach pollen to their bodies, which fertilizes the next orchid they visit. Some species produce scents that mimic sex pheromones.
This video shows a striking example of this form of deception: a solitary bee, lured by chemical compounds that smell like a pheromone from a female insect, attempts to copulate with the flower.
Other orchids, such as the Narrow-leaved Helleborine, rely on food deception: they falsely advertise pollen or nectar by the shape of their flowers, colour, scent, or pollen-like structures.
Floral deception in orchids has puzzled biologists including Charles Darwin, who found it difficult to explain the evolutionary advantages of flowers with no food rewards.
Plants that produce pollen and nectar attract a wide variety of food-seeking pollinators – mostly bees and other insects – while deceptive orchids attract only males of one or a few species. However, research has shown that these plants have higher pollen transport efficiency than plants with multiple pollinators; less pollen is lost or deposited in flowers of the wrong species.
This strategy must be evolutionarily advantageous, since approximately one-third of all orchids are believed to deceive insect pollinators one way or another.
For more on the pollination of the Narrow-leaved Helleborine, see The pollination of European orchids.