By Athayde Tonhasca
The broad-leaved helleborine (Epipactis helleborine) is found throughout UK and much of Europe and Asia in all sorts of habitats, including urban and disturbed areas. This orchid was introduced to America, where it is viewed as an invasive species in some places. Despite its common occurrence and being the source of a reasonable supply of nectar, the broad-leaved helleborine is often ignored by insects, a fact noted by Charles Darwin.
The orchid’s small, inconspicuous, greenish/purplish flowers are not exactly good marketing for attracting bees and other pollinators. But one group of insects are keen visitors: wasps, in particular the European (Vespula germanica) and the common wasp (V. vulgaris).
Adult wasps feed mostly on carbohydrates, which they get from nectar – or from your sugary drink, if you give them a chance. But the nectar of broad-leaved helleborines is special: it’s laced with chemical compounds, some of them with narcotic properties. It also contains ethanol and other alcohols, possibly as the result of fermentation by yeasts and bacteria. This chemical cocktail is toxic or repellent to many visitors, but not to wasps: they lap it up. Unavoidably, a concoction of opioid and morphine derivatives plus alcohol, even in minute amounts, has consequences for its consumers. Wasps become intoxicated and sluggish after a few sips, which suits the orchid very well. They spend more time on the flower, staggering about and thus increasing their chances of ending up with a pollinium (a sticky mass of pollen grains) glued to their heads. Watch it. Nobody knows if wasps are hungover afterwards.
This orchid has another trick up its sleeve besides inebriating nectar; it also lures wasps with false promises of prey for their larvae. It turns out that these flowers release chemicals that mimic green-leaf volatiles, which are produced by plant tissues when they are damaged by herbivores. Wasps are attracted to green-leaf volatiles in the hope of finding some juicy caterpillars chomping on the host plant. When a wasp gets to the flower, its attention is diverted to the sugary nectar, so the scent scam is forgotten.
Orchids are highly diverse: with approximately 25,000 described species, they make up about 10% of all flowering plants. About one third of orchids do not offer food rewards – nectar or pollen – to visiting pollinators. Instead, they have evolved all sorts of tricks to attract insects. Some flowers have the shape, colours or scents of food-rewarding plants; they may bait male insects by resembling female counterparts, or by releasing pheromone mimics; sometimes they charm visitors that are seeking a place to lay their eggs. Or by using a combination of artifices, as it is the case of the broad-leaved helleborine.
These deceiving orchids attract only a handful of insects that respond to specific chemical or visual cues, so many potential pollinators are excluded. But the strategy pays. Pollen is transported more efficiently for deceptive species than for those with multiple pollinators. This means that more pollen is taken to another flower of the same species, and less is dropped or deposited on the wrong flower.
Deception works for orchids, but how about their cheated visitors? Sometimes they are rewarded, but often they get nothing. We don’t have much information about the insects’ side of this relationship. They must benefit somehow, or at the very least they are not significantly harmed. This matters to wasps, as they pollinate around 5 % of all known orchid species.
Orchids provoke much fascination for their biology, diversity and exoticism. This level of attention has helped us appreciate better the role of wasps. Most of them don’t collect pollen, and their lack of body hairs – compared to bees – does not allow for many pollen grains to attach to their bodies. But if we go by their contribution to orchids’ reproduction, these important but often maligned insects have much to reveal about their part in pollination services.