Readers’ wildlife photos and stories: Toxic nectar!

Today we have another entomological tale from reader Athayde Tonhasca Júnior. The prose is his and the photos are credited (click to enlarge photos):

Toxic Nectar

In 401 BC, an army of Greek mercenaries led by Commander Xenophon crossed Anatolia (modern day Turkey) to seize the throne of Persia. Xenophon kept a diary of the expedition, entitled Anabasis, or ‘The March of the Ten Thousand’, which today is a classic of ancient Greek literature. Among many battles and other adventures, the commander described one curious episode. His troops came across a supply of honey, and some of the men went for it with gusto. In no time they regretted it: the soldiers could not stand up, and were assailed by bouts of vomiting and diarrhoea. ‘So they lay, hundreds of them, as if there had been a great defeat, a prey to the cruellest despondency. But the next day, none had died; and almost at the same hour of the day at which they had eaten they recovered their senses, and on the third or fourth day got on their legs again like convalescents after a severe course of medical treatment.’ (Anabasis, Book IV).

The Romans had their own taste of Anatolian honey, this time with grimmer consequences. In 97 BC, General Pompey the Great led an army across Turkey in pursuit of king Mithridates of Pontus, an old enemy of Rome. The local people, known as the Heptacomitae, withdrew. But they left a gift for Pompey’s men, possibly on Mithridates’ orders. The geographer and historian Strabo tells us what happened: ‘The Heptacomitae cut down three maniples [around 1,500 soldiers] of Pompey’s army when they were passing through the mountainous country; for they mixed bowls of the crazing honey which is yielded by the tree-twigs, and placed them in the roads, and then, when the soldiers drank the mixture and lost their senses, they attacked them and easily disposed of them.’

Strabo’s ‘crazing honey’ that incapacitated those Greek and Roman troops is known today as ‘mad honey’. It comes from nectar produced by the common rhododendron (Rhododendron ponticum), which is endemic and abundant in northern Turkey, the Black Sea region. This plant is full of grayanotoxins, a group of toxic substances that protect it against herbivores, but also accumulate in the nectar.

The bees that produce mad honey, the Caucasian (Apis mellifera caucasia) and Anatolian (Apis mellifera anatoliaca) honey bee subspecies, seem to be resistant to grayanotoxins. But other subspecies are not: they die, or become paralysed, sluggish or erratic after consuming R. ponticum nectar – although they learn to avoid rhododendron flowers. For reasons not yet known, common rhododendron growing outside its native range has lower levels of grayanotoxins, so mad honey is not a problem.

Rhododendron honey is eaten in tiny amounts by local people for its perceived medicinal, hallucinogenic or aphrodisiac properties. But an adventurous gourmet taking even a spoonful of the stuff risks being struck by a long list of unpleasant and dangerous clinical symptoms. Indeed, it is not uncommon for people in Turkey – some of them tourists – to end up in the hospital after experimenting with mad honey.

So if you find yourself in Anatolia for your holidays, don’t be too bold in exploring exotic local products. You may share the fate of Pompey’s soldiers.

A plant that secretes toxic nectar may seem to be engaged in self-harm, as this sugar-rich substance is the main incentive for pollinators to pay a visit to its flowers. But hundreds of plant species produce nectar laced with secondary compounds such as alkaloids, terpenes and phenolics, which are often noxious or unpalatable. So there must a reason for this apparent paradox. Toxicity may be a way of excluding inefficient pollinators, reserving the metabolically expensive nectar for a few specialists that are immune to secondary compounds. Many of these chemicals defend plants against pathogens and herbivores, so a few dead pollinators may be acceptable collateral damage.

Toxic honey may be a side effect of nectar with protective properties, but the broad-leaved helleborine orchid () intoxicates flower visitors for its own benefit.

This orchid is found throughout much of Europe and Asia in all sorts of habitats. It was introduced to America, where it is viewed as an invasive species in some states. Despite packing 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: social wasps such as 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 chemicals 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 tipsy wasps at work. Nobody knows if the wasps are hungover afterwards.

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 visitors. 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 their female counterparts, or by releasing pheromone mimics.

It’s no wonder orchids were the subject of Darwin’s second book, published in 1862 (On the Various Contrivances by Which British and Foreign Orchids Are Fertilised by Insects, and On the Good Effects of Intercrossing). Darwin worked with orchids to test some evolutionary ideas such as coevolution. He could never have imagined that his studies would inspire H.G. Wells (1866-1946) to write the tale of a Mr Winter-Wedderburn, who buys a strange orchid and tells his housekeeper: ‘There are such queer things about orchids. Darwin studied their fertilisation, and showed that the whole structure of an ordinary orchid flower was contrived in order that moths might carry the pollen from plant to plant’. But science turns to horror when the orchid flowers: it produces a scent that makes Winter-Wedderburn pass out. The orchid wraps its roots around his neck to suck his blood, but luckily the housekeeper is on hand to rescue the unfortunate gardener from the vampire plant. Wells’ story was translated into several languages and inspired numerous imitators into a new genre of science/horror fiction that is still around today: the man-killing plants. if you are old enough, you may remember the hungry orchid from Little Shop of Horrors.

The reception of Wells’ story reflects our fascination with Nature and its mysterious ways. Certainly much more remains to be discovered about plants and their pollinators, so many a fantastic tale can yet be written.