Sweet, sweet poison

By Athayde Tonhasca

In 401 BC, an army of Greek mercenaries led by 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 come across an abundance of honey, and some of the men went for it with gusto. In no time they regretted their excess: they could not stand up, vomiting and suffering from diarrhoea. Xenophon’s army had been overpowered by honey. ‘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 march of the Ten Thousand. Image in the public domain

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 an unpleasant 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. This plant is full of grayanotoxins, a group of toxic substances that protect it against herbivores, but also accumulate in the nectar. 

Common rhododendron, a source of mad honey © Rasbak, Wikipedia Creative Commons

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. For reasons not yet known, invasive plants have lower levels of grayanotoxins than native plants, so mad honey is not a problem here in the UK.

A plant that secretes toxic nectar may seem to be engaged in self-harm, as this sugar-rich substance is the main incentive for pollinating insects 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 noxious or unpalatable to pollinators. And rhododendron’s mad honey is not unique: pure honey from mountain laurel (Kalmia latifolia) and bog-rosemary (Andromeda polifolia), for example, have a range of possible ill effects on people, including death. 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. It may also discourage nectar robbers, that is, flower visitors who avoid contact with the floral reproductive structures and therefore do not pollinate. Some of these chemicals may have antimicrobial properties that preserve the nectar. Perhaps toxicity is just incidental: secondary compounds act as chemical defences against herbivores, so seepage into nectar may be unavoidable. Poisoning a few pollinators would be a small price to pay for immunity from plant grazers. 

Sources of honey to be avoided: mountain laurel and bog-rosemary © Vlmastra (L), Pmau (R), Wikipedia Creative Commons

The bees that produce mad honey, the Caucasian (Apis mellifera caucasia) and Anatolian (Apis mellifera anatoliaca) honey bee subspecies, seem to be immune to grayanotoxins. So, too, are bumble bees, who take full advantage of rhododendron’s profusion of nectar-rich flowers. But other honey bee subspecies and some solitary bees are not: they die, or become paralysed, sluggish or erratic after consuming R. ponticum nectar (although honey bees learn to avoid rhododendron flowers). 

The common rhododendron is an incredibly efficient pioneer species. Each plant produces more than a million small seeds that are dispersed by the wind over large distances. It forms a dense canopy that shades out other plants, and it may release chemicals that inhibit the germination and growth of other seedlings. And thanks to grayanotoxins and other phenolic compounds, rhododendron has few grazers and sap feeders. Not surprisingly, this plant is a serious invasive wherever it grows unchecked. It is threatening natural and semi-natural habitats in Britain, much of Western Europe, New Zealand, and even in its native range in the southern Black Sea Basin. 

A Perthshire native woodland invaded by rhododendron © Lorne Gill/NatureScot

Because of its nectar, common rhododendron may contribute to the reduction of pollinators’ populations either by intoxicating some bee species or by decreasing food availability for others (invaded areas have fewer alternative flowers). It may also shift local species composition, leaving fewer solitary bees and more bumble bees. Indeed, colonies of the white-tailed bumble bee (Bombus lucorum) and the common carder bee (Bombus pascuorum) occur at higher density in areas invaded by common rhododendron when compared to uninvaded areas. We have no idea of the long term consequences of these disturbances to pollinator communities, but they are not likely to be trivial. Toxic nectar is one more reason for keeping common rhododendron on the list of bad gardening ideas.