Lethal foliage is all around you. Every year in the United States, 3900 people are injured by electrical sockets, but more than 68,800 are poisoned by plants. And they join some of history’s luminaries: Nancy Hanks Lincoln, mother of America’s most-adored President, died after drinking milk from cows that had eaten white snakeroot (so did more than half the population of neighbouring Dubois County, Indiana). Socrates was executed with hemlock, and an innocuous-looking blue flower—monkshood—brought down the mighty Emperor Claudius (with the help of his murderous wife, Agrippina).
Here in New Zealand the toll is much lower, but around 75 people nevertheless need treatment each year after a brush with malignant greenery. We have around 100 poisonous species here, of which about two-thirds are introduced: black nightshade, oleander, hemlock, foxglove, laburnum and castor oil plant are but a few.
More than 30 of our native plants are toxic to humans and/or livestock to some degree, though some manufacture poison only during the growing season. Many more harbour venom to deter insects.
Some are very familiar: there’s a liver toxin in the leaves of ngaio which can kill livestock, but no people are known to have succumbed. Kowhai seeds can be toxic, but you need to go to a bit of trouble. Bracken shoots are carcinogenic, and dozens of cattle died in the central North Island between 1950 and 1970 after being press-ganged into trampling bracken prior to land clearance. Turutu, poroporo and karaka wield poisonous berries (raw karaka kernels can paralyse humans, but Maori perfected a way to neutralise their alkaloid poisons by steaming and steeping), but possibly the most Machiavellian is tutu (Coriaria arborea).
Two varieties of tutu have done well out of forest clearance, thriving in the light created by roads, tracks and clearings. This places them squarely in the path of walkers, and tutu is responsible for a number of deaths.
The most recent fatality was an adult male in 1989, but it caused particular havoc among the children of early settlers, who were lured to its dark berries. Maori knew of the tutu’s devilry: they carefully removed the toxic seeds from its fleshy fruit—the only safe part of the plant. They also strained the berries for juice, which was administered as a medicine, added to bland foods and used as the base of a sweet wine.
In April 2012, three tourists had a narrow escape after eating “hundreds” of tutu berries on a tramp in Kahurangi National Park. After a couple of hours, the trio developed nausea and stopped into a garden centre to show staff a photo they’d taken of the berries. Recognising tutu at once, the staff rushed the three to Accident & Emergency. Two developed racing pulses and suffered seizures, and even after successful treatment, they were left with memory loss.
More frequently, tutu claims livestock the first two sheep liberated by Captain James Cook in 1773 died just days after release in Queen Charlotte Sound. Pioneering farmers blamed tutu for stock losses of up to 75 per cent, and even in 1971 it was still estimated to kill between five and 10 per cent of South Island high-country herds. It is also, apparently, irresistible to elephants. The Nelson Examiner reported in 1869 that a circus elephant succumbed to tutu in Otago: “The poor animal fed heartily upon this for four hours, then went to a stream and took a long drink, turned, reeled, fell and died in three hours.”
In 1956, it claimed another:
The Bullen Brothers circus lost a star attraction after it ate tutu while watering beside the Mangawhero River. Its bones still lie behind the railway houses at Ohakune Junction. Only barbiturates and a quick-witted vet, David Marshall, saved two more elephants that ate tutu while being transported through the Buller Gorge in the 1960s. Marshall later recounted piles of vomit up to his waist.
Tutu is laced with a toxin, described in 1870 as tutin, that attacks the body’s glycine receptor, a critical component of the central nervous system that helps to regulate neurotransmission in the spinal cord and brain stem. It’s a powerful convulsant and can strike in the most circuitous and unanticipated way: passion vine hoppers which have presumably evolved an immunity—will often suck tutu sap, and honey bees will sometimes ‘milk’ those vine hoppers for honeydew. When they do, the tutin ends up in the bees’ honey, triggering outbreaks of toxic honey poisoning in humans. Fortunately, it’s a rare concert of circumstance—aggravated by drought—and is more likely when victims eat comb honey straight from a stricken hive.
Then there’s Urtica ferox, the stinging nettle, or ongaonga. Anyone who’s brushed carelessly against its barbed hairs knows its fierce burn. On Boxing Day 1961, two young hunters stumbled into a thick swathe of nettles in the Ruahine Range. Within an hour, one was struggling to breathe, then he went blind. He died five hours later. His companion suffered similar symptoms, but survived. Many more people have been laid low, sometimes for days. Ongaonga’s leaves bristle with hollow stinging hairs called trichomes. They’re filled with histamine and virulent irritants, and when you brush against them, they actually pump those toxins into you in a pernicious hypodermic action. (If you’ve been stung, look for dock or plantain leaves to apply. Sometimes the underside of fern fronds, if they’re laden with spores, can also alleviate the pain.)
All these toxins are expensive for a plant to make. There has to be a very clear benefit to justify the cost in terms of energy. Typically, plants become toxic because they mean to discourage browsers, bacteria, fungi or competition. The New Zealand forest is nowadays full of browsers—possums, deer, pigs, goats, feral stock—but they were introduced by humans far too late to have any bearing on the evolution of native plants. So what turned our death-dealing flora bad?
Take a longer look, and you notice that most of our poisonous plants—certainly the most virulent—are relatively low-growing, and enjoy a bit of headspace: clearings, margins, scrub and grasslands, where they’re among the first to colonise new real estate. Historically, storms, eruptions, fires and slips would have always left ample habitat for them, but those contestable open spaces had their perils.
Moa were among the most formidable of browsers: they came built with beaks like secateurs and could deal with fibrous twigs, bark and even flax leaves. Brawny gizzards full of grinding stones—gastroliths—meant they could glean nutrition from pretty much anything. Plants responded in a number of ways: some grew spines, or defensive twiggy thickets, some hid themselves, and others developed poisons. (Still more made themselves look like poisonous species—a ruse to score the benefits of toxicity without incurring the expense.)
Just seven of 32 listed poisonous native plants grow higher than 10 metres, and even they have to survive the seedling/sapling stage to get there. Analysis of moa coprolites—fossilised poo—in 2009 found that more than half of the moa’s diet species were under 30 centimetres in height. Given that there were at least 11 moa species (and many more flightless rails, which have also mostly become extinct) that occupied practically every lowland niche going, they represented a vast browsing biomass that co-evolved with our native flora. Browsers and the browsed employed ever more sophisticated traits to either discourage or perpetrate herbivory, depending on which side they were on.
Tutu’s poison is nowadays a shield held up to a ghost. But it still works on elephants…