Such sweet poison
The strange story of New Zealand's deadliest plant (9 minute read)
Last week, I received some book vouchers, so I decided to pay a visit to the lovely Schrödinger’s Books in Petone. Although I already have far too many books on my to-be-read pile, I can always fit in one more. This time, though, I quickly decided that I wouldn’t add to the pile. Instead, I’d get a field guide, not something I would need to sit down and read, but something I could simply open at random, read a page, then close again.
I soon had it narrowed down to a book on New Zealand fungi, and a foraging guide which covered both plants and fungi. In the end I went with the foraging guide – Foraging New Zealand by Peter Langlands. I had a quick look at it, and all the plants were familiar to me, but many of them were species I didn’t know were edible. Also, the book made my botanist’s heart happy – the plants were arranged by botanical family. This means that closely related plants are grouped together, which is also how they are arranged in my mind. So it’s easy for me to use.
I’m confident to trust my life to my plant identification skills, but I also know that careless foraging can be hazardous. Most people are aware that foraging wild mushrooms can be dangerous if they don’t know what they’re doing, but may be less aware of the dangers of plants. In fact, there are many more dangerous plants than there are dangerous mushrooms, and some are dangerous in very small amounts. Despite this, it’s mushrooms which provoke greater dread1, which is a source of great frustration to some of the mushroom experts I know.
New Zealand is home to a few truly deadly plants. I’ve written about our delightful but dangerous giant nettle, but we have others as well. Mostly, these are plants you have to eat for them to harm you, but one, known as tutu, occasionally poisons people in a most insidious way. So, since foraging is on my mind, I thought I’d share the strange story of one of our most dangerous plants.
Tutu is one of the native species which has benefitted from the arrival of people in New Zealand. It’s a shrub which prefers to grow in full sunlight and thrives on disturbance. In more remote areas, it is often found growing on the edges of streams and rivers, but it grows equally well on roadside banks. Although it’s not related to species such as beans, clover and gorse, is has something in common with them. Tutu has nodules on its roots containing bacteria. These bacteria take in nitrogen from the atmosphere and convert it to a form which plants can use, allowing the plants to thrive in areas with low fertility.
The name tutu usually applies to a single species, but there are six other native species which are closely related. They also have the same bacteria in their roots and the same toxins. They all belong to a group (or genus) known as Coriaria, which has a profoundly odd distribution. New Zealand has almost half of the world’s species. Although there are none in Australia, there are a few in other islands in the region and into Asia – Fiji, Vanuatu, Samoa, New Guinea, the Philippines, Taiwan and Japan. There are also a few in the Himalayas, one in the Mediterranean and one found from central Mexico to Patagonia.
A distribution so odd prompted much discussion and debate among scientists. Usually, related species grow near each other, because at some point in the past, they’d evolved from the same ancestral species. There are examples where related species are found in different parts of the world, but there was no other group of related species found in so many different places, with such large gaps between.
In the early part of the 20th century, scientists were grappling with the question of whether the continents were in fixed locations, or moved in some way over very long periods of time. They had recognised since the late 19th century that land masses must have once been linked, and not everyone was content to assume that the separation was caused by the biblical flood. The predominant idea was that there had once been land bridges between continents, but that the land had since sunk. However, a few were leaning towards the controversial idea proposed by a German scientist, Alfred Wegener. In 1912, he suggested that the continents had once all been joined in a single super-continent, and had then drifted apart. His idea, however, suffered from a major weakness. He could not give a convincing explanation of how they moved.
While some scientists gave his idea serious consideration, others dismissed his work as delirious ravings. It wasn’t until the 1960s that scientists began a serious investigation into the movement of the Earth’s crust. Then, they determined that the continents had indeed moved, and had once been joined in the way Wegener had originally proposed.
Among those to take him seriously before the idea became accepted was British botanist Ronald Good. In the late 1920s, he was working on plant groups which had unusual distributions, such as those found on widely separated continents. He noted that Wegener had cited plant distributions as evidence for the movement of continents, but that Wegener hadn’t chosen the best plants as examples (to be fair, Wegener’s main area of research was meteorology and his research centred on Greenland ice cores, so he had limited opportunity to develop his botanical skills).
To Good, one plant group stood out – Coriaria. Its distribution, he stated, could only be explained by great changes in the positions of land and sea over time. He dismissed the idea of fixed continents and land bridges, and suggested that some kind of continental displacement must have occurred. While not endorsing Wegener’s work (which may have been a career-limiting move at the time), he did note that an acceptable theory for the movement of continents would be supported by the distribution of plants such as Coriaria.
To New Zealanders, though, Coriaria is more than a botanical curiosity. It can be a deadly threat. All parts of all species which have been tested, except for the flower petals, contain a potent nerve toxin, named tutin. The point about the flower petals is crucial, because the berries of Coriara are as odd as everything else about this species. They aren’t berries at all, but the flower petals which have swollen and turned black. Since the flower petals don’t contain the toxin, they are actually safe to eat, but – and it’s a big BUT – the seeds are deadly. Pluck these apparent berries and eat them, and you’re in trouble, because you’ll swallow the seeds as well. Those who survive often suffer lasting ill-effects, such as difficulties with memory.
Māori well understood the dangers, but also recognised its value. They used to squeeze the berries of tutu in a tightly-woven bag, to trap the seeds and create a juice which could safely be drunk or used to make a jelly when boiled with seaweed. The plant was also used medicinally in a number of ways – mostly externally, for obvious reasons.
Early Europeans in New Zealand had many deadly encounters with tutu. The first two sheep introduced to New Zealand were dead within days of being released, the first of many sheep and cattle to be killed by grazing on the plant. Other animals were poisoned as well, including more than one circus elephant. There is even a record of trout being killed by beetles which had fed on tutu then fallen into streams.
Nor were human deaths uncommon. Both adults and children found the clusters of glossy black berries tempting, and there are a number of deaths, potentially dozens, reported from the 19th century in various sources I’ve found2. The earliest European settlers in Nelson had a very narrow escape, when they began picking the berries soon after arriving. Local Māori intervened immediately – in one settler’s account they knocked them out of our hands as we lifted them to our lips. The Māori showed the settlers how to strain the berries and that the juice was safe to drink, then communicated with very emphatic sign language that they must not eat the seeds. It’s notable in the settlers account that they believed this was for “superstitious” reasons, only later realising that the Māori had saved their lives.
More recently, people have become less inclined to eat the fruit of plants that they can’t identify. Every few years there are reports of people eating the berries, or occasionally the shoots, and ending up hospital. However, direct poisonings – from people actually eating tutu – are rare.
Here’s where things get really weird. Tutu has another way of poisoning people, and it’s so complicated that it’s hard to believe.
In the late 19th century, an insect known as the passion vine hopper was accidentally introduced to New Zealand from Australia. If you’re a gardener living in warmer parts of New Zealand, you probably know it, because it’s quite distinctive. The juveniles are small and round, with disproportionately large, white, fluffy tails. The adults have pretty, transparent wings. And both adults and juveniles jump if you disturb them. The passion vine hopper is something of a pest, because it sucks sap from the stems of plants. It’s not fussy either, feeding on a wide range of plants, including tutu, which doesn’t seem to do it any harm.
How does this lead to human poisoning? Bear with me, there are still a couple of steps to go.
Sap-sucking insects have straw-like mouthparts which they insert into plants. They end up taking in more of the sap than they need, so they excrete a sugary liquid known as honeydew. Many animals feed on honeydew, including native birds such as kākā and tūī, as well as introduced insects such as wasps and bees. When the passion vine hopper feeds on tutu, the honeydew it excretes contains more than sugar. It also contains a toxin from the tutu. So the animals feeding on the honeydew also eat the toxin.
I haven’t come across reports of any animal being sickened or killed as a result directly eating the toxic honeydew, but when it is gathered by bees, it ends up in their honey. And if people eat the honey, they can be poisoned and even killed. Within a few years of the passion vine hopper arriving in New Zealand, poisoned honey had become a serious problem, and it took decades for scientists to determine the cause. These days, the risk is well-recognised among beekeepers, who must follow specific rules to keep the toxin out of honey. Poisonings are now rare, but they still happen. In 2008, 22 people were poisoned by a batch of toxic honey, with a number hospitalised and suffering seizures, although fortunately nobody died.
I’m going to enjoy my foraging book, but I’m also confident that I know how to identify what I’m eating. I urge anyone planning to forage in New Zealand to learn to identify tutu and its relatives, because it’s not a forgiving plant.
This is an example of what Peter Sandman calls outrage, which is the degree to which a risk upsets people. There isn’t a strong relationship between how likely something is to kill us and how upset or outraged we are about it – I wrote about this topic late last year if you want to know more.
I’ve found newspaper reports, 19th century scientific publications and modern reviews, but it’s getting late and I don’t have time to add them up and cross-reference so I’m not duplicating records, although the nerd in me really wants to.
Quite fascinating - thank you
That was super interesting, thank you. I lived on a farm years ago and I am familiar with tutu. Down on the farm they referred to it as "toot", which makes it sound misleadingly benign.