Invasive species week: Monday 15th May

the weird and the (not so) wonderful

We take a closer look at some of the more unusual characteristics of invasive species in the UK. From colour changing water weeds, to tree patrolling caterpillars, to smelling like a skunk – discover some interesting facts.

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American Skunk Cabbage

Quite possibly the smelliest invasive species in the UK

American skunk cabbage is predominantly recognised for its large leathery leaves and bright yellow flowers but get up close and you’ll remember it most for it’s intoxicating foul smell. Its name comes from the putrid odour the flowers produce in spring, and it will come as no surprise that it resembles the smell a skunk emits. Prolific in swamp forests and associated wetlands, when not in flower, it may be confused with the native Water plantain (Alisma plantago) which is fairly common in slow flowing habitats.

American skunk cabbage

Azolla (or Water fern)

Impressive colour changing water weed

Azolla has clusters of small leaves forming rosettes up to a few cm in diameter. The colour change happens in autumn when the leaves turn a deep shade of red, creating a dramatic dense auburn carpet on the surface of the water which can be up to 30cm deep. For this reason it is often called Mosquito fern as it is said that mosquitoes are unable to hatch, but can also called Fairy moss. Considered to be of tropical origin; however, it is thought to have evolved a cold-tolerant strain since its introduction to the UK.


Cinnabar moths

Biological control of Common ragwort

Ragwort is one of the most frequent causes of plant poisoning of livestock in Britain, with equines and bovines being more susceptible than others – particularly the young. But Ragwort has a natural predator – the Cinnabar moth (Tyria jacobaeae) which feed almost exclusively on Ragwort, eventually killing it off. The caterpillars are distinctive, black with gold stripes, and grow up to 30mm long.

It’s worth noting that the caterpillars absorb toxicity from the Ragwort they consume and can cause a rash if handled.It can also be poisonous to people and has been suspected of causing liver damage in those who seek to pull the plant without the benefit of protective clothing. Ragwort acts as a cumulative poison, eventually destroying the liver, and a small intake of ragwort over a long period can be just as damaging as a large intake on a single occasion.

Oak processionary moth caterpillars

A living toxic train

Oak processionary moth caterpillars (OPM) are a species of moth whose caterpillars nest on oak trees during the spring. They move from tree to tree in a visually stunning writhing train, that can often be seen travelling down one tree, along the ground and up the next tree. The hairs on these caterpillars contain an irritating substance called thaumetopoein, which can cause health risks to pets and humans, and as these hairs are incredibly fine, they are difficult to spot. The greatest risk period is between June and August, so keep a lookout for this highly toxic species.


intoxicating ‘mad’ honey

Rhododendron contains grayanotoxin which is a toxic substance found in the pollen of the plant. Grayanotoxin poisoning is the most common form of poisoning in humans eating honey made by bees foraging on rhododendron. Small doses cause light-headedness and hallucinations. In large doses it is overtly toxic and induces the range of symptoms.