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Invasive Species in the UK – Spiders, Scorpions, and Slugs

Noble False Widow Spider

Noble False Widow Spider

Invasive Arachnid and Mollusc Species

Did you know that venomous spiders, scorpions and giant slugs are now living in the UK? If you suffer from arachnophobia or are an avid gardener, this might not be welcome news to you, but there are indeed invasive arachnid and mollusc species that have now made their homes in our country.

Invasive species are a problem in many countries around the world, and because of their generally small size, arachnids and molluscs can be among the hardest to keep out and get rid of once they have arrived.

While some invasive species have been deliberately introduced, usually as a form of pest control, many have arrived here accidentally on ships and planes, often in packing crates or in shipments of fruit.

While generally much smaller than introduced animal species, they can have just as big an impact on local ecosystems and can out-compete our native species by eating their food supply, taking over their territory, introducing previously unknown diseases and parasites or even by preying on them. So what species of spider, scorpion and slug have been introduced into the UK?

Noble False Black Widow Spider

Did you know that there are around a dozen species of spiders in Britain that bite? Even if you are one of those people who are terrified of spiders, you have probably consoled yourself with the thought that, at least in this country, they don’t bite you.

Luckily, although some of the British spiders may give a painful nip, their bite is unlikely to cause any real problems unless you are allergic or extremely frail. However, there is an invasive spider species that has made its home in the UK that has a bite that packs a bit more of a punch, and that is the noble false widow spider, or Steatoda nobilis.

This uninvited invader has actually been a UK resident for quite some time as they first arrived in the country over a hundred years ago in cargoes of fruit imported from the Canary Islands and Madeira.

Their presence was first recorded in Torquay on the English Riviera in 1879, and there are now recorded populations in Devon, Dorset, Hampshire and Essex.

They are also moving further north, possibly due to warmer temperatures, and sightings in northern counties are on the increase. They are known as false widow spiders because of their superficial resemblance to the much more venomous true black widow, but they are more brown than black in colouring, with creamy markings on their bulbous abdomens and reddy-orange legs.

The females are bigger than the males and can grow to around 32 mm if you include the legs. Noble false widow spiders generally live for between one and two years and eat small insects and flies that get captured in their webs.

They have poor eyesight and use vibrations to locate where their prey has become trapped and to move around.

It is also the female noble false widow spider that has the more potent bite, although they usually stay on their webs which they spin under window sills or in cracks in masonry and walls.

On the whole, their bite does not cause very severe symptoms in humans, but in April 2012, a woman in the Bournemouth area was bitten on the hand during the night and nearly lost her hand due to the spider’s toxic venom.

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Although noble false widow spider’s usually live outside, it is thought that the female spider was brought into the house on a sheet that had been drying on the line.

European yellow-tailed scorpion

European yellow-tailed scorpion

European Yellow-Tailed Scorpion

The European yellow-tailed scorpion, or Euscorpius flavicaudis, is another invasive arachnid species that has been in residence in the UK since the 19th century.

They are thought to have arrived on masonry and building materials being shipped from Italy, and the first colony of these invasive scorpions was reported in Sheerness in the 1860s. This scorpion colony in Sheerness is still the most famous one in the country, but there are now populations of European yellow-tailed scorpions in and around the London area and North Devon as well as around the Isle of Sheppey.

There are now thought to be as many as 13,000 European yellow-tailed scorpions in the UK, and they mainly live in cracks in bricks and masonry on sunny, south-facing walls in isolated spots such as docks, railway stations and disused buildings.

Scorpions are not insects, but are arachnids like spiders and mites and have eight legs, not six. European yellow-tailed scorpions are a small species and are generally no more than 5 cm long.

They have a brown body with paler brown legs and a yellow tail and feed on insects and small invertebrates. They do have a sting, but although it is painful it rarely causes any severe symptoms in humans unless they are allergic to the sting or are very frail.

But in reality, they very rarely sting people because they do not live in houses or around human populations. You are only likely to get stung if you attempt to handle one, and if you get stung it will probably swell and become red around the wound.

Very unusually for an invasive species, there are moves to protect these invasive scorpion colonies because they do not appear to have any detrimental effects on native species, increase biodiversity and pose little threat to humans.

Spanish Slug

One of the latest invasive species to invade our shores is the Spanish slug, or Arion flagellus, usually brought in on consignments of vegetables being imported from Spain. Now I know that all creatures evolve to fill an important niche in an ecosystem, but how come snails seem to be regarded as cute and whereas slugs are not?

No slugs are regarded favourably by gardeners or farmers due to their voracious appetite for garden plants and vegetables, but these recently arrived olive green Spanish slugs are much larger than our native slugs and can grow to be as much as 4 inches long, and they also produce hundreds more eggs than our British slugs.

To make matters worse, they are breeding with British slugs to produce a tough hybrid that can make short shrift of any lettuce patch they find themselves in. Once established, they are very hard to eradicate, as they are very hardy and can survive some very tough conditions.

They then start breeding in great numbers when conditions improve. Organic farmers and gardeners have a particularly hard time getting rid of them because they are not allowed to use any chemical pesticides on their crops, and all they can do is encourage the birds that prey on them and cut any unnecessary vegetation where the Spanish slugs may lurk right back.

Spanish slugs have also brought diseases and parasites into the UK with them, which our native slugs have little defence against. The British slugs cannot fight off these unfamiliar pathogens because they self-fertilise, meaning they are inbred, which does not give them the required resilience, and it is feared that some native species may go extinct because of this invasion of Spanish slugs.

This invasion of slugs also poses a threat to our domestic pets, as dogs that eat slugs and snails can contract a disease called lungworm. The symptoms to look out for in your dog are vomiting, coughing, problems with breathing, nosebleeds and tiredness; if not successfully treated, a lungworm infection can even lead to death.

They can also pose a threat to humans, as when numbers of them are squashed by vehicles on the road, their remains create large, very slippery patches that have caused cars to crash. To add to the unpleasantness, Spanish slugs are cannibalistic, and the slippery mess on the road is added to when they come to eat the dead ones.

So while the European yellow-tailed scorpion is regarded as a fairly benign presence in Britain, the spread of the noble false widow spider is feared because of its venomous bite and the giant Spanish slug because of the destruction they can wreak in gardens and on commercials crops.

The arrival of any introduced species can cause difficulties for native ecosystems and can also cost huge amounts of money trying to eradicate them or make good any damage they cause. How the problem of invasive species is dealt with is a hotly debated issue, but hopefully the spread of these arachnids and molluscs can be successfully halted or contained.


pat o shea on February 19, 2020:

slugs learn i noticed that when i put though i know i shouldn't slug pellets around new Pentstimom plant they died for about a week then they stopped dying and I have had no more problem with them and that was months ago maybe they react and perceive the plants as fatal.

AnimalWrites (author) from Planet Earth on July 14, 2012:

Thanks for reading and commenting Sammi - slugs and snails will eat most plants and can destroy whole gardens. Maybe this time they did bite off more than they could chew?

Sammi on July 13, 2012:

Every year in my garden ,I get a few opium poppies in my Borders . This year I had three healthy plants with large buds , that practically disappeared within two nights . Leaves buds and stems eaten , a slime trail remained to show the guilty culprit ( slug or snail ? ) .This has never happened before & I am now wondering if some mollusc is suffering cold turkey curled up in a damp spot in my border !

AnimalWrites (author) from Planet Earth on July 12, 2012:

Too true aviannovice - it's amazing what can hitch a free ride around the world. Thanks for reading the hub and commenting

Deb Hirt from Stillwater, OK on July 11, 2012:

It's pretty amazing what travels around the world!

AnimalWrites (author) from Planet Earth on July 10, 2012:

I agree with you JKenny that slugs play an important role in the environment, but for some reason they just turn me a bit and I don't generally have a problem with creepy-crawlies. A bit like cockroaches (which don't bother me) for some reason slugs just don't seem to be appreciated by the majority of the human race. Thanks for reading the hub and leaving a comment

AnimalWrites (author) from Planet Earth on July 10, 2012:

Thanks for reading and commenting sharkfacts. You probably never saw a slug in Spain because they have all emigrated over here to the UK where they get lots more rain lol! I have looked up whether slugs are edible like snails, and they are but unless cooked properly they can pass on a parasite that they pick up from rat faeces. Said parasite if ingested will crawl from your stomach to your brain and make it swell. Apparently Ray Mears also does not advocate eating slugs because they may have fed on toxic mushrooms

James Kenny from Birmingham, England on July 10, 2012:

To be honest I don't mind any of the three, although I would probably don a pair of gloves before deciding to pick up a slug. People are often too quick to dismiss slugs as repulsive. But they do perform important ecological jobs like helping to speed up the decomposition of leaves (on the ground) as well as providing food for birds like thrushes and mammals such as hedgehogs and badgers. I think Chris Packham wrote an article about the benefits of slugs and snails in Gardeners World, but I can't remember which issue it was, I know it was a recent one though. Thanks AnimalWrites.

sharkfacts from UK on July 09, 2012:

Well I don't like spiders, scorpions or slugs! Yuk! But this was a fascinating read as I am interested in invasive species and how they interact with local populations. All the time I was in Spain I never once saw a slug, but there were plenty snails which my neighbours used to collect and eat. Always one way to get rid of them out the garden, give them to the neighbours!

AnimalWrites (author) from Planet Earth on July 09, 2012:

I agree with you Judi Bee, as I really don't mind spiders and scorpions but I really don't like slugs. I'm not frightened of them or anything, but for some reason they are a bit revolting. Which is a shame because they are a living creature like all others and have evolved to fill an evolutionary niche so native slugs are necessary to the good health of an ecosystem

Judi Brown from UK on July 08, 2012:

I can live with the venomous spiders and scorpions but the thought of big, fat slugs makes me feel very queasy. And they cause car crashes! I always knew slugs were slippery customers.

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