Their Latin name is Arion vulgaris, but they’re better known as the nightmare of many gardeners under their common name, ‘Spanish slug’.
Sometimes in the media, they’re called: killer slug, killer snail or cannibal slug.
The Spanish slug is closely related to common red slugs (Arion rufus) and can mate with them.
The species are therefore difficult to distinguish.
Slug population booms: What are the reasons?
Some people claim that if Spanish slugs were to breed with local species, a sort of ‘super slug’ that could be immune to drought and poison would be created.
Newspapers like these kinds of stories:
‘Foreign invaders wipe out local species and there’s no remedy. Thus, we’re doomed!’
This is just not the case.
There are many remedies, and it’s worthwhile taking a close look at the real reasons behind the population booms:
The loss of biodiversity and the extinction of natural enemies.
Most gardeners try slug pellets or beer traps, but at best these methods work for a short time, and often a second, even stronger, wave of slugs comes in.
Unfortunately, violent slug control can only alleviate the symptoms, but this often leads to negative long-term consequences.
Spanish slugs grow up to 4–6 inches (10-15 cm) in length and are very variable in their coloring.
The most common are brownish to reddish, but there are also light yellow to black specimens.
The Spanish slug can mate with the black slug (Arion ater).
It’s therefore difficult to distinguish them from the common red slugs (Arion rufus) and black slugs (Arion ater).
These three species can mate with each other, which further impedes a clear separation.
However, red and black slugs occur mainly in forests.
Rarely can they be found in the garden, which is where Spanish slugs prefer to stay.
If you encounter a reddish-brown slug in your garden, it’s most likely a Spanish slug.
One clear distinguishing feature between this group of slugs and other slug species is their ability to curl up to protect themselves against their enemies.
Most other slugs, such as the leopard slug, can’t do this. They can contract their body, but aren’t able to curl up.
The Spanish slug loves to hang around humans.
Where Did Spanish Slugs Come From?
The Spanish slug was first seen in 1954, in Great Britain.
New studies have shown that this slug is not related to species in Spain or Portugal, as was believed for a long time, so it might be more appropriate to call it the ‘British slug’.
Nowadays, it’s believed that the Spanish slug originates somewhere in southern France, from where it’s likely to have spread through most of Europe.
Since the 1990s, the Spanish slug has rapidly conquered more and more areas and has invaded nearly every garden on the Continent.
Because slugs can multiply explosively under favorable conditions, they’ve become a major horticultural pest.
Their exact spread across the Continent is still a research issue and isn’t yet clearly understood.
The common opinion that the Spanish slug was introduced to Northern Europe in the 1960s through salads from Spain is fake.
For a long time, Arion vulgaris was confused with Arion lusitanicus. A slug that lives in Portugal and Spain.
Recent studies have proven that the Spanish slug has only recently arrived in Spain.
Genetic tests indicate that it’s closely related to Central European slugs.
The first documented sightings are from Great Britain (1954), France (1955) and Switzerland (1956).
In Germany, a specimen was sighted for the first time in 1969 on the north side of the Rhine near Basel.
However, these sightings don’t allow conclusions about their true origin and spread to be drawn.
The slugs’ migration north was probably also favored by climate change and by humans.
It’s believed that they spread mostly embedded in horticultural soils and potted plants distributed around Europe.
As the name ‘Spanish slug’ is misleading, in the future it would be better to find a new name or to stick to its Latin name, Arion vulgaris.
The slugs also like mushrooms.
Which Plants Do Spanish Slugs Like to Eat?
The Spanish slug eats mainly fresh plants and carrion.
There are even some plants that they like more than others:
Under normal conditions, these creatures are nocturnal and hide in damp, shady places during sunny and dry weather, as they risk dehydration and coming across natural enemies.
Only in humid conditions can they also be found during the day.
The bigger a slug becomes, the more active and drought-resistant it will usually be.
It’s assumed that a slug can travel up to 25 yards (ca. 23 m) per night.
The slugs move at a speed of about 3 meters per hour, so it’s not enough just to throw them over the fence after collecting or to put them some 100 yards (meters) away in bushes.
Collected slugs should be taken at least 1,000 yards (meters) away.
They’d then need more than a month to return and are likely to find another spot on their way.
In the long run, it’s recommended to take them even further away.
The best place would be a river or stream that they can’t overcome too easily.
The optimal place would be an island!
Spanish slugs are mating on autumn leaves.
Why Can They Multiply So Quickly?
Slugs, like all snails, are hermaphrodite, which means they have both sexes and can even fertilize themselves in extreme situations.
A Spanish slug can lay up to 500 eggs, with up to 225 in a single spot.
The eggs of a Spanish slug.
The Spanish slug usually dies soon after laying its eggs because, under normal conditions, the species has only one reproductive cycle.
The eggs are buried in damp, dark places, normally directly on the soil but protected from enemies and the weather as far as possible.
Often, they’re stored just below stones, logs, and pots or in the shelter of shrubs and bushes.
Occasionally, you can find their white eggs in cavities under garden fleece or deadwood.
Video: How to Find and Deal with Slug Eggs
In autumn or early spring, it’s worth taking a closer look around and bringing the eggs to the light, where they’re prey for birds and other hungry garden residents.
Depending on the weather conditions, the tiny slugs hatch after a few weeks.
The eggs can also survive the winter and then hatch in springtime.
In addition, some adults can try to hibernate and lay their eggs in spring.
Video: Close-up of Leopard Slugs Hatching
How Can Spanish Slugs Be Controlled?
This is best done by a combination of many methods, as described above.
One often-underestimated means is the installation of slug fences, which are insurmountable to the slugs.
5 thoughts on “Spanish Slugs: Who Are They? Identify & Control +++”
Thanks for thorough background info on these creatures but would you mind listing in order of effectiveness how to get rid of them? They are devastating my plants and veg. I have tried mostly picking them off and also bio slug pellets but am now totally overrun with them. Help!
Thanks for thorough background info on these creatures but would you mind listing in order of effectiveness how to get rid of them? They are devastating my plants and veg. I have tried mostly picking them off and also bio slug pellets but am now totally overrun with them.
Help!
Hello Dot,
thank you for your comment!
Here you find some help: how to control slugs.
May you enjoy your garden!
Alex
How do slugs end up in the sink even though plugs are in overnight in Spain?
Hi Alex,
I wonder if there are nematodes for this species?
Thanks,
Dot
Hi Dot,
to my knowledge nematodes only work for slugs that live underground (Agriolimacidae – Deroceras).
Here you can find more information on nematodes: nematodes against slugs.
May you find a peaceful way to protect your plants!
Alex