When I was 12 I read a book which involved an encounter with terrifying mutated slugs that fed on birds. So you can imagine my horror when 17 years later, I came across the Spanish slug, which is capable of terrorising bird nests. In our latest article of Norway’s invasive species, we look at what other forms of havoc this slug wreaks.

What are they?

Considered the most destructive pest slug in Europe, the Spanish slug, or Arion lusitanicus, or Arion vulgaris, or sometimes Geoff (there’s some controversy over the name, thanks to the fact that the Arion genus contains up to 50 species and they all look a lot like one another) is between 7-15cm long and can weigh up to 15kilos if it’s sitting on a dog. They were originally thought to be from the Iberian peninsula, hence their name, but it appears that the slug doesn’t appear in Spain anywhere south of Catalonia, a controversy which recently ignited political unrest throughout the region. They are an incredibly slimy species, leaving trails wherever they go, however identification upon sight is made difficult by the fact that they can be a variety of colours, including yellowish, grey, reddish or brown, as can many of there aforementioned close relatives.

How did they get here?

Geoff can now be found throughout the whole of Norway, with the exception of Troms and Finnmark. Soil and plants are the slugs food source and egg-laying sites, which means that potted plants have spread them throughout most of Europe. They don’t seem to discriminate between urban and natural habitats, which mean they can pop up pretty much anywhere. Those guys you seen covering the paths at twilight every day? Probably them.

What do they do?

They eat birds. Yeah I wasn’t joking about that. In a recent review of Geoff’s attack on birds, injuries of nestlings included “bleeding wounds, holes in the stomach with viscera exposed, vast skin lesions on wings, back, neck or head, partially eaten muscles or bills, even loss of eyes”.

But whilst horrifying, I should not pretend that this is one of the major threats the slugs pose. They are more likely to cause declines in native slug species before we see any impacts on bird populations. However they are a real threat to Norwegian crops. Like many successful invasive species, they will feed on a wide range of organisms, so while there has been no quantitative study on the economic impact of the slugs yet, there have been reported attacks on everything from potato fields to garden plants to sunflowers.

How do we stop them?

Prevention is almost impossible, as this would involve detailed inspection of all imported plants, which would potentially do little damage to a species which is already well established in Norway.

It is possible to reduce local populations by killing slugs. Decapitation or storage in the freezer are effective methods. Use of homemade explosives is considered excessive, and to be honest if that’s an option then you’re overqualified for pest removal. Elimination of eggs is also recommended, and slug fences (see here for an example) can help keep them out of garden patches. However the techniques are small-scale and labour intensive, and therefore not particularly helpful for the agriculture industry.

On a wider scale, some foreign parasites and beetle have been introduced as control measures, and slug pellets and other molluscides have been used in Europe. However the effectiveness is not proven yet, and some may have negative impacts on native species, and many molluscidal compounds are banned in Scandinavia.

For more information on the Spanish slug, we invite you to read the following studies

Invasive Alien Species Fact Sheet – Arion vulagris by the Online Database of the North European and Baltic Network on Invasive Alien Species

Arion slugs as nest predators of small passerine species – a review by Katarzyna Turzanska and Justyna Chachulska