Undoubtedly, some species, usually the most abundant ones, are pestilential. Loathsomeness, on the other hand, like beauty, may be in the eye of the beholder.
Not the culprits
Perhaps the species at the top of the gardener’s hit list will be the large black slug and its four close relatives – collectively known as large roundback slugs. On damp, cool evenings it can be difficult to avoid treading on the abundant beasts. Due to their size and colour they are well-recognised and have become the scapegoats for all damage caused by any slugs.
In fact, these slugs are pretty neutral as far as gardeners are concerned. They may attack some vegetables or flowers but far prefer decaying organic matter. Why spend energy on breaking down and digesting fresh green matter when you get enough nutrients more easily by clearing up dog droppings? Their main role is recycling organic matter.
Species such as the green cellar slug actively avoid plants in favour of algae, fungus and lichen. The leopard slug is an omnivore which preys on small snails and slugs in a gardener-friendly diet.
Small nuisances
The big pests for gardeners are the smaller species, often present in great numbers. Hiding by day, they emerge at night to do their business. Also pestilential are small snails, many under 5mm in diameter, generally overlooked by gardeners but at least as damaging as small slugs. But, to paraphrase the musical hall song
She was poor but she was honest,
It’s the snails that eats the hostas and the slugs that gets the blame.
The Large Black Slug (Arion ater) is a native species seen throughout the British Isles, not always black. Some juveniles are pale yellow, and the dark adult colouration starts on the back gradually working down to the foot fringe (a skirt-like feature around the bottom of the sides).
Slugs are hermaphrodite and self-mating is possible. For the large black, self-mating seems almost obligatory. If an individual has a particular genetic feature, it will likely pass to its offspring. So slugs in one place resemble each other. For example, a population of large black slugs found at Storr in Skye comprised just white individuals.
The slug that rocks
The large black slug has one curious characteristic, useful in separating it from the other four species in the group. When irritated, mature individuals rock from side to side in a motion difficult to describe, recognisable if you see it (see video below).
To irritate the slug just stroke it gently on the mantle (use a stick for this – you do not know where the slug has been). If you have a rocker, it is almost certainly the native large black. This species is becoming uncommon in gardens, suffering from competition from the other four species unwittingly introduced by gardeners.
Other large roundbacks
I often visited Wicken in Cambridgeshire in the mid 2010s. The large roundback slugs in the village were exceptionally variable in colour but, as soon as one reached the fen they were all black. The variability away from the fen suggested species other than the large black, their uniform appearance on the fen suggesting a closed homogeneous self-mating population.
The Large Red Slug (Arion rufus) is usually orange and the foot fringe much brighter than the body. Taxonomists debate whether the red and black slugs are the same species. The pendulum swings from one view to the other and I don’t doubt will swing back before long.
The Green-soled Slug (Arion flagellus) is widespread and typically brown. The sole is plain with a faint greenish tinge and no lines going in from the foot fringe. It has been found recently on the fen and it is likely to replace the large black slug in the next few years.
The Vulgar Slug (Arion vulgaris) is also present in Wicken. First recorded in Britain in 1964, it became widespread in the early 2010s. The Daily Mail made dire predictions about this ‘new’, invasive species: it could cause the extinction of our native slugs; devastate food crops in fields and gardens; it was a cannibal; it fed on rabbits. (Actually true, but they didn’t say the rabbits had to be dead first.)
It has been named Plague Slug, Cannibal Slug, Stealth Slug, Spanish Stealth Slug, Lusitanian Slug and Iberian Slug, suffering more negative misinformation than other species. But why let facts interfere with a good story?
The beast is usually brownish but varies. Look for the colour of the inner rim of the breathing pore (on the right side of the mantle) – it is almost always very dark or black.
A species new to science
That brings us to the last of the five species – Stella Davies’ slug (Arion sp. Davies), which was almost called the Wicken Slug. Among the large roundbacks I saw in Wicken village, several were an unusual grey colour.
They were clearly not the large black slug (did not rock), nor the large red slug (foot fringe no brighter than the body) nor the green-soled slug (tubercles too tightly packed). That left the vulgar slug – except for the unusual colouration.
I sent some specimens to Ben Rowson, keeper of molluscs at the National Museum of Wales. Initially unsure, he subjected it to DNA analysis with surprising results. It was different from all other large roundbacks found in Britain and Europe. Thus we had a species new to science.
What to name it?
All that was needed was to publish a formal description and name it – perhaps the Wicken Slug, Arion wickensii.
However Ben was, at the time, studying some preserved specimens bequeathed from lifelong slug enthusiast Stella Davies. Some were an exact match to the Wicken Slug. Her slug was therefore the first found and so bears her name. It is unknown how widespread this species is now. DNA seems the only reliable way of identifying it.
The Wicken slug is now preserved in the National Museum of Wales; its image is in Ben’s guide.
Up-to-date distribution maps of these species are on the NBN Atlas site, click here. This national treasure is free to use and gives access to records of all species (plants, animals, fungi, not just slugs).
It would be interesting to know whether Wicken still has the same variety of species, and to discover more about Stella Davies’ Slug – not just in Wicken but everywhere.
I hope I have given enough hints about what to look for, how to identify slugs and especially, as this video shows, how they rock.
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