A Great Black Slug (red form) on patrol

Earlier this week, after an impressive thunderstorm, I popped into the garden and was astonished by the number of slugs gathered beneath the bird feeder, gobbling up the suet and the dried mealworms. I crouched down for a closer look . With their frilly tomato-coloured skirts, I assumed that these were the red form of the Large Black Slug, a native British slug which is mainly interested in eating decaying matter, and which is often found in compost heaps. One of the slugs was dragging along a mass of gravel attached to its back end, so I gently removed the encumbrance. The slug stopped, drew in its tentacles one by one, and contracted into a little dome.

Slug in defensive posture

Slugs are descended from snails, and so a slug in distress will try to withdraw into its shell. Sadly, most slugs no longer have any shell at all, and so they have to make do with sheltering under their mantle, which is slightly thicker than the rest of their skin. Their only other defence is their rather revolting mucus, which they can produce in prodigious quantities if attacked by a curious fox or cat.

However, just lately I have been regarding my slugs with a little more anxiety than previously. The headlines in the papers haven’t helped.

‘Can science stop the invasion of the giant killer slugs?’ (Guardian October 2013)

‘Catch the killer slugs! Spanish molluscs ‘on rampage’ in Britain’s gardens’ (Metro October 2013)

It appears that a close relative of our Large Black Slug, the Spanish Slug (Arion vulgaris), was identified in Norwich last year. The worry is that because it lays three times as many eggs as ‘our’ slug, it will out compete it. The Spanish Slug also has very cosmopolitan tastes – roadkill, dog faeces and even other slugs can form part of the menu, which does nothing for its public image.  At the moment, the Spanish Slug is vulnerable in this country because it can’t survive frost. But what, the scientists ask, will happen if it interbreeds with our slug, for whom winter holds no terrors? The Spanish Slug and the Great Black Slug are so closely related that they can only be told apart by having their genitalia dissected. For the non-scientist, there seems to be no easy way of distinguishing them from one another.

A Spanish Slug (Arion vulgaris) – image from the Slugwatch website (see below)

A Great Black Slug (Red Form) – Arion ater

So, what to do? Well, in my case, nothing. My slugs are innocent until proven guilty. I’ve got no intention of drowning them, desiccating them with salt or boiling them on the off chance that they are the Spanish species, and slug pellets are out of the question – I have too many frogs and ground beetles who eat the young slugs to risk putting down poison.

We have a great fear of ‘invasive species’ – maybe it’s partly to do with living on an island. Sometimes, the creatures and plants that set up home here do become a problem. But generally, they become absorbed into the great ecological community after a while, and everything settles down again. With climate change, we can expect to see more and more species making their home here that would not previously have survived, and at the same time other species will move. The creatures that suffer most are likely to be our upland animals and plants, who will eventually run out of places to go. Everything is changing, at an unprecedented rate, creating opportunities for some species, and sounding a death knell for others. The Spanish Slug saga is not the first of its kind, and will not be the last.

For anyone interested in more details on the Spanish Slug, or the other slug species, I can recommend the  Slugwatch website for lots of interesting information.