“Invasion of the giant cannibal Scandinavian slugs: Cambridge gardens under siege from unstoppable monster molluscs” - so said the Cambridge News in September 2014. The truth is slightly less horrific, but still pretty dramatic.
The slug in question, Arion vulgaris, variously known as the Lusitanian, Vulgar or Spanish Slug (and now, Scandinavian Slug!), has been in Britain for at least 50 years, it’s no larger than several other species, it’s no more prone to cannibalism than our native slugs (of which, more later), it’s from the Mediterranean not Scandinavia, and it’s had a good year but is probably no more a threat to gardens than several much smaller species. This species and its relatives have an annual life cycle, breeding in summer and autumn then dying off. Newly hatched youngsters are sensitive to frosts and to drying out - so a mild winter and a wet summer are good for them. They are hermaphrodites, and each can lay up to 400 eggs, so they can build up large populations quickly.
This is one of many species of slug which have arrived, or become much more abundant and widespread in Britain in the last century. When, as a student, I helped write the standard guide to slugs back in 1983, we included 30 species. The latest guide published this year, has 43. I can’t think of any other group of plants or animals which has changed so much in 30 years.
The slug which first captured my interest was the Durham Slug. In my first week at university, on my first visit to the science library, the first journal I took off the shelf was the Bulletin of the British Museum. It fell open at a paper which referred to a slug new to Britain from 24 North Bailey Durham. At the time, I was living about 6 doors away! I spent quite a few evenings in my three years there studying this attractive and variable species, which was little-known and didn’t even have a valid scientific name at the time. We now know that it was first recognised in southern Ireland in 1893, then forgotten for 60 years till it turned up in Durham. I didn’t see the Durham Slug in central England till the 2000s, but in recent years I’ve found it at Flitwick Moor, Harlestone Firs and last year, at Cambourne (with its bigger ‘Scandinavian’ relatives).