Spanish Slug

• Although it is called the Spanish slug, genetic studies have found no clear native populations on the Iberian Peninsula and its true origin remains disputed.
• A single slug can lay several hundred eggs and, being hermaphroditic, any one individual introduced can found a population.
• It is one of the worst vegetable-plot and garden pests in Europe, and a first-rank invader that has spread across almost the whole continent in recent decades with the trade in plants and soil.
• Its origin is a curious story: for years it was said to come from the Iberian Peninsula — hence its popular name, the Spanish slug. Later genetic studies have not confirmed that Iberian origin, and today it is considered more likely to come from central or western Europe.
• It hybridises with native slugs of the same genus, which complicates its identification and adds a conservation problem for the local species.
• It is hermaphroditic and highly prolific: a single slug can lay several hundred eggs.
• When disturbed it contracts into a hemispherical ball and rocks from side to side, a characteristic behaviour.
• Its mucus is thick and sticky, which makes it unappetising to many predators and also makes it awkward to handle.

Skeleton type

Invertebrate - no rigid skeleton (mollusc)

Habitat and environment

Gardens, vegetable plots, verges and damp grassland

Sexual maturity

2-3 months

Gestation / incubation

3-4 weeks

Litter / clutch size

200-400 eggs per clutch

Sources consulted: Zemanova et al. (2016), trabajos sobre el origen de Arion vulgaris · CABI Compendium · Rowson et al., Slugs of Britain and Ireland

Best time: Spring and autumn, on damp nights or after rain