Some large arionid slugs are now considered to be important agricultural pests in many areas of Continental Europe, where 'slug plagues' have become a common phenomenon. However, identification of arionid taxa is problematic, complicated by common colour varieties and morphology, confounding attempts to map their distribution and differentiate pests and non-pests. We have used a PCR-based molecular approach combined with anatomical investigations, first as an aid to identification and subsequently as a tool for investigating the ecology and reproductive strategies and phylogeny of this group. Our evidence suggests that the large arionids are divisible into four distinct taxonomic groups, of which three appear capable of generating pests, distinguishable from their congeners on the basis of reproductive strategy and life-history data. Using this integrated approach we have produced a concordance between colour varieties and taxa, allowing use of varietal distribution records from the early 19th century to the present day. Combining these data with our own pan-European survey has made it possible to identify the rapid recent advance of some taxa throughout the UK. Their life-history and colonization profiles closely parallel those of more strictly Continental species shortly before they began to produce major outbreaks and became serious agricultural pests, suggesting that these taxa deserve further attention from agriculturalists. Future studies may be assisted by the taxon-specific molecular markers we have identified.