Body size and the colonisation of cereal crops by the invasive slug Arion lusitanicus

The invasive slug Arion lusitanicus lives in undisturbed stands of grass or herbaceous plants from where it disperses into crops. Factors that determined the dispersal of slugs from a grassy orchard (O) into an adjacent cereal stand were investigated. Slug-feeding activity, abundance and body weight were established along a 120 m transect across an orchard and a cereal crop. Slugs in the source population in the orchard were more abundant and lighter in 2008 than 2009. The slugs that left the orchard and moved into the cereal crop were heavier than those that remained in the orchard and the distance they travelled was positively correlated with their weight. The abundance of slugs in the cereal crop decreased with the increase in the distance from the source population, more so in 2008, when slugs were light in weight and accumulated near the edge of the orchard, than in 2009 when the slugs were heavier and distributed more evenly. The feeding activity of the slugs leaving the orchard and distance they dispersed also increased during the course of the season. Long distance dispersal is achieved by crawling, the speed of which increased with slug size. Seasonal and annual variation in body size thus affected the distribution of slugs in the cereal crop and the distance they dispersed.