Recolonization by Slugs: Vertical and Horizontal Dispersal by the Field Slug, Deroceras reticulatum

SIMPLE SUMMARY: Slugs are persistent pests. Following control treatments, the numbers of slugs can rapidly return to pre-treatment levels and it is often assumed they are migrating in to the site from adjacent areas. By comparing plots with restricted access to those with open access over a 32-month period we were able to compare the importance of migration from adjacent areas with migration from the soil below the plots. For Deroceras reticulatum, which is a major pest species, a large proportion of immigration came from soil below the undisturbed grassland plots. The importance of this inactive subpopulation, below the soil surface, needs further study. ABSTRACT: Following treatment with molluscicides or other controls, slugs can recolonize a site very quickly, but the proportion of the colonizing slugs moving from adjacent areas (horizontal dispersal) and the proportion from within the soil (vertical dispersal) has not previously been established. At a grassland site, barriers were used to exclude and trap slugs in order to estimate horizontal and vertical movement over a period of 32 months. For the first 15 months vertical movement made a significant contribution to the slugs recolonizing a grassland area. The ecological mechanisms occurring and the implications for the control of slugs are discussed.