Spatial distribution and home-range of the pest slug Arion lusitanicus (Mollusca: Pulmonata)

The density and distribution of egg-batches, juveniles and adult Arion lusitanicus, as well as individual home-ranges, were investigated over two seasons. The study was performed on grassland which was mown twice a year and received no herbicides or pesticides, with neighbouring field and stream-side vegetation. Female-mature slugs preferred restricted areas closely neighbouring the taller vegetation of the stream-side vegetation and field, but less-favourable parts of the site were also inhabited in the second year when population densities increased and weather conditions were more favourable. It was postulated that slugs could have home-ranges, as homing behaviour is a well-known phenomenon. The application of a long-lasting individual marking technique enabled calculations of home-ranges (convex polygons) for slugs for the first time. Home-range size averaged 45.4 m² when the population density was low in 1995, and was found to be negatively correlated with density, as it decreased to 12.4 m² in 1996, when population density was high. Mating sites were found to be distinctly closer to the geometric centres of activity than the sites used for egg-deposition. Egg-batches were aggregated under the shelter traps (50 × 50 cm squares of hardboard covered with polystyrene) and the pattern of distribution of egg-batches persisted in the young hatchlings. A number of foreign A. lusitanicus were introduced into the unfavourable centre of the site, and by two to four weeks after release their distribution exactly resembled that of the local ones. In all the various aspects of behaviour investigated, large individual differences were found, suggesting a great plasticity in this generalist species. The importance of the species‘ dispersal capacity in relation to general ecological and applied aspects of control is discussed in the context of the present findings.