Assessment of slug populations in grassland with permanent refuge traps

Abstract: Surface refuge traps are recommended by researchers, consultants and manufacturersof molluscicides to monitor slug density-activity in arable crops, but pastureland differs fromarable land with respect to many characteristics which play a major role in the slugs’ biology.Here we report on the performance of non baited mat refuge traps permanently placed at the sameposition over more than two years in established pasture, for the assessment of slug numbers andbiomass, in comparison with soil sampling and flooding over three days. Despite the availabilityof alternative shelters provided by the vegetation, a great many slugs and slugs’ eggs wereregistered in the traps over the year and over a wide range of temperatures under the traps.Overall, traps showed the same trends as in arable land: traps showed a bias towards the largerindividuals and underestimated the numbers of the smallest slugs of each species (Derocerasreticulatum (Müller), Deroceras panormitanum (Lessona & Pollonera), Deroceras laeve (Müller)and Arion intermedius (Normand).