Effects of slug size on predation by Pterostichus melanarius (Coleoptera: Carabidae)

The carabid predator Pterostichus melanarius (Illiger) is known to consume slugs in arable Welds and may be a significant natural slug control agent. However, any quantitative study of the dynamics of this predator-prey relationship will require information on the size range of the slugs consumed. Individual P. melanarius were confined with individual slugs, Deroceras reticulatum (Muller), of different sizes (ranging form newly hatched to adults) in arenas. They readily attacked, killed and consumed slugs weighing <40 mg. Slugs >40 mg were killed by the beetles after prolonged exposure to attack where the slugs had no means of escape. In a second experiment, in larger laboratory arenas, P. melanarius demonstrated a strong preference for the smallest individuals, when offered a choice of slugs <40 mg. The implications of these data are discussed with regard to optimal foraging theory and the potential impact of P. melanarius on slug populations in the field.