1. Interactions between the polyphagous carabid predator Pterostichus melanarius (Illiger) and slugs were investigated from July to September 1992, before and after harvesting a rape crop. The experimental site comprised a long-term field study of the effects of different forms of cultivation (ploughing vs. non-inversion tillage), and methods of straw disposal (baling vs. incorporation of chopped straw) upon invertebrate populations and crop yields. Direct-drilling was also included as a no-tillage base-line. 2. Beetles (total 2078) were collected by pitfall trapping twice weekly. Each beetle was dissected, and its crop contents weighed and tested by enzyme-linked immunosorbent assay (ELISA) to determine the concentration and quantity of slug haemolymph it contained, Slugs [Deroceras reticulatum (Muller) and Arion intermedius Normand] were extracted from soil samples by gradual flooding, to estimate both numbers and biomass. 3. Significantly more P. melanarius were trapped in direct-drilled plots than in the tilled treatments. Within the tilled treatments, greater numbers of beetles were trapped where straw was incorporated by non-inversion tillage. 4. Crop weights were significantly greater in beetles from direct-drilled plots than in those from tilled treatments, as were both the concentrations and quantities of slug haemolymph they contained. Overall, approximate to 84% of beetles contained slug remains. 5. Greatest concentrations and quantities of slug remains were detected prior to the disposal of rape residues at the end of July, by baling or shallow incorporation in the soil. Cultivation had both short- and long-term effects upon the proportion of the diet of the beetles that was slugs. 6. Slug biomass declined following disposal of rape residues and it was only after this time that significant treatment differences emerged. 7. Positive relationships were found between the biomass of slugs in the soil and numbers of beetles trapped, the proportion of the beetles' diet that was slugs and the quantities of slug haemolymph in beetle crops. 8. Our results strongly suggest aggregation of P. melanarius ius to areas of high slug biomass in the soil and preferential feeding in such areas upon slugs. As this carabid is probably the commonest large predatory beetle in arable crops in Britain, these results clearly identify P. melanarius as a potentially important slug control agent.
