Effects of cultivation techniques and methods of straw disposal on predation by Pterostichus melanarius (Coleoptera: Carabidae) upon slugs (Gastropoda: Pulmonata) in an arable field.
Interactions between the polyphagous carabid predator Pterostichus melanarius and slugs were investigated from July to September 1992, before and after harvesting a rape crop in the southern UK. 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) on invertebrate populations and crop yields. Direct-drilling was also included as a no-tillage base-line. 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 and Arion intermedius) were extracted from soil samples by gradual flooding to estimate both numbers and biomass. 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. The results strongly suggested aggregation of P. melanarius to areas of high slug biomass in the soil and preferential feeding in such areas on slugs. As this carabid is probably the most common large predatory beetle in arable crops in Britain, P. melanarius is clearly indicated to be a potentially important slug control agent.