Predation by the carabid beetles Pterostichus madidus and Nebria brevicollis is affected by size and condition of the prey slug Deroceras reticulatum

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Abstract
Summary: 1 Slugs are important pests in many agricultural crops but molluscicides commonly used to control slugs affect non‐target organisms. Encouraging biological control may help to reduce molluscicide use, but the efficiency of potential natural enemies needs to be investigated.2 Serological tests have shown that certain carabid species consume slugs. These techniques, however, do not distinguish between scavenging and true predation, nor do they provide information on the size or other characteristics of the prey consumed. The study reported here was undertaken to establish whether scavenging of dead slugs might be an important factor contributing to positive serological test results.3 Both Pterostichus madidus (Fabricius) and Nebria brevicollis (Fabricius) consumed Deroceras reticulatum (Müller) under laboratory conditions. Dead slugs were scavenged in preference to injured or healthy slugs.4 Only small, live slugs (< 0.11 g) were killed by both beetle species, which may, therefore, be incapable of killing larger slugs.5 These generalist beetle species appeared unable to overcome the defence mucus produced by slugs. The data suggest that positive serological results from field collected beetles may reflect scavenging rather than predation on live or injured slugs.