Feeding preferences of a predatory beetle (Pterostichus madidus) for slugs exposed to lethal and sub-lethal dosages of metaldehyde

Carabid beetles are species rich and abundant in arable habitats worldwide and, due to their predatory polyphagous nutrition, potentially important as natural pest control agents (Kromp, 1999). Diversification of agricultural field margins with techniques such as sowing wildflower mixes can enhance populations of arthropods such as carabid beetles (Asteraki et al., 1995). Management of this type has been shown to reduce densities of pest insects in close proximity to field edges (e.g., Lys & Nentwig, 1994). However, an associated increase of populations of pest species of slug adjacent to sown wildflower margins has been documented (Frank, 1998). Under such circumstances, it has been suggested that chemical protection should be increased at the edges of crops (Friedl & Frank, 1998). This scenario provides slug predators with a range of potential prey items as a consequence of sub-lethal and lethal doses of molluscicides, in combination with unaffected living animals and those that have died from non-chemical causes. Applications of slug pellets to crops have been shown to dramatically depress carabid populations (Purvis & Bannon, 1992), but little is known about effects of molluscicides on arthropod foraging behaviour. Slug pellets are fed on directly by carabids and can even provide good control of some species (Kelly & Martin, 1989). Lack of sensitivity to such baits by other carabids may result from the poor attractiveness of the pellets as food items (Buchs et al., 1989). However, there is a requirement for more knowledge of beetle preferences for slugs that have consumed pellets. Investigations of beetle-slug interactions may be considered timely due to new approaches towards field margin management and the suggestion that carabid beetles such as Pterostichus sp. are potential biocontrol agents for slugs (Symondson & Liddell, 1993). This study examines the feeding behaviour of the carabid beetle Pterostichus madidus (F.) (Carabidae, Pterostichini), which can reduce densities of slugs in the field (Asteraki, 1993). Laboratory observations of the frequency and duration of beetle attacks on living and dead individuals of the grey field slug (Deroceras reticulatum (Muller) Limacidae) are used to assess the influence of lethal and sub-lethal dosages of metaldehyde ingested by the slugs on prey selection.