Effects of nutrient and insecticide treatments on invertebrate numbers and predation on slugs in an upland grassland: A monoclonal antibody-based approach

Slugs can have profound effects upon plant community structure and, by selective grazing, act as ecosystem engineers. Experimental treatments on upland grasslands that included nutrient addition and use of an insecticide were shown to have significant effects on invertebrate communities, estimated by pitfall traps and Tullgren funnel extractions, including potential slug predators. Nutrient enhancement (nitrogen and lime) significantly increased the depth of soil surface thatch, but this did not affect slug numbers or biomass. However, nutrient enhancement did increase numbers of some slug predators. Invertebrates that fed on slugs in the vicinity were first identified by screening a wide range of potential predators captured by pitfall trapping and hand searching. We then tested the invertebrates for slug remains in their guts using a mollusc-specific monoclonal antibody and enzyme-linked immunosorbent assays. The primary slug predators (those that tested positive for slug antigen) were shown to be species of carabid and staphylinid beetle, some of which had not been recorded to eat slugs before. Other slug predators included Silphidae, Araneae and Opiliones. When time was included as a factor, there was no relationship between slug predator numbers and slug numbers, nor were numbers of other invertebrates a significant co-variable. It was predicted that slug numbers should be greatest where there was more surface organic matter (nutrient enhancement treatment), and lowest where slug predators were reduced (insecticide treatment). The lack of any overall effect of treatment on slug numbers possibly suggests that the greater numbers of potential slug predators in the nutrient enhancement treatment suppressed any increase in the slug population, emphasizing the care needed in interpreting such field data.