Spatial dynamics of predation by carabid beetles on slugs

Abstract

Summary1. An explicitly spatial sampling approach was employed to test the null hypothesis that the predation on slugs by the carabid beetle Pterostichus melanarius (Illiger) was opportunistic. 2. The beetles and slugs were sampled across a nested series of grids of sampling points, in a ®eld of winter wheat during June and July 1997. 3. The spatial distribution of all slugs in June was found to change with the scale of the sampling grid, from random on the 0.25 m scale, through aggregation at 1 m, to random at 4 m. At the highest scale of 16 m, the slugs were signi®cantly spatially aggregated. 4. The distribution of beetles in June was also spatially dynamic, with randomness observed at the 4 m and 8 m scales. At 16 m, signi®cant aggregation was observed. 5. The dynamic distributions of slugs and beetles, at 16 m, were found not to be associated with, and thus were not determined by, soil or crop factors. 6. Comparison of slug and beetle populations showed, however, that the distributions at 16 m were dynamically associated with each other. In June where there were many slugs there were also many carabids, whilst in July where there were many carabids there were few slugs. 7. Approximately 11% of the beetles sampled across the 16 m grid in June and July were found to have ingested slug protein, following intensive enzyme-linked immunosorbent assay (ELISA) testing. 8. The spatial distribution of these slug-positive beetles was signi®cantly associated with the distribution of the larger slug classes, over 25 mg. Where there were many large slugs in June there were many slug-positive beetles. Conversely, in July few large slugs were found where there were many slug-positive beetles. 9. Parametric analysis revealed that these changes in the large slug class, at each sampling point between June and July (growth), were negatively related to the local numbers of slug-positive beetles, and that growth declined as the local numbers of beetles increased. 10. These ®ndings suggest that predation was not opportunistic, but direct and dynamic, falsifying the null hypothesis. Moreover, this predation elicited signi®cant changes in the spatial distribution and local density of the slugs, in a manner that may be termed spatially density dependent.

Effects of Cattle on the Abundance and Composition of Carabid Beetles in Temperate Grasslands

“…All but one of the carabid species captured are known to be carnivorous as adults; the exception being Amara obesa, a species that is mainly herbivorous as an adult, and eats grasshopper eggs as a larva (Lindroth 1969). Insects can be regulated by plants (Ayal 1994), and will migrate to areas of high food availability (Bohan et al 2000). Upper elevation sites produced more plant biomass, which possibly provided more food resources for the herbivorous adult A. obesa.…”

The quality of aphids as food for generalist predators: implications for natural control of aphids

“…A third example may be that of control of field slugs Deroceras reticulatum by carabid beetles in the UK. Bohan et al (2000) found "spatial density dependence" between the slugs and the carabid beetle Pterostichus melanarius, resulting in significant local reductions in slug densities. However, laboratory tests have demonstrated low food quality of these slugs for P. melanarius and other carabid species (Mair & Port, 2001a, b;Fawki et al, 2005).…”

The potential of carabid beetles (Coleoptera) to reduce slug damage to oilseed rape in the laboratory

“…Although P. melanarius is a generalist predator, a reduction in slug population growth rate was observed in field areas with high densities of P. mela narius (Bohan et al, 2000), which further shows that slugs were a single important prey type for P. melanarius. The fact that one type of prey plays a significant role for a generalist predator may be common in agroecosystems, where food diversity for carabid predators appears to be scarce (Van Dijk, 1996).…”

“…P. melanarius was detected as a slug predator in the field, using immunological methods (Tod, 1973;Ayre & Port, 1996;Symondson et al, 1996). P. melanarius seems to be a potentially important slug control agent as this bee tle's spatial distribution was found to be significantly associated with slug distribution in winter wheat (Bohan et al, 2000). Moreover Symondson et al (2002) observed a strong relationship between the crop weights of P. melanarius and slug densities over five years, indicating that slugs were a major part of the diet of the beetles.…”