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.

No abstract

Although communities of generalist invertebrate predators are known to be capable of suppressing pests, little is known about the long-term population dynamics of individual species of generalist predators and single classes of prey in the field. We present evidence of a dynamic interaction between a generalist insect predator, the carabid beetle Pterostichus melanarius, and its slug prey.We analyzed the numbers of slugs and beetles in an arable field over a 5-yr period, during the main activity period of the beetles from June to September. The field contained 25 plots comprising five replicates of five cultural treatments. An index of the nutritional status of the 8497 beetles collected was obtained by weighing the crop of each beetle (a measure of total prey availability per predator).There was a strong relationship between the crop mass of the beetles and slug numbers in the soil, indicating that slugs were a major part of the diet of the beetles. The change in the beetle population from year to year was strongly related to both slug numbers in the soil and the crop mass of the beetles. This indicated that the slugs influenced the nutritional status, and hence the reproductive success, of the beetles. The predators had a significant effect on slug population growth between years but not between months within years. The temporal effect of these processes was a between-year coupling of beetle and slug population dynamics, buffered by feeding on other prey. The relationship appears to be similar to that between mammalian predators and limited numbers of prey species in the subarctic, where one prey species forms a substantial proportion of the total available food resources. Such relationships may be common in agroecosystems, where species diversity is low, and may be a significant factor driving periodic fluctuations in the abundance of both predators and pests.

The intensity of seed predation the invasive tropical legume Leucaena leucocephala by the bruchid Acanthoscelides macropthalmus was investigated in south-eastern Queensland, Australia. The number of seeds damaged by A. macropthalmus as a proportion of total seeds available was found to increase the longer the pods remained on the tree. Seed predation ranged from a mean of 10.75% of seeds on pods that remained on the plant for 1 month and increased to 53.54% for pods that remained of the plant for 4 months. The low bruchid populations at high pod densities results in 'predator satiation'. However, pods dehisce over time and the proportion of pods available over time to the bruchid correspondingly declines. By the time bruchid densities build up, most pods have dehisced and the seeds consequently escape predation. As a result the number of seeds lost to bruchid damage increases only marginally over time. Despite the levels of seed predation observed over the course of the study, the number of seeds in the soil seedbank almost doubled over time increasing from 8.5 seeds m -3 to 15.5 seeds m -3 over a 4-month period. Levels of seed predation and addition of seeds to the soil seedbank were not correlated. The taxonomic (subspecies) status and apparency of host plants as measured by plant and patch traits (average plant height, density of podding plants and patch size) did not influence levels of seed predation. Pre-dispersal seed predation studies need to take into account the pod/seed retention behaviour of the plant. The ability of the bruchid to regulate the invasiveness of Leucaena through influencing its demography is likely to be diminished if the insect populations cannot increase rapidly enough to use the seeds before pod dehiscence.

Summary.Levels of damage by mixed natural infestations of the leaf‐feeding chrysomelid beetles, Phyllodecta vulgatissima (L.) (the blue willow beetle) and Galerucella lineola (Fab.) (the brown willow beetle), were determined in replicated field plots of 24 Salix clones at Long Ashton (Bristol, UK) during 1993–94.Over the same period, the host plant preferences of both chrysomelids were investigated in a standard multiple‐choice laboratory procedure, where beetles were enclosed in Petri dishes with leaf discs cut from young pot‐grown trees propagated from shoot cuttings taken from 20 of the 24 willow clones represented in the field study. The laboratory experiments indicated that P. vulgatissima and G. lineola had similar host plant preferences in the range of willows examined (r >0.85). In both field and laboratory, the least preferred Salix clones and hybrids were those of 5. eriocephala, followed by S. purpurea, S. burjatica, S. dasyclados and S. triandra. Clones of S. eriocephala and S. purpurea were frequently rejected altogether in laboratory tests. Most preferred were clones of S. viminalis and several hybrids of S. viminalis, S. aurita, S. caprea and S. cinerea. These results substantiate the reports that P. vulgatissima and G. lineola are deterred from feeding on willows which have relatively high concentrations of phenolic (salicylate) glucosides in the leaves. The least preferred willows, particularly S. eriocephala, S. purpurea and S. burjatica, could be of great potential value in plant breeding for resistance to these willow beetle pests.