Short-term effects of vegetation management on epigeal predatory arthropods in organic farming systems
Predatory arthropods such as carabid beetles and spiders are generally thought to be favoured by crop management that results in dense vegetation. In grassland, silage production increases vegetarian density compared to grazed grass but involves disturbance from cutting, while intercropping with clover increases the vegetation density of vegetable crops, but causes yield loss through interspecific competition. The first part of this thesis compares the predatory arthropod fauna of silage with grazed grass. There were small differences between the carabid assemblages of the two habitats before the first silage cut, when the contrast between silage and grazed grass was greatest, and a general reduction in species richness following silage cutting. However, the responses of individual species varied greatly. Agonum muelleri and Calathus fuscipes were associated with grazing management, but no species were consistently favoured by silage production. For Loricera pilicornis, this may have been due to cool weather creating similar conditions in silage and grazed grass, while for Nebria brevicollis, which was susceptible to disturbance, the timing of cutting determined its distribution. Spiders showed a more uniform association with structurally complex vegetation and were thus generally found in higher numbers under silage management. The second part of the thesis investigates daily movements of predatory arthropods between dense vegetation, such as clover intercropped vegetables, and adjacent open, weeded crops. The results suggested that nocturnal carabids and spiders of the genus Erigone sheltered in dense vegetation by day and moved into the open at night. Thus intercropping only parts of a vegetable crop would enhance predatory arthropod activity throughout the crop, while reducing interspecific competition. These results showed that the relationship between increased vegetation density and predatory arthropod activity is less important in the habitats studied than other factors, such as disturbance and daily movements in determining the distribution of these animals.