Reduced numbers of generalist arthropod predators after crop management.
Generalist arthropod predators act as natural enemies of insect pests in agro-ecosystems. Crop management activities may cause a reduction in arthropod densities, either directly through mortality and emigration, or indirectly through habitat disruption. Our aim was to quantify direct mortality caused by mechanical crop treatments. For some treatments, we also quantified emigration and other indirect effects, such as population declines caused by the impact of habitat deterioration on reproduction and survival. Direct mortality was determined by measuring predator densities simultaneously in control and treatment plots using closed emergence traps immediately following a mechanical treatment. Treatments consisted of the following crop management activities: ploughing, non-inversion tillage, superficial soil loosening, mechanical weed control and grass cutting. Predator densities were measured a second time 5-26 days after treatment to quantify emigration and indirect effects. All treatments had a negative influence on one or more arthropod taxa. Direct mortality caused a 25-60% reduction in arthropod densities. Overall, spiders were more vulnerable to mechanical crop treatment than carabid and staphylinid beetles. Intensive soil cultivation, such as ploughing and soil loosening, did not kill more arthropods than weed harrowing and grass cutting. We estimated the cumulative effects of mortality, emigration and indirect effects 3 weeks following treatment. The cumulative effect was greater than direct arthropod mortality, suggesting delayed effects of habitat disruption. Grass cutting caused spiders and staphylinid beetles to move out of the crop, except in one case when the grass was left to dry, suggesting an important role of organic material or structural elements for arthropod persistence. Our results also suggest that arthropod predators aggregate in undisturbed or less disrupted habitats. Synthesis and applications. Mechanical operations in arable crops and grass cutting cause mortality and emigration of generalist arthropod predators, especially spiders. Effects can be species-specific and are likely to be affected by the timing of management. In grassland, cutting and removal have greater adverse impacts than leaving cuttings that contribute to habitat structure. Adjacent, less-disturbed refuge areas are colonized by predators following husbandry events, demonstrating significant spatial dynamics among farmland arthropods. The negative effects of crop husbandry operations might be mitigated by the provision of refuge areas within and adjacent to fields and by maintaining crop and landscape diversity.