Farm-scale spatiotemporal dynamics of predatory beetles in arable crops.

Abstract

1. The spatial dynamics of farmland invertebrates can provide essential information relevant to their management for pest control and biodiversity conservation in sustainable agriculture. Carabid beetles are one of the most important groups contributing to biological control in arable fields. Previous studies have focused on spatial dynamics within single fields and years. In this study we examined their larger scale, long-term dynamics, thereby taking into account the impact of changes in crop rotation and the influence of field size.
2. The spatial distributions of four beetle species were investigated at an unprecedented spatial scale in a grid of 973 pitfall trap locations across six fields encompassing 64 ha of arable land. Week-long trapping was conducted four times in the first year and twice in the two following years. 
3. All species showed strong aggregation but the size and location of patches differed among species. The distribution of Pterostichus melanarius was stable within and between years, with a single large patch close to the field boundaries. Patches of Poecilus cupreus were also located close to field boundaries but their location changed between years. Pterostichus madidus and Philonthus cognatus distributions extended across field boundaries and were less stable, with patch locations changing between years. 
4. Synthesis and applications . The spatial extent of a population patch for a given species was species-specific. Species overwintering in field boundaries remained in proximity to these throughout the summer, whereas patches of mid-field overwintering species were more extensive. Patches were generally stable within years but varied for some species between years. Species therefore differ in their response to crop management practices and consequently blanket management approaches for these important generalist predators of crop pests are inappropriate. For spatially stable species (e.g. Pterostichus melanarius) it may be possible to determine their specific habitat requirements and to devise predictive and protective measures to preserve populations or manipulate them at the farm-scale. More mobile species may be better at responding to pest aggregations at the farm-scale. However, operations that deplete populations, e.g. soil cultivations, should be spatially and temporally desynchronized at the farm-scale to conserve populations and enable functional biocontrol.