Crop type, crop management and grass margins affect the abundance and the nutritional state of seed-eating carabid species in arable landscapes

Promoting the ecosystem service of crop pest biological control requires to understand the drivers of the spatio-temporal distribution of natural enemies in agricultural landscapes. In this study, we analysed the functional role of different habitat types that are widespread in mosaics of arable farmlands for three abundant weed seed-eating carabid species (Poecilus cupreus (L.), Amara similata (Gyll.) and Pseudoophonus rufipes (DeG.)) Carabids were sampled in an experimental design of 5 replicates for each of three habitat combinations, i.e. oilseed rape fields adjacent to winter cereal fields, oilseed rape fields adjacent to grassy field margins and winter cereals fields adjacent to grassy field margins. The effect of habitat type (crop type and semi- natural habitat), habitat management (insecticide use) and of the adjacency to a grassy field boundary on the activity-density and the nutritional state of were tested for the three species. Our results indicate that oilseed rape is of prime importance for the two spring breeders P. cupreus and A. similata in terms of activity density and nutritional state, although high levels of insecticide use had a detrimental effect on the nutritional state of individuals. Grassy field margins were used by the three species at least during the beginning of their activity period but, during the peak of activity of the two spring breeders, no positive effect of the adjacency of a grassy field boundary could be detected on the abundance and the nutritional state of individuals in the adjacent crop. Conversely, grassy field margins had a positive effect on P. rufipes which peak of activity occurred after crop harvest. This study highlights the importance of complementation and spillover processes in the functioning of populations living in agricultural shifting mosaics. Particular attention should thus be paid to the spatial distribution of cropping systems at various spatial scales if we are to enhance populations of organisms of benefit to agriculture.