Managing agricultural landscapes to enhance biological control requires an understanding of the spatio-temporal dynamics of beneficial arthropod populations. In agricultural landscapes, functional heterogeneity results from both the spatial pattern of landscape elements and their rapidly changing of habitat suitability. This suitability dynamics results first from the within-year dynamics of vegetation growth and crop management practices, and second from the year-to-year dynamics of crop rotations.

We investigated the impacts of cropped habitat dynamics on population recruitment, distribution and movements of a beneficial predatory carabid beetle (Pterostichus melanarius). Our hypothesis is that populations are sustained by the beetles mobility that enables them to exploit the temporal complementarity of different adjacent crops and boundaries.

The study was conducted in a mosaic of five adjacent fields of cereals and maize (total area 30 ha) in Western France. The mosaic was surrounded by roads isolating the population. Three types of sampling were done weekly from May to September 2009: 1) the field productivity in terms of post-emerged adults with enclosed emergence arenas; 2) the overall activity-density of carabid beetles in open pitfall-traps and 3) the exchanges between fields and their field boundaries and adjacent fields, with directional barrier traps. 

Results/Conclusions
On average, in the cereal fields, emergent adults are eight times more abundant ( ~45 indiv/m²) than in maize fields ( ~5 indiv/m²), suggesting that cereals acted as a productive source of predatory carabids early in the season. The low productivity of maize may be due to the destructive effect of spring soil tillage on late larvae stages and pupae. In spring, the activity-density rapidly increased in cereals, explained at thirty percent by local emergences. Interception traps showed that carabid movements were more important between crop fields than between fields and their boundaries. In the early season, carabid beetles moved significantly from maize to adjacent cereal due to the low habitat quality in maize. Later in the season, after cereals harvest and the closure of maize canopy, maize fields appeared as high quality habitat, massively colonized by carabids.

This medium landscape-scale study reveals that different crops act as complementary habitats in the season. Despite the apparently sink role of maize early in the season, it appears as a refuge during the reproductive period for within-cereals emerged adults. Therefore, the crop mosaic spatial and temporal structure is shown to play a crucial role for the viability of crop carabids populations.