Flowering cover crops in winter increase pest control but not trophic link diversity

In agrosystems, the increase in non-crop plant diversity by habitat management in or around arable fields contributes to improved Conservation Biological Control. During winter, plant flower are often used as mono specific ground cover and are expected to die before flowering as a result of recurrent frost events. Decreases in minimal temperature due to climate change offers new possibilities for plants used in such sown cover crops to mature and flowers. Changes in plant phenology thus constitute an important environmental change with expected consequences for ecosystem functioning, such as biological control. In Brittany, where winter agricultural landscape is dominated by a mosaic of cereal and sown cover crops, we assessed the consequences of mustard (Synapis alba) flowering cover crops (MFCC) on aphid parasitism and food web structure in plots adjoining cereal crops, in contrast to plots close to spontaneous non-crop plants (SNCP) of the same field. Overall, aphid parasitism rate at the field scale was strong (60-70%), being 13% higher adjacent to the MFCC than closer to SNCP. In addition, there was no change in food web structure between the two distinct zones, enabling us to hypothesize that MFCC mostly constituted an alimentary patch. The positive effect on parasitism rate was significant but weak, as floral nectar of mustard is known to be of poor quality for parasitoids. Results highlight the potential advantages of adapting practices in response to actual changes in agrosystems. Increase floral diversity in sown cover crops could constitute a complementary method in management programs, by providing more alternative food resources, alternative hosts, and climatic refuge to enhance the Conservation Biological Control of parasitoid populations.