Within-field floral resources have the potential to increase parasitism rates in winter oilseed rape pests more than resources at field margins

Pollen beetles (Brassicogethes spp.) and stem flea beetles (Psylliodes chrysocephala) are among the main pests of oilseed rape crops in Europe, causing high yield losses. Increasing insecticide resistance and environmental concerns have highlighted the need for alternatives to chemical control, such as conservation biological control. Conservation biological control can be enhanced by providing resources to beneficial organisms in and around crop fields. Flower strips are a promising tool to provide nectar resources for parasitoids. In addition, the planting of such resources in field margins should be reinforced by the absence of insecticide application. However, the presence and amount of weeds within the field could mitigate the effect of flower strips and needs to be explored. We surveyed 25 oilseed rape fields in 2019 and 2020. Ten of these fields were adjacent to a sown wildflower strip. We compared an eco-friendly strategy (wildflower strip without insecticide in spring) to a conventional approach (no wildflower strip and chemical control of pests). We assessed the parasitism of stem flea beetle and pollen beetle larvae in the spring, at distances of 5 m and 30 m from the wildflower strip or the natural field margin. We performed botanical surveys in wildflower strips and natural field margins and within the field, to assess the availability of accessible nectar resources to parasitoids. Stem flea beetle parasitism rates were not affected by the strategy used, but the eco-friendly strategy had a weak positive effect on pollen beetle parasitism. Unexpectedly, the amount of accessible nectar did not differ significantly between wildflower strips in the eco-friendly strategy and natural herbaceous field margins in the conventional strategy. The amount of accessible nectar provided by weeds in the field was positively related only to pollen beetle parasitism rates. By contrast, the amount of accessible nectar for both spontaneous field margin and flower strips had no effect on the rates of parasitism of either species. Parasitism rate decreased significantly with decreasing distance to the field edge, for both species. Our results suggest that further studies are required: (i) to improve the composition of wildflower strips, so as to provide more accessible nectar resources at the start of spring and (ii) to explore more management strategy options, notably on in-field resources management such as unsown patches or intercropping.