Crop species diversity levels with attract and reward strategies to enhance Pieris brassicae parasitism rate by Cotesia glomeratain strip intercropping

Summary

1. Crop diversification through inter- and mixed cropping practices has been shown to enhance conservation biological control of herbivorous pest insects. However, distinct cropping practices are usually tested independently, which makes assessing the relative impact of specific measures difficult.2. We assessed parasitism rates of released large cabbage white (Pieris brassicae) caterpillars by parasitoids on white cabbage (Brassica oleracea) plants in six different cropping systems: four different strip- cropping designs, a pixel cropping de-sign and a monoculture. These cropping designs differed in the number of crops included, the use of parasitoid-attractive cultivars in concurrence with a cash cultivar, the use of nectar- providing crops for adult parasitoids and the spatial arrangement of the crops.3. Parasitism rate by the main P. brassicae parasitoid Cotesia glomerata was enhanced by strip cropping of white cabbage with wheat, and even further enhanced by the inclusion of four more main crops. Contrastingly, C. glomerata parasitism rate was lower in the most intensive crop mixture, that is pixel cropping, than in any of the strip-cropping designs. The use of attractive cultivars or rewarding floral resources within a strip-cropping set-up did not significantly further enhance C. glomerata parasitism rate.4. Synthesis and applications: Our results show that increasing the number of crops in the agroecosystem enhances parasitism rates, whereas the additional inclusion of more subtle measures to attract or reward parasitoids shows mostly marginal effects. We argue that strip cropping can readily be employed as (part of a) pest control strategy, but trait- based measures still need further development. How to implement attract and/or reward strategies in large-scale agricultural systems should be further evaluated in the context of crop diversification schemes.