Flower availability drives effects of wildflower strips on ground-dwelling natural enemies and crop yield

Wildflower strips have been heralded as a promising way to enhance ecosystem services by providing organisms which may help make farming less dependent on external inputs. However, recent studies show inconsistent effects on delivery of ecosystem services and crop yield, warranting a more detailed analysis of the factors determining the effects of wildflower strips. We examined how the natural enemy groups of spider, carabid beetle and staphylinid beetle, as well as aphid pest and crop yield respond to wildflower strips. We furthermore determined whether the response of natural enemies, aphids and crop yield depends on flower cover and species richness, and how this is influenced by fertilizer and insecticide applications to the crop in 16 winter wheat fields in the Netherlands. We used an experimental approach with a nested design that included all combinations of wildflower strips (present/absent), fertilizer application (yes/no) and insecticide application (yes/no). Presence of wildflower strips did not affect ground-dwelling natural enemies, aphids or crop yield. However, flower availability across wildflower strips and control margins was positively related to the abundance of the pooled number of examined natural enemies, spiders and carabid beetles. Positive effects in the crop were observed over limited distances; up to 5 m from the edge for spiders and wheat yield. The effects of flower availability and on-field management practices on natural enemies, aphids and wheat yield did not interact suggesting that, in our study, effects of flowers were not influenced by insecticide or fertilizer applications but were mainly additive. Our study indicates that cover and richness of wildflowers in field margin habitat, rather than establishment of wildflower strips per se, drove increases in natural enemies and crop yield. This suggests that more attention should be given to the optimization of establishment success of seed mixtures and management practices enhancing wildflower cover and diversity. Furthermore, biodiversity enhancing management of the herbaceous vegetation in linear landscape elements may represent a cost-effective alternative to boost ecosystem services regulating crop production in agricultural landscapes.