1. The structural complexity of agricultural landscapes influences the local biodiversity and associated ecosystem services. Hence, developing effective biodiversity management requires a better understanding of the relative importance of local and landscape changes, especially for functionally important organisms such as hoverflies benefiting from flowering plants.
2. We examined hoverfly (Diptera: Syrphidae) communities in broad and narrow sown flower strips, in naturally developed grassy strips and in wheat fields (as a control). We also investigated the effects of these four habitat types on syrphid occurrence in the adjacent wheat fields.
3. The relative influence of local vs. landscape effects was tested by selecting study sites along a gradient of structural complexity from simple landscapes (∼100% arable land) to complex landscapes (up to 70% semi-natural habitats such as fallows, field margins, hedges and grassland). Landscape complexity was assessed within landscape sectors of 0·5–4·0 km radius around strips.
4. Syrphid density and in particular, the density of aphidophagous species, was higher in narrow and broad sown flower strips compared to grassy strips and wheat–wheat boundary controls at the milk-ripening stage of the wheat. In addition, species richness of aphidophagous syrphids within wheat fields adjacent to broad sown flower strips was higher at the wheat peak-ripening stage. This indicates a spillover between habitats and a positive effect of these sown flower strips on potential biocontrol of cereal aphids. Flower densities and syrphid diversity and density, respectively, were closely related.
5. Species richness and abundance in the sown flower strips increased as the proportion of arable land in the surrounding landscape increased, suggesting that within structurally simple landscapes (at 0·5–1 km radius around the sites) syrphid flies concentrated on the most rewarding resources within the sown flower strips. Sown flower strips were more effective at increasing syrphid species richness and abundance in simple landscapes, presumably because the creation of flower resources made the greatest difference in such homogeneous, intensively managed arable landscapes.
6. Synthesis and applications. Agri-environment schemes should take the surrounding landscape characteristics into account when considering using sown flower strips to enhance syrphid density and diversity, and their biocontrol function, in arable landscapes. Creating locally such flower strips is more effective in simple landscapes containing a high proportion of arable land, while in complex landscapes, keeping the overall diversity is important.