Organic farming and landscape factors affect abundance and richness of hoverflies (Diptera, Syrphidae) in grasslands

1. Hoverflies provide ecosystem services including pollination, pest regulation and waste decomposition, but not enough is known about how they are influenced by organic farming and landscape composition, and structure to effectively manage them in different landscapes. 2. By conducting transect walks and pan-trapping, we investigated how hoverfly abundance/richness were related to organic farming, floral abundance, land-use cover and hedgerows in nine pairs of organic and conventional dairy farms in the Republic of Ireland. 3. We found more flowers on organic farms which attracted more hoverfly species. Organic farming also increased hoverfly abundance and richness beyond the benefits from increased floral resources, indicating that other organic management practices benefit hoverflies. This was evident regardless of the surrounding landscape composition. 4. Hoverfly abundance and richness were most strongly related to land use within 4 km around sites, indicating the importance of large spatial scales. Homogenous grassland landscapes (i.e. high grassland cover) supported higher hoverfly abundance and richness, possibly because they contain abundant macrohabitats suitable for the proliferation of grassland hoverflies. Hedgerow structure was also important, with hoverfly abundances greater on sites surrounded by a higher total cover of hedgerows, but with lower total hedgerow lengths (i.e. greater hedgerow volumes) within 1 km. 5. Our findings illustrate complex relationships between hoverflies and local and landscape factors, and they contrast with previous findings for other pollinators. We encourage increased floral resources and organic farming as a means of conserving hoverflies in grassland systems. Future management of grassland hoverflies needs to account for large spatial scales and the possibility that hedgerows greatly impact hoverfly diversity in hedgerow-dominated landscapes.