Hoverfly assemblages in urban farms compared to urban parks in the city of Geneva

This study aims to assess whether urban farms are good providers of ecosystem services compared to other green infrastructures. To assess two services (pollinating and pest control services), hoverflies were monitored in three urban farms (Budé, Lignon and Tissot) and three urban parks (Franchises, Trembley and Beaulieu) in the canton of Geneva in 2017. The results show a large abundance of hoverflies in the parks and farms, particularly at the end of June. In terms of hoverfly diversity, 81 species were recorded, with 10 species accounting for around 80 % of the captures. The Shannon index shows that hoverfly diversity is significantly higher in the parks than in the farms at three out of 6 sampling dates. Of the five most abundant species in parks and farms taken together, three are aphidiphagous and can therefore play a biocontrol role in crops. The average proportion of aphidiphagous individuals is significantly higher in the farms than in the parks. In conclusion, studying hoverflies in urban environments has permitted to highlight differences in function and community composition between apparently similar habitats. However, we recommend limiting the sampling period to the month of June, which would reduce the work involved in sampling with Malaise traps, while retaining significant discriminatory power and limiting impact on syrphid fly communities. In order to encourage hoverflies in the city, it is crucial to maintain several stepping stone habitats within the urban matrix, to favor structures that will give them access to diversified food resources and to adopt maintenance practices that have the least impact on these communities. The introduction of native hedges and aquatic habitats could also be a source of diversification for the pool of species found in the city.