Local and landscape management effects on syrphid fly guilds: flower strips, farming practice and hedges

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During the last decades, agriculture has transformed into highly effective and economically optimized production systems with extended cultivation areas and increased usage of fertilizers and pesticides. This development is accompanied by the loss and fragmentation of remaining semi-natural habitats, thereby negatively affecting farmland biodiversity, resulting in species loss and declining ecosystem services such as biological pest control, and pollination. Agri-environmental schemes such as organic farming practices and habitat management can help to mitigate these negative effects, while it is increasingly recognised that attention should be paid to both local and landscape scales. Improving habitat diversity on the landscape scale can enhance biodiversity at the local scale, compensating for intensified local land use. In this thesis, we analysed the effects of local habitat management (sown flower strips, forest edges and hedges) and farming practices (extensive vs. intensive management) on syrphid fly guilds. In addition, we focused on the influence of landscape scale parameters such as the proportion of arable land as well as the proportion of mass-flowering oilseed rape in the surrounding of study sites. Syrphid flies represent one of the biggest groups of the order Diptera and occur in a wide range of habitats in agricultural landscapes. While adult syrphids flies are pollen and nectar feeders, their larvae show different feeding strategies ranging from predators of aphids, bacteria feeders and phytophagous to fungivorous species. Aphidophagous species such as Episyrphus balteatus or Sphaerophoria scripta represent the most frequently occurring syrphid fly species in agricultural landscapes, preying on a wide range of aphids species (e.g. Sitobion avenae, Rhopalosiphum padi, Metopolophium dirhodum), and can play an important role in the suppression of cereal aphid outbreaks. Here, we studied the effect of naturally occurring grassy strips and sown flower strips on syrphid flies in winter wheat fields, which were located along a gradient of landscape complexity (ranging from 30 to 100 % arable land and at multiple spatial scales ranging from 0.5 to 4 km radii). Analyses on the impact of extensive vs. intensive farming practices on syrphid flies were carried out in two European countries (South-Sweden and North-Germany), in each region by comparing four fields at low and four fields at high levels of agricultural intensification. In addition, syrphid flies were analysed by comparing forest edges, forest-connected hedges and isolated hedges adjacent to crop fields (winter wheat and oilseed rape) with respect to the influence of varying proportions of oilseed rape at the landscape scale. The results showed that sown flower strips increased syrphid fly abundance. Syrphid species richness was increased in wheat fields adjacent to sown flower strips. Furthermore, species richness and abundance of syrphid flies in sown flower strips increased as the proportion of arable land in the surrounding landscape increased, indicating a local concentration effect to highly rewarding pollen and nectar resources. Total and aphidophagous syrphid abundances were generally higher in the German than the Swedish study region. Aphidophagous syrphid abundance was higher in high intensity managed fields being located in landscapes with high proportions of arable land, while non-aphidophagous syrphid flies showed higher abundances in low intensity managed fields being located in landscapes with low proportions of arable land. Furthermore, syrphids in the German region appeared earlier in the season, therewith possibly allowing for a better predator-prey synchronization. Syrphid flies appeared to profit from high amounts of pollen and nectar resources in oilseed rape fields. Aphidophagous syrphid abundance in hedges and forest edges showed contrasting responses to the neighbouring crop, with low abundance when neighboured by oilseed rape fields (dilution) and higher abundance when neighboured by winter wheat fields (concentration) at high proportions of oilseed rape fields at the landscape scale. Aphidophagous syrphid fly abundance was increased in forest-connected hedges as well as in crop fields which lay adjacent to forest-connected hedges, indicating noncrop-crop spillover with potentially positive effects on local bicontrol potential. In conclusion, results showed that local habitat management can enhance diversity and abundance of syrphid flies, thereby potentially improving local biological control of cereal aphids. On the landscape scale, results support the idea that environmental schemes are more effective in structurally simple compared to complex landscapes owing to the concentration of highly dispersive organisms such as syrphid flies in resource-rich habitats. The relative abundance of aphidophagous syrphids varied largely between German and Swedish study regions indicating a changing role of species identity along latitudes. Syrphid fly guilds (aphidophagous vs. non-aphidophagous) were contrastingly affected by management type (extensive vs. intensive farming). Aphidophagous syrphid fly abundance in semi-natural habitats is mediated by the percentage of oilseed rape at the landscape scale, depending on local crop identity. The positive influence of forest-connected hedges on syrphids calls for group-specific habitat management practices in order to enhance biodiversity and related important ecosystem services such as biological pest control and pollination in the agricultural landscape.