Non-rotational set-aside fields improve reproductive success of cavity-nesting bees and wasps at the landscape scale, but have no effect on other wild bees and hoverflies in mid-summer

Abstract

Wild bees, hoverflies and wasps are valuable ecosystem service providers in agricultural systems through pollination and biological control, and their species richness, abundance and reproductive success well indicate ecosystem health. However, they are often limited by foraging and nesting resources that are major drivers behind their steep decline. Although agri-environmental measurements improve resources for these groups, their wider landscape-scale impacts are basically unknown. Here, we questioned whether 2–3 years old, sown set-aside fields could have a potential in enhancing pollinator and predatory wasp communities at the landscape scale in a Central European agroecosystem. We measured reproductive success and parasitism of cavity-nesting bees and wasps by trapnests throughout the vegetation period, and sampled bee and hoverfly communities by colored pan traps in mid-summer, comparing landscapes with and without set-aside fields. We measured the effects of increasing distance from set-aside fields, the effects of share of different habitats and flower resources, and the effects of increasing set-aside field percentage area at landscape scale. The landscape scale effects of set-aside fields varied among taxa and/or at different time scales. Reproductive success of cavity-nesting bees and wasps was positively related to the presence of set-aside fields in the landscape and together with the number of cavity-nesting bee and wasp genera increased with the proportion of set-aside fields. Species richness or abundance of bees or hoverflies showed no difference between landscape plots with or without set-aside fields in the mid-summer period according to the pan trap samples, and flower abundance and distance from the set-aside fields had only a limited effect. Set-aside fields had no effect on either the most abundant wild bee species or on composition of species of intermediate abundance. Our results suggest that reproductive success of cavity-nesting bees and wasps can be enhanced by additional flower resources and nesting habitats through set-aside fields at the landscape scale. Other wild bees and hoverflies can be less sensitive to the presence of set-aside fields according to our results that might need different conservation approaches. But this might also suggest that such landscape-scale benefits of set-aside field management might be measureable only with samplings covering the whole vegetation period. We argue that well-defined measures specific to spatial scale and target groups are mandatory and should be adapted to the different histories and local contexts of agricultural landscapes in Europe to strengthen ecosystem service provider insects and have the highest benefit for agricultural production.

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