Landscape floral resources provided by rapeseed correlate with next-year reproduction of cavity-nesting pollinators in a national participatory monitoring program

Context Wild pollinators depend on floral resources available in the landscape, partly provided by mass flowering crops (MFCs), such as rapeseed or sunflower. MFCs are however often grown conventionally, implying insecticide use, with potential negative effects on pollinators. Objectives To understand whether and to what extent these crops could contribute to the maintenance of pollinator populations, we investigated the inter-annual correlation between MFC resources and the reproduction of cavity-nesting pollinators (solitary bees and wasps) at a national scale. Methods We studied a standardized citizen science dataset, in which farmers collected data on the abundance of sealed tubes in trap nests, between 2012 and 2017, in nearly 600 fields distributed across France. We modelled the relation between nesting and landscape resources of the current and previous year, taking local farming practices into account. Results Pollinator nesting was positively correlated with the quantity of rapeseed floral resources the year preceding observations, as well as with the area of permanent meadows. On the contrary, we found more variable relations with sunflower floral resources, depending on the type of sealing material, hence likely on the phenology of pollinators. Our models also confirm that local practices should be accounted for when assessing the influence of the landscape context, although their effects were difficult to interpret. Conclusions Solitary bee and wasp reproduction is likely to be positively and durably affected by rapeseed cover. Moderate areas of rapeseed may help maintain pollinators, in combination with semi-natural habitats, which provide more diverse and stable food and nesting sites.