Floral resources, energetic value and pesticide residues in larval provisions collected by Osmia bicornis in oilseed rape dominated landscape

Abstract Pollinators in agricultural landscapes are facing global decline and the main pressures include food scarcity and pesticide usage. Although intensive agricultural landscapes are poor habitats for wild pollinators, the mass flowering crops may provide important food resources, albeit monofloral and short-term, which in addition may contain pesticide residues. We explored how the landscape composition with a different proportion of oilseed rape (6%-65%) around Osmia bicornis nests affects floral diversity, contamination with pesticides, and energetic value of provisions collected by bees as food for their offspring. The bees collected pollen from 28 floral taxa (6-15 per nest) and provisions were dominated by Brassica napus (6.0-54.2%), Quercus (1.2-19.4%) and Ranunculus (0.4-42.7%) found in all 12 nests, but also by Poaceae (1.2-59.9%, 11 nests) and Acer (0.6-42%, 8 nests). Residues of 12 pesticides were found in provisions, with acetamiprid, azoxystrobine, boscalid, and dimethoate being the most frequently detected at concentrations up to 1.2, 198.4, 16.9 and 17.8 ng/g, respectively. Floral diversity and energetic value of provisions, but not the Pesticide Risk Index depended on landscape structure. Moreover, pollen diversity decreased, and energetic value increased with landscape diversity. Thus, even a structurally simple landscape may provide diverse food for O. bicornis if the nest is located close to a single but resource-diverse patch. Both B. napus and non-crop pollen were correlated with pesticide concentrations.