Moderate use of pesticides in allotment gardens as indicated by their residues in larval food provisions of Osmia solitary bees

Exposure to pesticides may jeopardize pollinators, but it has rarely been examined in urban landscapes. We used larval food provisions (mix of pollen and nectar) of mason bees (Osmia spp.) nested in artificial shelters as matrices to investigate the rate of pesticide pollution in twelve allotment sites in Prague and Brno cities. Seven commercial croplands (fruit orchards, oilseed rape fields) were selected for comparison. In total, 79 samples of larval provisions were analysed using a validated UPLC-MS/MS method for the presence of 262 pesticides and/or important metabolites. Fifty-six pesticide residues were detected in the allotments and 74 in cropland sites. Fifty pesticide compounds were common to both landscapes. Fewer residues were found in the allotments (14.3 +/- 0.9, mean +/- SE) than in croplands (21.5 +/- 1.1, mean +/- SE; df = 17.4; t = -2.9; p < 0.01), and they were present at low levels up to 16 ppb. In cropland sites, ten residues occurred at high individual concentrations (mean per site > 10 ppb). The highest levels were observed for the fungicide pyrimethanil (mean = 1,989 ppb, maximum = 3,983 ppb) and among insecticides for thiacloprid (mean = 136 ppb, maximum = 170 ppb). The results supported the presumption of a lower risk of pesticide exposure in urban gardens than in croplands. Given the limited knowledge on the sublethal effects of pesticides and especially pesticide mixtures on solitary bee development, the regulation or control of pesticide use in allotment gardens is recommended.