Concentrations of pesticides in multiple matrices to measure exposure of wild bees visiting pollinator hedgerows in northern California

To better understand the exposure of wild bees to pesticides in an agricultural landscape, samples were collected from fields in northern California. Hedgerows are known to provide habitat for wild bees, but these bees may also be exposed to pesticides from nearby agricultural fields. The study included eight hedgerow sites located in an intensively managed agricultural landscape that includes almonds, (wine) grapes, rice, tomatoes, and walnuts. In addition to collecting both wild bees and honey bees, soil, flowers, and silicone passive sampling devices (PSD; staked near the hedgerows to sample the air) were also included. Sampling was conducted from April to June 2016, to coincide with peak bloom and bee activity for comparison with pesticide use records from the surrounding landscape. PSDs were extracted via sonication with an organic solvent while the other matrices (soil, flowers, bees) were extracted via pressurized liquid extraction and solid phase extraction cleanup. Samples were analyzed for 168 pesticides and degradates using both gas and liquid chromatography-tandem mass spectrometry. Overall, 38 pesticides were detected in all matrices (10 insecticides and degradates, 12 fungicides, 15 herbicides and degradates, and 1 plant growth regulator). The number and type of pesticides detected varied by matrix; 24 compounds were detected in the PSDs, 24 in soil, 15 in flowers, 18 in native bees, and 10 in honeybees. These results can help determine which matrices are best at estimating wild bee pesticide exposure and which pesticides should be targeted for future effects work.