Reduced species richness of native bees in field margins associated with neonicotinoid concentrations in non-target soils
The authors examined the impacts of neonicotinoid treated seeds on native bee abundance and richness in conservation areas in Missouri that include cropped fields and uncultivated fields. They evaluated routes of exposure in fields and margins by sampling soil and vegetation for neonicotinoids and several fungicides, along with native bee community surveys. Sampling took place in and on the margins of fields planted with either seed treated corn (clothianidin) or soy (imidacloprid), with comparisons made to uncultivated reference fields.
Soil samples were taken pre-seeding, post-seeding, mid-growing, and during harvest. In-field samples were taken between rows, and margin samples were taken roughly 2 meters from the field edge. Vegetation and native bee samples were collected three times (all but the pre-seeding time) throughout the growing season.
Neonicotinoids were detected in 87% to 100% of soils collected from treated seed fields; 53% to 93% of treated seed field margins; 22% to 56% of untreated field soils; and 33% to 56% of untreated field margins (values in uncultivated field soils are likely due to use in the area). Concentrations varied throughout the study, with clothianidin in treated field soils showing the greatest values. Neonicotinoids were rarely detected in field margin vegetation of either treated or uncultivated fields (though the authors note that their composite plant samples had fairly high levels of detection which may have obscured neonicotinoid detections). Fungicides were more commonly identified in field margin vegetation.
Variation in field margin bee abundance was best explained by time of year and floral richness, with no significant effect of pesticide presence. The primary statistical analysis excluded the larger bee species collected because they act as long-distance foragers. This pattern held when larger bees were included in the analysis. Bee richness was significantly impacted by time of year, neonicotinoid soil concentration, and total fungicide concentrations in field margin vegetation. There was also a significant interaction between neonicotinoid soil concentration and fungicide vegetation concentrations, suggesting that insecticides and fungicides may be acting synergistically to reduce bee richness.
The authors suggest that soil exposure is an overlooked route of neonicotinoid exposure, especially in ground-nesting native bees that are likely exposed through both oral and contact routes. This research shows that field margins that contain neonicotinoids in soils and fungicides in margin plants may experience reductions in overall native bee species richness despite maintaining abundant bee communities. They note that the fields included in conservation areas are typically smaller and have more adjacent natural habitat than typical conventional agriculture, which may help buffer the effects of pesticides on bee species.
