Neonicotinoid and atrazine content in forage sources available to honey bees found near corn field margins collected around and after neonicotinoid-treated seed planting in southwestern Ontario, Canada.

In 2013, flowering plant sources available to honey bees were surveyed around the perimeter of 18 corn fields located in southwestern Ontario. The forage resources identified were compared to the taxonomic composition of bee-collected pollen from hives placed near the corn fields during seven sampling periods from late April, around neonicotinoid-treated seed planting, until late June. During planting, honey bees collected pollen from mainly woody species with Acer spp., Salix spp., Populus spp. and Rosaceae predominating. Brassicaceae and Taraxacum spp. were the most frequently recovered herbaceous plant sources throughout May, albeit at low frequency. By mid-June, bees shifted to collecting pollen mainly from herbaceous flowers. Neonicotinoid and atrazine content were measured in all bee-collected pollen and two herbaceous (T. officinale and Barbarea vulgaris) and two tree species (Salix spp. and Crateagus spp.) that were used as indicator plants to assess the potential exposure of bees originating from this ecosystem at different timings from 2013 to 2015. Neonicotinoid residues in bee-collected pollen ranged from 1.6 to 62.1 ng/g, which were greater than observed in pollen-producing organs from indicator plants. During corn pollination, no tree pollen was found, and pollen from Trifolium, Lotus and Parthenocissus spp. was preferred. Corn and soybean pollen was less favoured, collected only when other species were not available; corn pollen generally had concentrations less than 5 ng/g of total neonicotinoids. Neonicotinoid-contaminated pollen was collected from bee forage resources near agricultural fields, but exposure concentration was greater from other sources such as field and planter dust during foraging.