Risks of exposure to systemic insecticides in agricultural soil in Ontario, Canada for the hoary squash bee (Peponapis pruinosa) and other ground-nesting bee species

Insecticide exposure is an environmental factor of concern relating to pollinator health. Recent increases in use of systemic insecticides, particularly neonicotinoids, have led to considerable research into the potential impacts for bees of exposure to these insecticide residues via the nectar and pollen of treated crops or contaminated wild flowers. However, although the majority of bee species are ground-nesting, the risk of exposure to insecticides in soil has not yet been evaluated. Here we use the hoary squash bee (Peponapis pruinosa) as a model system to provide the first evaluation of the risk of exposure to insecticide residues in soil to ground-nesting solitary bee species. The evaluation assessed agrochemical residues from the nectar, pollen and soil from Cucurbita-crops (e.g. squash and pumpkin) grown in southern Ontario in 2016, and soil insecticide residue data collected from field crops by the Ontario government. Systemic insecticide residues were rarely detected in the nectar and pollen of Cucurbita-crops in 2016, and hazard assessment deemed these potential routes of exposure to be non-hazardous for honey bee lethal dose endpoints. In contrast, quantifiable pesticide residues were frequently detected in soil suggesting this route of exposure to be hazardous for honey bee lethal dose endpoints, leading to further assessment using probabilistic risk assessments. Concentrations of clothianidin, imidacloprid, thiamethoxam (neonicotinoid insecticides) and chlorantraniliprole (an anthranilic diamide insecticide) in soil samples were plotted to produce an environmental exposure distribution for each insecticide. A honey bee LC50 and a solitary bee LC50 endpoint were converted to exposure endpoints using the amount of soil excavated by hoary squash bees during nest construction, for both acute and chronic exposure scenarios. The probability of exceedance of each exposure endpoint was calculated and compared to an acceptable risk threshold (i.e. 5% exceedance). In the acute exposure scenario, risk to hoary squash bees was below acceptable threshold levels for all residues evaluated using the honey bee LC50, but exceeded the threshold for clothianidin and imidacloprid using the solitary bee LC50. In the chronic exposure scenario, risk from exposure to clothianidin and imidacloprid exceeded the threshold for both the honey bee and solitary bee LC50s, and for chlorantraniliprole risk only exceeded the threshold for the solitary bee LC50. Using the hoary squash bee as a model, the wider implications for other ground-nesting bees that pollinate crops was explored using insecticide residues from soil samples taken from Ontario field crops. Probabilistic risk assessments suggest that risk to ground-nesting bees is high from clothianidin residues in soils, even when exposure is acute, and substantial for both thiamethoxam and imidacloprid under chronic exposure scenarios. These results demonstrate the urgent need to consider direct exposure to pesticides in soil for ground-nesting bees as part of risk assessments and provides a potential model for evaluating risk from this type of exposure.