Abstract
Using the hoary squash bee (Peponapis pruinosa) as a model, we provide the first probabilistic risk assessment of exposure to systemic insecticides in soil for ground-nesting bees. To assess risk in acute and chronic exposure scenarios in Cucurbita and field crops, concentrations of clothianidin, thiamethoxam and imidacloprid (neonicotinoids) and chlorantraniliprole (anthranilic diamide) in cropped soil were plotted to produce an environmental exposure distribution for each insecticide. The probability of exceedance of several exposure endpoints (LC50s) was compared to an acceptable risk threshold (5%). In Cucurbita crops, under acute exposure, risk to hoary squash bees was below 5% for honey bee LC50s for all residues evaluated but exceeded 5% for clothianidin and imidacloprid using a solitary bee LC50. For Cucurbita crops in the chronic exposure scenario, exposure risks for clothianidin and imidacloprid exceeded 5% for all endpoints, and exposure risk for chlorantraniliprole was below 5% for all endpoints. In field crops, risk to ground-nesting bees was high from clothianidin in all exposure scenarios and high for thiamethoxam and imidacloprid under chronic exposure scenarios. Risk assessments for ground-nesting bees should include exposure impacts from pesticides in soil and could use the hoary squash bee as an ecotoxicology model.
Highlights
Global insect pollinator declines are being driven by multiple interacting environmental stressors, including land-use intensification, pathogens, invasive species and climate change, and may threaten the production of crops that depend directly or indirectly on the pollination services that bees provide[1,2]
As a first step in assessing risk, we evaluated the concentrations of all pesticides found in composite samples of soil (n = 29), nectar (n = 25), and pollen (n = 25) taken from 18 farms growing Cucurbita crops
Residues of 7 insecticides, 6 fungicides, and 2 herbicides were detected in samples taken from Cucurbita crops (Table S3)
Summary
Global insect pollinator declines are being driven by multiple interacting environmental stressors, including land-use intensification, pathogens, invasive species and climate change, and may threaten the production of crops that depend directly or indirectly on the pollination services that bees provide[1,2]. Thiamethoxam breaks down quickly to clothianidin as one of its metabolites[6,7] They are effective against pests, neonicotinoid insecticides are of environmental concern because of their relatively high toxicity to (non-target) insects, their systemic nature, their persistence, and their extensive use in agriculture[8,9]. Neonicotinoid residues have been found in the nectar and pollen of Cucurbita flowers[10,11], and in agricultural soil[3,7], where they have been found to persist across seasons[9,12,13] Both adult and larval stages may be exposed to pesticide residues consumed in nectar and pollen. Adult male ground-nesting bees have little exposure to soil as they do not participate in nest construction[14], but they might have greater exposure to residues on plants than adult females because they may spend more time in flowers (e.g. residing inside them overnight)
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