Are cover crops a potential threat for pollinators due to clothianidin residues in floral resources?

Abstract

Cover crops are now mandatory in areas at risk of nitrogen leaching into groundwater. Many late-flowering entomophilous species used in these cover crops provide critical floral resources (pollen and nectar) for pollinators in early autumn. However, pesticide residues such as neonicotinoids can trans- fer to cover plants and their floral resources, posing a potential threat to pollinators. We investigated the transfer of clothianidin, a neonicotinoid insecticide, from soil to plants and floral resources in three com- mon cover species (phacelia, white mustard, and faba bean). The study was conducted both in fields conditions (three years after clothianidin treatment on sugar beet) and under controlled growth chamber conditions (74 days post-treatment). We analysed clothianidin concentrations in soils, vegetative parts (stems and leaves), flowers, pollen, and fruits. Our results demonstrated that clothianidin persisted across the soil–plant–floral resource continuum in both field (soil concentration of 2.79 ng g−1 ) and growth cham- ber (soil concentrations of 34.4–106 ng g−1 ) experi- ments. Clothianidin residues accumulated in floral resources (ranging from 0.55 to 66.0 ng g−1 in pollen) posed a potential risk to pollinators through pollen consumption exposure (hazard quotient close to 1), and an even greater risk through soil contact exposure (hazard quotient > 1, reaching up to several hundred). These findings highlight that neonicotinoid residues in soil, and to a lesser extent in cover crop pollen, may threaten pollinator health.