Synergistic Lemna Duckweed and Microbial Transformation of Imidacloprid and Thiacloprid Neonicotinoids

Neonicotinoids are the most widely used insecticides in the world and are commonly measured in aquatic environments, including freshwater wetlands. We report for the first time the synergistic transformation of neonicotinoids by a Lemna duckweed and microbial system collected from an agricultural pond in Iowa, USA. Imidacloprid and thiacloprid were removed at statistically indistinguishable rates (0.63 ± 0.07 and 0.62 ± 0.05 day–1, respectively) from hydroponic medium only when in the presence of both duckweed and its associated microbial community. As evidence for this duckweed–microbial synergy, experiments with surface-sterilized duckweed, duckweed-associated microbes, pond water microbes alone, and two other plant species (Typha sp. and Ceratophyllum demersum) did not yield significant neonicotinoid removal beyond initial biomass sorption. Degradation of imidacloprid and thiacloprid by the duckweed–microbial system generated multiple, known neonicotinoid metabolites (desnitro-imidacloprid, imidacloprid urea, thiacloprid amide, and 6-chloronicotinic acid). Measured metabolites with increased insect or vertebrate toxicity were either absent (imidacloprid olefin) or present only in small amounts (desnitro-imidacloprid; <1% of the parent). The neonicotinoid parent and metabolite mass balance did not fully account for total neonicotinoid removal, suggesting mineralization and/or other unidentified transformation products with unknown toxicity. This novel duckweed- and microbe-facilitated neonicotinoid degradation may represent an important contribution to the environmental fate of neonicotinoids.