Quantum Yields for Direct Photolysis of Neonicotinoid Insecticides in Water: Implications for Exposure to Nontarget Aquatic Organisms

Environmental fate processes of neonicotinoid insecticides are of significant interest, given the serious threats these chemicals can pose to nontarget organisms such as pollinators (e.g., bees). Direct photolysis was investigated using a laboratory photoreactor approximating full-spectrum sunlight to predict the aquatic fate of neonicotinoids. Quantum yields (ϕ_c) were 0.019 ± 0.001, 0.013 ± 0.001, 0.0092 ± 0.0005, 0.0022 ± 0.0003, and 0.0013 ± 0.0002 for thiamethoxam, clothianidin, imidacloprid, acetamiprid, and thiacloprid, respectively. On the basis of these values, estimated half-lives were 0.2–1.5 days for different seasons in surface waters at temperate latitudes for thiamethoxam, consistent with the 0.98 day half-life observed experimentally outdoors at Winnipeg, Manitoba, Canada (50°N), for thiamethoxam in summer. Light attenuation through shallow clear surface waters (e.g., by natural organic matter) indicated that photolysis of thiamethoxam at depths greater than 8 cm was negligible, which may help explain reports of its environmental persistence.