John Wiley & Sons, Ltd.

Field scale examination of neonicotinoid insecticide persistence in soil as a result of seed treatment use in commercial maize (corn) fields in Southwestern Ontario

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Neonicotinoid insecticides, especially as seed treatments, have raised concerns about environmental loading and impacts on pollinators, biodiversity and ecosystems. We measured concentrations of neonicotinoid residues in the top 5 cm of soil before planting of maize (corn) in 18 commercial fields with a history of neonicotinoid seed treatment use in southwestern Ontario in 2013 and 2014 using LC‐ESI(+)‐MS/MS. A simple calculator based on first order kinetics, incorporating crop rotation, planting date and seed treatment history from our subject fields, was used to estimate dissipation rate from the seed zone. The estimated half‐life (the time taken for 50% of the insecticide to have dissipated by all mechanisms) based on 8 yr of crop history was 0.64 (range 0.25 ‐ 1.59) yr, and 0.57 (range 0.24 ‐ 2.12) yr for 2013 and 2014, respectively. In fields where neonicotinoid residues were measured in both years, the estimated mean half‐life between 2013 and 2014 was 0.4 (range 0.27 ‐ 0.6) yr. If clothianidin and thiamethoxam were used annually as a seed treatment in a typical crop rotation of maize, soybean and winter wheat over several years, residues would plateau rather than continue to accumulate. Residues of neonicotinoid insecticides after 3‐4 years of repeated annual use tend to plateau to a mean concentration of less than 6 ng/g in agricultural soils in southwestern Ontario. This article is protected by copyright. All rights reserved

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