Transport of Neonicotinoid Pesticide Thiamethoxam Under Field Conditions.

Water solubility and systemicity make neonicotinoid insecticides ideal for crop protection via seed dressing, yet these chemical characteristics, combined with a high toxicity to non-target invertebrates (e.g. honeybees), elicit concern of environmental transport. Neonicotinoids are detected in soil and surface water bodies throughout North America; however, no study has proven a direct connection to planted seed dressings in the field. To address this gap, this study sought to quantify physical transport of the neonicotinoid thiamethoxam (TMX) from corn seed coats under field conditions. Six runoff plots were erected, planted with neonicotinoid coated corn seeds, and divided into three plots containing viable plants, and three plots controlled for plant growth to understand the influence of living plants on TMX transport. TMX concentrations were recorded in bulk soil samples collected at corn stages V3, V5, VT, and R6, and in plot drainage throughout the growing season. In terms of pesticide concentrations, storm-generated runoff was the most dominant mechanism for field transport of TMX (1.72±0.605 μg L^-1; no plant), followed by shallow (70 cm) subsurface lateral flow (0.570±0.170 μg L^-1; no plant), and deep (110 cm) gravity drainage (0.170±0.265 μg L^-1; with plant). Plots containing viable plants generally transported less TMX and had earlier peak concentrations (runoff: 19 days and 5 cm of rain; subsurface flow: 30 days and 13 cm of rain) compared to plant-controlled plots (runoff: 40 days and 17 cm of rain; subsurface flow: 53 days and 19 cm of rain). Soil samples also confirmed lateral movement of TMX by the V5 corn stage (36 days) and vertical movement as early as the V3 corn stage (23 days). These data show, for the first time, that neonicotinoids in the field can become transported from seed coatings via multiple mechanisms.