Transport and Fate of a Neonicotinoid Pesticide from Corn Seed Coatings.

Neonicotinoids are the most widely used class of insecticides worldwide which coat the seeds of almost every major crop in the United States, including corn, cotton, and soybeans. However this class of pesticides has come under increasing scrutiny for both their effects on non-target organisms, including a potential linkage to recent declines in bee populations, and the concern that these moderate to highly soluble compounds may leach through soils and enter ground and surface waters. The physical transport and fate of thiamethoxam (TMX) were investigated in a greenhouse study where TMX-coated corn seeds were grown in sandy and clayey soil columns. TMX concentration was measured 12 times in leachate over a 35 day growth period in eighteen 20 cm and eighteen 60 cm columns (20 cm-ID PVC) with and without viable plant growth. Twenty four 60 cm columns were also constructed to measure TMX concentrations in plant leaves, stems, roots, seed casts, rhizosphere, root soil, and bulk soil at three corn development stages (V1, V3, and V5). Results show that, by the V5 corn stage 46-56% of introduced TMX in soil remained in the 0-30 cm section of bulk soil for both sandy and clay soils. TMX concentrations were higher in both V5 clay and sand columns in the 30-45 cm bulk soil section (5.0±0.63 ppb and 10±1.7 ppb respectively) compared to those of the same texture without viable plant growth (4.3±4.5 ppb and 2.0±1.4 ppb, respectively) indicating an effect of the corn plant on downward migration of the pesticide (p=0.018; p=0.025). Consequently, 60 cm clay columns with viable corn plants yielded less leachate over the growth period compared to those controlled for plant growth (p=0.037). Results suggest that corn growth and development may act as both a retardant and a conduit for advection of TMX from seed coats.