Effect of nozzle selection on deposition of thiamethoxam in Actara® spray drift and implications for off-field risk assessment

Off-target drift during pesticide spray applications represents a potential pathway for the introduction of active ingredient into field-adjacent water, soils, and/or vegetation. This study investigated the extent of downwind spray drift deposition of thiamethoxam (as a model insecticide) from an application of Actara® 25WG using standard nozzles (TeeJet XR11003, DG11004, and AIXR11002) onto a fallow field test site in the Midwestern USA. Single broadcast applications at a target rate of 96 g a.i./ha were made uniformly via tractor boom to a mowed stubble plot at a spray volume of 93.5 L/ha. Sampling devices (stainless steel disks, filter paper, and stainless steel rods) were located upwind of the spray swath (as negative control samples), within the spray swath (filter paper only), and downwind (all samplers), perpendicular to the spray swath from 12.5 to 400 ft. (3.8 to 122 m) from the edge of the treated field. Comparison of measured residues from the three types of samplers indicated that filter paper generally had greater variability in results than metal disks. When nozzles were compared, the AIXR11002 air induction nozzle produced less off-field deposition than other nozzles tested. Measured downwind concentrations of thiamethoxam from disk samplers were used to predict distances for mitigating potential effects to honey bees. Based on field-derived models, downwind distances from the spray swath required to reduce exposure levels below levels of concern for honey bees varied from <1 ft. to 148 ft. (0.3 to 45 m) depending on the hazard endpoint and nozzle used. These distances were considerably lower than those predicted using the AgDRIFT model, particularly for distances further downwind. At 400 ft. (122 m), AgDRIFT over-predicted the calculated concentrations by up to a factor of 4.8, 7.2, and 10 for DG11004, XR11003, and AIXR11002, respectively. These data suggest that the AgDRIFT model is less reliable for predicting spray deposition at further downwind distances, with implications for risk assessment.