EVALUATING THE EFFECTS OF HERBICIDE DRIFT ON NONTARGET TERRESTRIAL PLANTS: A CASE STUDY WITH MESOTRIONE

Nature of exposure is a fundamental driver in nontarget terrestrial plant risk assessment for pesticides; consequently a novel study was designed to generate field-based drift exposure and evaluate corresponding biological effects of the herbicide mesotrione. The approach used a combination of US guideline drift reduction technology and vegetative vigor approaches. In each of 3 independent replicate spray application trials, 10 pots each of lettuce and tomato were placed at distances of 10, 20, 30, 40, and 50 ft (similar to 3, 6, 9, 12, and 15 m) from the downwind edge of the spray boom. Each application was conducted using a commercial 60-ft (18-m) boom sprayer fitted with TeeJet 1 Technologies TTI110025 nozzles, with a nominal application rate of 0.2 lb a. i./A (224 g a. i./ha). The environmental conditions required by the protocol (air temperature 10-30 degrees C and wind perpendicular to the swath (+/- 30 degrees) blowing toward the plants at a mean wind speed of >= 10 mph [>= 4.5 m/s] measured at 2.0mabove the ground) were met for each application. Following exposure, plants were transferred to a greenhouse for the 21-d vegetative vigor phase of the study. Symptoms of phytotoxicity and plant height were assessed at 7, 14, and 21 d after treatment. On completion of the 21-d after treatment assessment, all plants were harvested and dried in an oven to determine shoot dry weight. The biological data indicated that no statistically significant effects were observed at a distance of 30 ft (similar to 9m) from mesotrione drift at wind speeds of >= 10 mph (10.9-12.4 mph); this endpoint (30 ft) is defined as the no observed effects distance (NOED). (C) 2017 SETAC