Winds of change, developing a non-target plant bioassay employing field-based pesticide drift exposure: A case study with atrazine.

A field-scale, spray drift study with atrazine was conducted to simultaneously measure spray drift deposition, airborne interception and corresponding biological effects on two sensitive plant species (cucumber and lettuce). Applications of AAtrex 4L (atrazine) were made using ultra-coarse nozzles (TeeJet TTI11004) under worst-case drift potential conditions of bare soil and high wind speeds (i.e. >10 mph; >16 kph). This study was replicated 4 times, each with two parallel spray swaths (92.5 ft or 28 m per swath) perpendicular to wind direction. Within each replicate application, three sampling lines were used to measure drift deposition (using stainless-steel discs) at distances out to 400 ft (122 m), airborne interception (using stainless-steel rods) at distances out to 75 ft (23 m), and potential direct plant effects at 5, 15, 25, 35, and 45 ft (1.5, 4.6, 7.6, 10.7, and 13.7 m) from the downwind edge of the spray swath. Corresponding upwind control discs and plants were also included in each replicate. Each replicate application targeted steady wind speeds between 10 and 15 mph (16 and 24 kph) within a 30-degree angle of the downwind field orientation. On average, each 10% increase in distance from the spray zone resulted in approximately 14% less ground-deposited atrazine. Between 7 and 41× more atrazine mass was collected from vertical rods (airborne drift), compared to horizontally placed stainless-steel discs (ground deposition). Cucumber and lettuce plants exposed to spray drift were monitored for biological effects over 21 days post-application according to standard protocols. Endpoints of survival, weight (biomass), and shoot length were evaluated by comparing distance groups to up-wind controls. Overall, when trials were combined, the aggregate lowest observable effect distance (LOED) was 5-ft (1.5 m) and the aggregate no observable effects distance (NOED) was 15-ft (4.6 m), with cucumbers affected more than lettuce.