Assessment of sampling methods for peak naled deposition distance from flight line during aerial application

Abstract

Pesticide deposition on natural waters and other surfaces creates a public health risk from their use. The Environmental Protection Agency (EPA) determines deposition with the AGricultural DISPersal model, which does not seem to be an appropriate tool for mosquito control applications. On the other hand, sufficient reliable data on ground depositions from mosquito control sprays to evaluate the estimates from models used by EPA does not exist. This study was conducted to compare different sampling methods to determine peak deposition distance from the flight line when spraying Dibrom® using a helicopter equipped with rotary ULV nozzles. The three replicated tests were conducted at two different sites, one surrounded by trees on all sides and a second site mainly open. At both sites, twenty sampling locations were established, 30.5 m apart, first at 30.5 m and last at 610 m from the flight line. Each location had a spinner mounted on a tripod 1.5 m above ground holding one slide for droplet characterization and one for spray flux, a filter paper attached to a cutting board on the ground, and a petri dish on the same board. Single-pass applications were made at an elevation of 46 m with a helicopter discharging Dibrom at 4.3 L/min, travel speed of 145 km/h and swath of 305 m to deliver Dibrom at 51 mL/ha. Tests on one site were completed in one day, with the first spray an hour before sunset and the last spray an hour after sunset. The droplets on the slides collected during the tests were measured using DropVision system and the amount of Dibrom on slides, filter paper, and petri dish was determined with fluorometry. The droplet size data for all tests and all locations indicate that volume median diameter (VMD) was within the acceptable range of droplet spectrum (9.0 – 22 µm VMD) for ULV adulticide applications. The results indicated a significant effect of sites and samplers on ground deposition. The difference in ground deposition at different distances from the flight line was not significant. The peak ground deposition measured with petri dishes occurred at 91 m from the flight line at the wooded site under low wind and at 520 m in the open field under higher winds. The peak deposition was 33.5% and 49.5% of the application rate at the wooded site and open site, respectively. The results indicated that only the petri dish was able to determine the peak deposition distance from the flight line.

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