The importance of appropriate drop size and density for successful mosquito adulticide applications mandates the necessity for accurate determination of drop spectra of a particular nozzle. There is considerable disparity between mass median diameter (MMD) determinations for flat-fan nozzles relative to the horizontal or vertical orientation of the microscope slide used to collect the drops. To remove this ambiguity, the definitive MMDs of flat-fan nozzles used in aerially applied mosquito control adulticides were determined by laser-diffraction–based characterization and analysis. These data were compared with previous data, and the impact of these data on aerial adult mosquito control was discussed. At The Florida Wind Tunnel for Mosquito Control, the Malvern Spraytec Spray Particle Analysis System was used to characterize the entire aerosol plume of the nozzles. Nozzle characterizations were carried out at aircraft operational wind speeds and pressures with nozzles mounted at 135° relative to the direction of air flow. The mean drop-diameter volumes (Dv) Dv(0.1), Dv(0.5), and Dv(0.9) with 95% confidence intervals for each scenario were determined. Characterizations of flat-fan nozzles of 80005 to 8005 for Orchex 796, Dibrom and a Permanone∶Orchex 796 mix (1∶1) resulted in no Dv(0.5) less than 50 µm and a maximum of 133 µm. The Dv(0.1) was greater than 25 µm for 52% of the nozzles and ranged from 14 to 42 µm. The Dv(0.9) ranged from 130 to 296 µm. There was a decrease in drop-diameter values (Dv(0.1), Dv(0.5), Dv(0.9)) relative to increased wind speed and/or pressure for any particular nozzle. Relative to characterizations with Orchex 796, drop-diameter values for Dibrom varied from the same to slightly larger, whereas the Permanone∶Orchex 796 mix values were larger except for 2. Relative to the goal of creating an aerosol cloud efficient in controlling adult mosquitoes, none of the nozzles were capable of producing a Dv(0.5) of less than 50 µm. Fifty percent of the spray was capable of causing visible damage to a car's finish. The concept that droplets larger than 25 µm are wasteful because they contain more malathion than required for kill suggests that for 52% of the nozzle configurations, those with Dv(0.1) greater than 25 µm, 90% of the spray is of little use in controlling mosquitoes. None of the flat-fan nozzle regimes tested will satisfy new label requirements of a Dv(0.5) and Dv(0.9) of around 50 and 100 µm, respectively, because of their high Dv(0.9).
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Vol. 22 • No. 4