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Field observations of less residual pest control from some low-volume applications indicate that deposits generated by fine droplets may be more susceptible to rain wash-off and solar degradation as compared to deposits of larger droplets. This implies that, in addition to spray deposition, pesticide residue distribution may also be an important factor in the efficacy of pest control. The objective of the study was to determine the effects of the droplet size on rain washoff and solar degradation of fine and coarse spray deposits.
Two experiments were conducted in five replications. Spray solutions were applied at 102–735 μm droplets, using a droplet generating system. In the first experiment, spray solution of copper hydroxide was applied on plastic card targets. The targets were exposed to 0.6–7.5 mm rain, and the remaining deposits were quantified by copper colorimetry. In the second experiment, spray solution containing a fluorescent tracer was applied on plastic and acetate targets. Sprayed targets were exposed to 0–27 MJ/m2 of solar radiation. Samples of the targets were collected at different time intervals and spray residues were measured by fluorometry. Droplet distribution patterns were characterized on water-sensitive targets.
There were clear differences among distribution patterns of various droplet sizes. While fine droplets gave more uniform deposition than the larger droplets, they also were more susceptible to rain washoff. The amount of washoff increased with the rainfall and for any amount of rain, the targets with smaller droplets were washed off at a faster rate. Deposit degradation increased with the intensity of solar radiation, however, there was no difference in degradation rate of some smaller droplet sizes. Green plastic cards, simulating leaves, showed similar degradation trends as transparent acetate cards.
droplet, rain washoff, solar degradation, colorimetry, fluorometry
Professor, University of Florida, IFAS, Lake Alfred, FL