Evaporation and Deposition Coverage Area of Droplets Containing Insecticides and Spray Additives on Hydrophilic, Hydrophobic, and Crabapple Leaf Surfaces

The efficiency of foliar spray applications is influenced by the evaporation and residual pattern of pesticide droplets on targets. Evaporation time and maximal coverage area of a single droplet from 246 to 886 m in size at relative humidity (RH) ranging from 30% to 90% were measured with sequential images under controlled conditions. Droplets were placed on targets inside an environmentally controlled chamber under a stereoscope and a high-definition digital camera. The spray mixtures used to form droplets included different combinations of water, a nonionic colloidal polymer drift retardant, an alkyl polyoxyethylene surfactant, and two commercially available insecticides. The droplet evaporation was investigated on crabapple leaf surfaces, and hydrophilic and hydrophobic glass slide surfaces. Adding surfactant into spray mixtures greatly increased droplet coverage area on the surfaces, while droplet evaporation time was greatly reduced. For a 343 m droplet on a crabapple leaf at 60% RH, the evaporation time decreased from 70 to 50 s and the maximal coverage area increased from 0.366 to 0.890 mm2 after the surfactant was added into the spray mixture containing water and insecticide. Adding the drift retardant into the spray mixture slightly increased the droplet evaporation time and decreased the droplet coverage area. In addition, changing the target surface from the hydrophilic slide to the hydrophobic slide greatly increased the droplet coverage area and reduced the droplet evaporation time. Increasing RH increased the droplet evaporation time greatly but did not change the coverage area. The droplet evaporation time and coverage area increased exponentially as the droplet size increased. Therefore, droplet size, surface characteristics of the target (waxy or non-waxy), RH, and chemical composition of the spray mixture (water alone, pesticide, additives) should be included as important factors that can affect the efficacy and efficiency of pesticide applications.