A series of 76 field trials in a vegetated habitat with caged adult Aedes taeniorhyncus mosquitoes determined that ground ultra-low volume treatment efficiency was significantly influenced by droplet size. Small droplets (7-microns volume median diameter [VMD]) produced the lowest mosquito mortality at all test distances (100-500 ft.). A VMD of 26 microns was effective at 100-300 ft., but was not effectual at 400-500 ft. Droplets in the 15-microns range were the most effective overall, at the distances evaluated.