To improve the coverage area ratio (agrochemical adhesion area / total area) in orchards sprayed by unmanned aerial vehicles (UAVs), and to clarify the ratio at which the application of a small amount of concentrated chemicals achieves effective control of pests, it is necessary to establish the technology. Using two types of UAVs, we sprayed water over a citrus orchard at the rate of 100 L/ha, and assessed the coverage area ratio of water-sensitive test papers placed in the orchard canopy at three heights (200, 100, 50 cm) in vertical and horizontal orientations. The median of the ratio ranged from 0.35% to 23.0% by unmanned helicopter and from 0.05% to 25.9% by multi-rotor sprayer, while ranged from 21.6% to 99.9% by air blast sprayer spraying 3000 L/ha. The ratio depended on the orientation of the test papers and the installation height, because UAVs can spray only from above the trees. These results show that it is necessary next to clarify the optimal method for spraying citrus orchards by UAVs.
