Effect of Vegetable Oil Adjuvant on Wetting, Drift, and Deposition of Pesticide Droplets from UAV Sprayers on Litchi Leaves

The spatial transportation of pesticide spray droplets and their deposition and retention on plant leaf surfaces are critical factors contributing to pesticide loss. Adding adjuvants to pesticide solutions to improve their wettability and deposition behavior can enhance the targeted deposition efficiency of pesticides sprayed by unmanned aerial vehicle (UAV) sprayers. In this study, Maifei (MF), a prevalent vegetable oil adjuvant, was selected to analyze its effects on the physicochemical properties of water and 10% difenoconazole water-dispersible granules (D) and the wetting performance of droplets on litchi leaves. The changes in the drift and deposition of the spray solutions with or without MF were tested using a UAV sprayer, DJI T40. The results indicated that the addition of MF to water or D significantly decreased the surface tension (by 58.33% and 23.10%, respectively), wetting time (by 97.81% and 90.95%, respectively), and contact angle (by 40.95% to 70.75% for the adaxial and abaxial surfaces of litchi leaves), achieving the best effects at a 1% MF addition. Moreover, during the drift test, the addition of 1% MF to the solutions significantly reduced the cumulative drift rate (CDR) (by 48.10%). Finally, owing to the weakened spray drift risk and improved wettability of the droplets on litchi leaves with a 1% MF addition, the droplet deposition and penetration in the litchi canopy significantly improved, demonstrating an increased droplet density of 38.17% for the middle layers of the litchi and 15.75% for the lower layers, corresponding to increased coverage by 59.49% and 12.78%, respectively. Hence, MF can improve the interfacial properties of the spray solution on litchi leaves, reduce the drift risk, and promote deposition, thereby facilitating the efficient transfer and deposition of pesticide droplets from UAV sprayers.