Mechanism And Field Efficacy Of Low Volume Spraying By Plant Protection UAV On Wheat

In recent years,using plant protection UAV to spray pesticides for controlling diseases and pests has developed rapidly in China due to the rural labor shortage,weak mechanization of plant protection links and rigid requirements for plant protection disaster prevention and mitigation.The prevention and control of wheat diseases and pests in the middle and late stages of wheat growth is very important for its yield formation and quality assurance.Although the current application of plant protection UAV has initially met the requirements for pest prevention and control in the middle and late stages of wheat,sometimes the quality of operations is difficult to guarantee.The reason is that the current use of plant protection UAV to spray pesticides is basically based on experience and lacks a system Supported by theory and data.Therefore,in this research,wheat plants are used as a test crop,plant protection UAVs are used as sprayers,wheat mid-and late-stage pests and diseases are used as control objects,and common wheat pesticides are used as test agents.The microstructure of wheat plants,the surface tension and spreading index of highconcentration solution of the formulation system,droplets distribution in wheat canopy,field efficacy,and the dynamics of pesticides in wheat plants and wheat heads applied by the highconcentration application method are investigated through a number of indoor and field experiments.An abundant data has been obtained.The main research contents and conclusions are shown as the following.The understanding of wheat’s surface microstructure is critical to guide spraying due to its hydrophobic.Scanning electron microscope(SEM)is used to observe the surface microstructure of wheat leaves and heads.The results show that:(1)The surface microstructure of different leaves is similar.Stomata and burrs are the main microstructure of the leaves,but there are very few burrs on the back of the leaves.(2)The surface microstructure of leaf sheath mainly includes vesicular cells and stomata.(3)Papillae and vesicle cells are the primary microstructure on the outside of wheat glume,and the inside are acupuncture structures.These acupunctures are thinner than the burrs on the leaves.(4)The stomatal length,spacing and stomatal band spacing of wheat surface are between 12.5～253μm.Thus,droplet size of plant protection should be≥12.5μm.At present,the droplet size(VMD)of commonly used plant protection UAV is between 150～300 m.Therefore,the application of plant protection UAV for spraying pesticides can meet the requirements of covering the stomata,and make pesticides penetrate better into the target crop.The studies surface tension and spreading of pesticide solution are important to ensure that pesticide droplets deposit on the target smoothly.In the test,seven aerial adjuvants are added into different high-concentration solutions respectively.The surface tension and spread index with and without aerial adjuvants are measured.The results show that:(1)Silicone-based aerial adjuvants can significantly reduce the surface tension of single-agent solutions and mixed solutions of various formulations(emulsions,suspensions,aqueous agents)and improve the spreading ability of these solutions on target crops.(2)After the concentration of aerial adjuvants reaches the critical micelle concentration(CMC),increasing the concentration of the adjuvants does not change the surface tension of the solution,but it can continue to increase the spreading index of the solution.(3)When the mixed pesticide solution contains more than two types of emulsifiable concentrate,the minimum additive concentration of the aerial adjuvants should be based on the minimum additive concentration of the single emulsifiable concentrate that is most difficult to wet and spread on the target.(4)The surface tension,spread index and droplet size of different formulation system solutions are significantly different.Therefore,in order to make the aerial adjuvants efficient in the pesticide application process,it is necessary to perform surface tension and spreading index tests regarding to the types and concentrations of the aerial adjuvants before spraying the high-concentration formulation solution.The research on the influence factors of droplet deposition and field efficacy is an indispensable application technology research step to ensure spray quality.The field trials are carried out in several experimental sites in two years.Impacts of UAV models,spraying volumes,application parameters(operating height,flight velocity and nozzle flow rate),aerial adjuvants,and the spraying time of within a day are observed on the effects of droplet deposition and field efficacy.The results show that:(1)The characteristics of droplet deposition in wheat canopy and control efficacy with different pests and diseases are differed between two types of UAV.The total deposition of the single-rotor and multi-rotor UAV in the canopy is similar,however,droplets of the single-rotor in the lower part of canopy are mor than that of multi-rotor,and droplets of multi-rotor in the upper part of canopy outweigh of single-rotor.No significant difference in the control efficacy of wheat aphids is observed7 days after the treatment(DAT)between single-rotor and multi-rotor.For the prevention effect of the disease,no significant difference is observed 10 DAT.For the diseases locating on the lower part of the canopy(Wheat powdery mildew),the disease index prevention effect of single-rotor treatment(89.84%)is significantly higher than that of multi-rotor treatment(80.43%)10 DAT.(2)Spraying volume has significant effects on droplet deposition in wheat canopy and control efficacy.The droplet deposition in the upper and lower canopies increases with the increase of the spraying volume.The control effects of wheat aphids 1 DAT and 3DAT increase with the increase of the spraying volume.Therefore,when the wheat aphids occur moderately,a good control effect can be obtained when the spraying volume is or more than 12 L/ha;when the pests and diseases occur severely,spraying volume is or more than15 L/ha.For the prevention of wheat disease in the middle and late stages,the preventive effect is greater than 92% when the spraying volume is or more than 9 L/ha 10 DAT.(3)Aerial spraying adjuvants have a significant effect on droplet deposition and field efficacy.The wheat aphids control effects of treatment with aerial spraying adjuvant are significantly higher than those treatments without aerial spraying adjuvant.In addition,Aerial spraying adjuvants can reduce the use of pesticides by 20%.(4)Operating height,flight velocity and nozzle flow rate have a significant effect on droplet deposition in wheat canopy and field efficacy.Flight velocity has a greater effect on the deposition of the upper droplets,and the operating height affect the deposition of droplets in the lower part of canopy.The order of the effects of the three factors on the control effect of wheat aphids 1 DAT and 3 DAT are flight velocity,nozzle flow rate,and operating height.(5)The application time within a day has a significant effect on the droplet deposition on upper canopy and the control effect of wheat aphids.The droplet deposition and the control effect of aphids in the morning and evening application treatment are significantly higher than those in the noon application treatment.Upper canopy of wheat is the target part in the middle and late stages of wheat.Therefore,the application of pesticides by UAV should be carried out in the morning and evening when the temperature is lower to maximize the effectiveness of pesticides.Mastering the dissipation of pesticides in crops used high-concentration-low-volume spraying method by UAV is the basis for ensuring the safety of UAV application.In the test,imidacloprid and tebuconazole are used as target objects.The digestion of these two pesticides in wheat heads and plants is studied.The test results show that:(1)Pesticide dissipation are similar between UAV and knapsack application.(2)The half-lives of tebuconazole and imidacloprid in wheat heads are 3.33 to 4.13 days and 3.90 to 5.20 days,and the half-lives in wheat leaves are 2.90 to 4.18 days and 2.60 to 4.40 days respectively.In conclusion,when using plant protection UAV to apply pesticides in wheat of middle and late stages in morning and late afternoon time could obtain satisfying control efficacy by choosing the appropriate pesticides and aerial spray adjuvants according to the levels of pests and diseases,followed appropriate spraying volume,operating height,flight velocity and nozzle flow rate.