Wheat is one of the major food crops in China; the planting area and yield account for 21.4% and 20.9%, respectively, in 2017 (Data from National Bureau of Statistics of China). However, there are many kinds of pests and diseases which harm the production of wheat. These pests and diseases are controlled basically by the application of chemical products.
The selection of the equipment to be used is a critical factor for chemical pest control. In China, more than 88% of sprayers are manually operated [1
], which include electric or manual air-pressure knapsack sprayer and knapsack mist-blower sprayer. The quality of the application depends mainly on the skill of the operators. These types of equipment are of low cost, easily maintained and adequate to control periodic and localized problems. However, applications with knapsack sprayer generally lead to high chemical exposure of the operators [2
] and postural discomfort [4
]. The operational farm size is increasing with the growth of agricultural co-operatives, land leasing and contract farming, while the labor force is declining by urbanization and rural–urban migration [1
], leading to these low-efficiency and labor-intensive equipment types no longer being suitable for crop protection. As one of the alternative equipment types, self-propelled boom sprayer appeared on the market, equipped with horizontal spray boom. These machines have relatively higher working efficiency, lower chemical exposure and higher deposition [5
]. However, the complicated terrain and small farm size with separated plots limit the use of boom sprayer in China. In recent decades, to adapt to this unique operating environment and meet the shortage supply of the crop protection equipment, unmanned aerial vehicles (UAV) for pesticide application have been developed quickly in China. Comparing with the manned agricultural aircraft, UAVs do not require navigation station or airport, and the edge of field can be its landing site [6
]. The low rate of no-load flight and less flight crew reduce the expenditure of operations and administration [6
]. Meanwhile, UAVs have short turning radius due to hover and turn around flexibly in the air, which are suitable for working in rough terrain and small plots with high efficiency [1
]. Comparing with the conventional ground crop protection machinery, UAVs operate with lower labor intensity, operator exposure and have a higher working efficiency, especially in rough terrain and small plots [1
]. According to the statistics data by the Chinese Ministry of Agriculture, nearly 14,000 crop protection UAVs are used in the country. The spraying area approached 5.5 million hectares in 2017.
Because of the broad prospect of application, UAVs have attracted plenty of scholar’s attention. In the aspect of optimizing operational altitudes and speeds, Qin et al. [7
] optimized the flying parameters for preventing plant hoppers, showing that a flight height of 1.5 m and a flying velocity of 5 m/s achieved the maximum lower layer deposition and the most uniform distribution for HyB-15L UAV sprayer (Gao Ke Xin Nong Co. Ltd., Shenzhen, Guangdong, China). The optimal parameters change with the type of UAV and the crop. In a spraying test of different shape (open center shape and round head shape) of circus trees, the 3W-LWS-Q60S UAV (Zhuhai Crop Guardian Aerial Plant Protection Co., Zhuhai, Guangdong, China) performs better when the working height is 1.0 m compared with 0.5 and 2 m [10
]. In the aspect of deposition uniformity, a multi-spraying swath test is conducted with different UAV sprayers [9
]. There is an obviously inconsistent amount of deposition in the longitudinal and lateral direction and this phenomenon has been reported in many studies [7
]. The control efficacy on pests and diseases is one of the most important evaluation indices of chemical application. Quite different from the conventional large volume application, the UAV sprayer belongs to low volume (LV, 4.7–46.7 L/ha) or ultra-low volume (ULV, 0–4.7 L/ha) [12
] spraying equipment with the spray volume in the range of 1–40 L/ha [6
]. Meanwhile, with the same active ingredient applied per acre, the chemical concentration of UAV is particularly high. Qin et al. [7
] studied the control efficacy of HyB-15L UAV with spraying Chlorpyrifos·Regent EC against plant hoppers and found that the insecticidal efficacy is 92% and 74% at 3 and 10 days after application, respectively. On the premise of guaranteed control efficacy, the working efficiency is another important evaluation index of application. Currently, UAVs mostly rely on semi-autonomous control with the flight altitude belonging to autonomous control. The control range is 200–300 m in visible distance with manual control [6
]. The payload capacity of the aerial application UAVs is generally 5–25 kg [6
]. Considering the limited payload and the flight range, the effective spray work rates of 2–5 ha/h can be achieved in a vineyard with a gasoline-powered helicopter (RMAX, Yamaha motor Co., Cypress, CA, USA) [11
]. The working efficiency of different UAVs in the grain-filling stage of wheat was studied by Wang et al. [9
], with the daily working area ranging from 13.4 to 18.0 ha in 8 h. Pesticide spray drift is an important environmental problem for aerial application. Compared with the manned agricultural aircraft [6
], the droplet drift of UAV is effectively reduced with the lower flight height [6
] and the downwash wind [17
]. According to Xue et al. [19
], under the wind speed of 3 m/s, 90% of drift droplets of Z-3 UAV are located within a range of 8 m of the target area. Similar to Xue et al., Wang et al. [17
] measured that 90% of drift droplets of a fuel powered single-rotor UAV are within 9.3–14.5 m under the wind speed of 0.76–5.5 m/s. Compared with the conventional boom sprayer [20
], the droplets drift distance of UAV sprayer would be further.
Despite these preceding studies, research is focused mainly on parameter optimization, droplet deposition and biological efficacy of one equipment. Few studies compare different kinds of crop protection equipment, especially including UAV sprayer. Under the same working condition, the comparison of different crop protection equipment on deposition, control efficacy and working efficiency is very important for equipment selection and application quality analysis. The main objective of this research was to compare the application quality of a battery motive 3WTXC8-5 six-rotor UAV with a 3WX-280H self-propelled boom sprayer and two conventional knapsack sprayers (3WBS-16A2 electric air-pressure knapsack sprayer and WFB-18 knapsack mist-blower sprayer). The comparison items included the spray deposition on the plants and run-off, uniformity and penetrability of the deposition, deposition characterization (including droplet size, number of spray deposits and the area of coverage), pesticide efficacy on wheat aphid and working efficiency. The experimental results show that the control efficacy of the UAV on wheat aphid was comparable to other spraying equipment with the working efficiency significantly higher than others. Unfortunately, UAV still have many problems on deposition uniformity and droplets penetrability.