## 3 Results and Discussion
### Control of Wheat Aphids  
The control efficacy of the four sprayers applying 70% Imidacloprid-Regent WDG on wheat aphids are indicated in Figure 6. Although the spray deposition characteristics of the four sprayers were significantly different from each other, the differences of control efficacy were not remarkable. The control efficacies on Day 7 after application were all beyond 70%. From the comparison of four sprayers, it was found that the SPB and KMB sprayers achieved the best control efficacy, while the efficacies were intermediate for EAP and UAV sprayer. Deposit structure plays a major role in toxin efficacy [24,38]. According to the results of the deposition, the SPB sprayer had the best deposition uniformity and penetrability, which benefit the control of wheat aphids, especially since wheat aphids tend to favor the lower portions of plants. The larger area of coverage and a larger number of spray deposits increased the odds of interaction between active ingredients and pests.  
Because UAV sprayers are still in the initial development stage in China, there are still many problems on the spraying system and working parameter. These problems lead to unevenness deposition and poor penetrability. The lower area of coverage and fewer spray deposits with the nonuniform deposition reduced the chance of aphids to be exposed to the insecticides. The control efficacy of UAV sprayer on Day 1 after application was significantly lower than other sprayers, which was only equal to 50.5% of the SPB sprayer. However, in time, the control efficacy increased. The reason could be that, with the movement of the aphids and the systemic action of the active ingredients, the chance of wheat aphids being exposed to the active ingredients increased. On Day 3 after application, the control efficacy increased to 60.9% and, on Day 7 after application, the control efficacy increased to 70.9%. Although it was also significantly lower than the SPB and KMB sprayers, it was still an acceptable result for farmers, especially in complex small plots or rice fields, where the SPB sprayer is hard to work and the KMB and EAP sprayer have a low work efficiency. Although the UAV sprayer, especially for the electrical multi-rotor UAV, used in the field, is an innovation, the low volume application is not a new technology. Many studies had been conducted to evaluate the feasibility of low  
Figure 6: Control efficacy (%) of four sprayers against wheat aphids in the field tests at Days 1, 3, 7 after treatment. DAT, days after treatment.  
volume application for pests and diseases control. In the laboratory, [PERSON] and [PERSON] [39] found that several spray deposits of 5-10 points/cm\({}^{2}\) on foliage after spraying were effective (90% mortality) against second and third instars. [PERSON] et al. [40] investigated the effect of fungicide spray number of spray deposits (points/cm\({}^{2}\)), droplet size and proximity of the spray deposits to fungal spores on the banana leaf surface. The test results suggest that the inhibition zones of two contact fungicides on the leaf surface extend beyond the visible edge of the spray droplet deposit and a mean droplet deposit density of 30 points/cm\({}^{2}\) can inhibit the germination of the ascospore below 1%. [PERSON] et al. [36] evaluated the efficacy of aerial electrostatic-charged sprays for season-long control of sweet potato whiteflies, and they concluded that the control efficacy of electrostatic-charged sprays with spray volume of 4.68 L/ha was comparable with those on cotton treated with conventional applications of 46.8 L/ha.