Effect of Unmanned Aerial Vehicle Flight Height on Droplet Distribution , Drift and Control of Cotton Aphids and Spider Mites

Unmanned aerial vehicles (UAVs), as emerging plant protection machinery, have the advantages of high operational efficiency, high speed, and low drift. The current study aimed to elucidate the characteristics of droplet distribution and drift, control efficiency on cotton aphids and spider mites, and attachment and absorption of cotton leaves during UAV spraying. Kromekote card and filter paper are used as samplers to collect droplets, and the droplet density, coverage rate, deposition, and drift percentage are statistically analyzed. The pooled results showed that the droplet uniformity, the droplet coverage rate, the deposition, and the drifting ability are higher when the UAV flight height was 2 m. The control effects by UAV spraying on cotton aphids and spider mites were 63.7% and 61.3%, respectively. These values are slightly inferior to those obtained through boom spraying. Cotton leaf attachment and absorption of spirodiclofen after UAV spraying were slightly lower than those after boom spraying, therefore, the control efficiency of cotton pests is slightly different. According to the different flight height operations by the UAV sprayer, the drift capability of the droplets at 2 m flight height was large, and the droplet uniformity and deposition were satisfactory. The research results could provide the theoretical basis and technical support for UAV operation.


Introduction
Cotton (Gossypium spp.) is the main industry of Xinjiang Uygur Autonomous Region, China, and is of great significance to the economic and social development of Xinjiang [1,2].Due to the geographical and climatic conditions in Xinjiang, continuous cropping of cotton has caused the frequent occurrence of cotton diseases and insect pests, which has led to losses in cotton yield and quality.Hence, strengthening phytochemical prevention and the control of cotton is significant in improving the yield and quality of cotton [3,4].In recent years, the use of extensive pesticide spraying methods to prevent and treat pests and diseases due to the backward concept of pesticide application has not only had low utilization but also produced large amounts of pesticide residues in crops, thereby causing severe pesticide residue contamination and soil residual contamination [5].The use of pesticides per unit area in China is 2.5 times than that of the world average, and the area Agronomy 2018, 8, 187 3 of 13

Experimental Site
The experiment was carried out in Beiquan town of Xinjiang Production and Construction Crops, Shihezi, Xinjiang Uygur Autonomous Region, China, during July 2017.The experimental field had middle level fertilizer and planted cotton for many years.Cotton (Xinluzao 67) was planted on 21 April 2017, using a mechanical cotton-picking planting model, with wide film planting six lines (10 cm + 66 cm), 225,000 cotton/ha, and drip irrigation under plastic film (Figure 1).
Agronomy 2018, 8, x FOR PEER REVIEW 3 of 13 middle level fertilizer and planted cotton for many y ears.Cotton (Xinluzao 67) was planted on 21 April 2017, using a mechanical cotton-picking planting model, with wide film planting six lines (10 cm + 66 cm), 225,000 cotton/ha, and drip irrigation under plastic film (Figure 1).

Experimental Design
The experiment consisted of three treatments (Table 2).Cotton defoliant spraying was carried out on 12 July 2017.Each treatment was conducted on an area of 1.33 hm 2 .Treatment 1 was conducted with the boom sprayer.Treatments 2 and 3 were low-height and low-volume spraying by the UAV at working heights of 1.5 and 2 m above the canopy.The weather during the experimental period is described in Table S1.

Evaluation of Droplet Deposition
The UAV spraying test aimed to study the droplet deposition and drifting regularity of the cotton plants by UAV spraying.Before spraying, a handheld GPS locator was used to determine the coordinates of each spraying zone.Five points (deposition area) were arranged on the line perpendicular to the spray belt in the test area, and five points (drift area) were arranged from the edge of the spray width, with 1 m between each point (determined according to the spray width).The sample layout was repeated thrice.The UAV flew through the center of the deposition area as shown in Figure 2. Before spraying, a stapler was used to attach kromekote cards and filter paper to cotton leaves in the cotton canopy.The kromekote cards and filter paper were placed at a height that was equivalent to the lower, middle, and upper thirds of the cotton canopy (Figure 2) [25].Kunshan Ultrasonic Instruments Co., Ltd., Kunshan, Jiangsu, China, and the rotary evaporators, RV10 D, were from IKA, Staufen im Breisgau, Germany.

Experimental Design
The experiment consisted of three treatments (Table 2).Cotton defoliant spraying was carried out on 12 July 2017.Each treatment was conducted on an area of 1.33 hm 2 .Treatment 1 was conducted with the boom sprayer.Treatments 2 and 3 were low -height and low -volume spraying by the UAV at working heights of 1.5 and 2 m above the canopy.The weather during the experimental period is described in Table S1.

Evaluation of Droplet Deposition
The UAV spraying test aimed to study the droplet deposition and drifting regularity of the cotton plants by UAV spraying.Before spraying, a handheld GPS locator was used to determine the coordinates of each spraying zone.Five points (deposition area) were arranged on the line perpendicular to the spray belt in the test area, and five points (drift area) were arranged from the edge of the spray width, with 1 m between each point (determined according to the spray width).The sample layout was repeated thrice.The UAV flew through the center of the deposition area as shown in Figure 2. Before spraying, a stapler was used to attach kromekote cards and filter paper to cotton leaves in the cotton canopy.The kromekote cards and filter paper were placed at a height that was equivalent to the lower, middle, and upper thirds of the cotton canopy (Figure 2) [25].The kromekote cards and filter papers were collected after the field experiment and placed in self-sealing bags.Each card was separately photographed on a light table using a high resolution digital-single-lens-reflex (DSLR) camera positioned at a 10 cm height above t he light table.Photographs of the sprayed cards were analyzed using Image J software (Image J 1.48, National Institutes of Health, Bethesda, MD, USA).Each card was cropped to remove the background area, changed into 8-bit format, and then individually threshold adjusted to ensure that only sprayed droplets were included in the sample analysis.Each image was analyzed for droplet number density and percent coverage.Coverage was determined as the percent cover age of the card from the blue dye of the deposited droplets [24].The kromekote cards and filter papers were collected after the field experiment and placed in self-sealing bags.Each card was separately photographed on a light table using a high resolution digital-single-lens-reflex (DSLR) camera positioned at a 10 cm height above the light table.Photographs of the sprayed cards were analyzed using Image J software (Image J 1.48, National Institutes of Health, Agronomy 2018, 8, 187 5 of 13 Bethesda, MD, USA).Each card was cropped to remove the background area, changed into 8-bit format, and then individually threshold adjusted to ensure that only sprayed droplets were included in the sample analysis.Each image was analyzed for droplet number density and percent coverage.Coverage was determined as the percent coverage of the card from the blue dye of the deposited droplets [24].
In the laboratory, each filter paper was washed with 5 mL of distilled water and kept in separate bags.Each bag was shaken for 1 min, and the papers were allowed to soak for 1 h.Afterwards, the washing solution was removed from the bag, and the colorant concentration was measured at 514 nm with an ELISA instrument.The solution obtained from washing unsprayed strips of filter papers was used as the baseline solution.Droplet distribution was determined in accordance with the standard curve of allura red.Equation ( 1) was used to calculate the deposition of droplets on the cotton plants [15]: where β dep is the droplet deposition in µL/cm 2 ; ρ smpl is the ELISA instrument reading of the sample; ρ blk is the ELISA instrument reading of the blanks (collector + dilution water); F cal is the relationship between the ELISA instrument reading and the allura red concentration in (µg/L)/unit number; V dii is the volume of the dilution liquid used to dilute tracer from collector, L; ρ spray is the concentration of the tracer in spray in g/L; and A col is the area of the filter paper in cm 2 .
The experiment ended after the droplets on the Kromekote card were dried (deposition area), sealed in a self-sealing bag, and the data were collected and processed in the laboratory.To characterize the uniformity of droplet deposition between collection points in the test, the uniformity of droplet deposition was measured by the CV of droplet deposition densities at different locations of the cotton plants in the UAV effective spray width.The CV of droplet deposition density was calculated by Equations ( 2) and (3).
where S is the sample standard deviation, X i is the number of droplets per unit area at each collection point, and X is the number of collection points per layer.
To characterize the distribution of the droplet drift between collection points in the test, the drift of droplets in the experiment was expressed by the spray drift percentage of the UAV [26].The droplet drift percentage was calculated by Equation (4): where β dep is the spray drift deposit in µL/cm 2 ; β dep% is the spray drift percentage in %; and β V is the spray volume in L/ha 2 .

Allura Red Concentration: Absorbance Demarcation
To ensure the accuracy of the measurement data, the allura red (20 mg) was dissolved in 100 mL of water and diluted to 1 L to a final concentration of 20 mg/L.The mother liquid was then diluted to 0.1, 0.2, 0.5, 1, 2, and 5 mg/L.The absorbance values were measured at 514 nm in triplicate per concentration by using the ELISA instrument.The standard curve of allura red was created on the basis of the absorbance values of the six solutions [12], as shown in Figure 3.
The linear fitting of the standard solution concentration and absorbance, correlation index R 2 = 0.993, and concentration and absorbance for the relationship was as follows: where A is the absorbance of the allura red solution and C is the concentration of the allura red solution in mg/L.

Control of Cotton Aphids and Spider Mites
The survey and recording of aphid and spider mite populations were performed in accordance with criteria for pesticide field efficacy tests.To investigate the control effect of pesticide application on aphid and spider mites, the parallel-jump method was used to survey the population numbers of cotton aphids and spider mites in each zone prior to spraying and at five days after spraying.Each zone was subjected to a five-point survey using five cotton plants for each point.The total numbers of aphids and spider mites for a 10-point survey using one cotton leaf for each point were counted.The overall control effect against aphids and cotton spider mites was quantified wi thout considering the types or growth stages of aphids and spider mites.The dropping rate and control effect were obtained on the basis of the population numbers of live insects in each zone before and after spraying in accordance with Equations ( 5) and (6).
where D is the decline rate of insect mouth; Na is the number of live insects before spraying; Nb is the number of live insects after spraying; CE is the control efficiency; Da is the decline rate of insect mouth in the treatment area; Db is the decline rate of insect mouth in the control area.

Sample Extraction and Purification
Cotton leaves were collected after spraying.Four grams of cotton leaves were accurately weighed, ground into powder (liquid nitrogen grinding), and placed into a 50 mL centrifuge tube.Five milliliters of ultrapure water and 10 mL of methanol (chromatographically pure) were added, shaken and extracted for 30 min.3 g of NaCI was added, vortexed for 1 min, and centrifuged at 3800 r/min for 5 min.One milliliter of supernatant was obtained and placed in a 2 mL centrifuge tube containing 50 mL PSA, placed in the vortex mixer for 1 min, and centrifuged at 1000 r/min for 3 min.The supernatant was filtered through a 0.22 μm organic membrane filter and then tested [27].

Chromatographic Conditions
The sample was analyzed with an Agilent 7890B GC equipped with an HP-5 30 m capillary column with a film thickness of 0.32 mm × 0.25 μm (Agilent Technologies, Palo Alto, CA, USA).The

Control of Cotton Aphids and Spider Mites
The survey and recording of aphid and spider mite populations were performed in accordance with criteria for pesticide field efficacy tests.To investigate the control effect of pesticide application on aphid and spider mites, the parallel-jump method was used to survey the population numbers of cotton aphids and spider mites in each zone prior to spraying and at five days after spraying.Each zone was subjected to a five-point survey using five cotton plants for each point.The total numbers of aphids and spider mites for a 10-point survey using one cotton leaf for each point were counted.The overall control effect against aphids and cotton spider mites was quantified without considering the types or growth stages of aphids and spider mites.The dropping rate and control effect were obtained on the basis of the population numbers of live insects in each zone before and after spraying in accordance with Equations ( 5) and (6).
where D is the decline rate of insect mouth; N a is the number of live insects before spraying; N b is the number of live insects after spraying; CE is the control efficiency; D a is the decline rate of insect mouth in the treatment area; D b is the decline rate of insect mouth in the control area.

Sample Extraction and Purification
Cotton leaves were collected after spraying.Four grams of cotton leaves were accurately weighed, ground into powder (liquid nitrogen grinding), and placed into a 50 mL centrifuge tube.Five milliliters of ultrapure water and 10 mL of methanol (chromatographically pure) were added, shaken and extracted for 30 min.3 g of NaCI was added, vortexed for 1 min, and centrifuged at 3800 r/min for 5 min.One milliliter of supernatant was obtained and placed in a 2 mL centrifuge tube containing 50 mL PSA, placed in the vortex mixer for 1 min, and centrifuged at 1000 r/min for 3 min.The supernatant was filtered through a 0.22 µm organic membrane filter and then tested [27].• C, respectively.The nitrogen carrier gas rate, hydrogen rate, air rate, and injection volume were 40.0 mL/min, 30 mL/min, 400 mL/min and 1.0 µL.Under the above chromatographic conditions, the retention time of acetamiprid was approximately 8.66 min, and the retention time of spirodiclofen was approximately 9.60 min.

Standard Curves of Spirodiclofen and Acetamiprid Concentration
A mother liquor of 1000 mg/L acetamiprid and spirodiclofen was prepared and diluted with methanol (chromatographically pure) to standard solutions of 2, 4, 6, 8, 10, and 12 mg/L, underwent gas chromatography FID injection, and was tested under 1.5.2 chromatographic conditions.A good linear relationship was observed between the peak areas and mass concentrations of acetamiprid and spirodiclofen.The standard curves were y = 0.79x + 0.2667 and y = 6.2229x + 1.139.The correlation coefficients were 0.9904 and 0.9966, as shown in Figure 4. respectively.The nitrogen carrier gas rate, hydrogen rate, air rate, and injection volume were 40.0 mL/min, 30 mL/min, 400 mL/min and 1.0 μL.Under the above chromatographic conditions, the retention time of acetamiprid was approximately 8.66 min, and the retention time of spirodiclofen was approximately 9.60 min.

Standard Curves of Spirodiclofen and Acetamiprid Concentration
A mother liquor of 1000 mg/L acetamiprid and spirodiclofen was prepared and diluted with methanol (chromatographically pure) to standard solutions of 2, 4, 6, 8, 10, and 12 mg/L, underwent gas chromatography FID injection, and was tested under 1.5.2 chromatographic conditions.A good linear relationship was observed between the peak areas and mass concentrations of acetamiprid and spirodiclofen.The standard curves were y = 0.79x + 0.2667 and y = 6.2229x + 1.139.The correlation coefficients were 0.9904 and 0.9966, as shown in Figure 4.

Determination of Recovery
Exactly 0.05, 0.5, and 5.0 mg/kg of acetamiprid standard solution were added to blank cotton leaves.Three parallel tests were performed for each solution.Extraction and purification were conducted according to the method of 1.5.1, with the determination of recovery and standard deviation (Table 3).The results showed that the average recovery of acetamiprid in the cotton leaves was 90.8%-101.5%, and the relative standard deviation was 3.7%-6.9%,which met the residue analysis requirements [28].Note : 1, 2, 3 me ans thre e paralle l te sts for e ach solution.

Statistics and Data Processing
Data were statistically analyzed using the software SPSS (SPSS Inc., an IBM Company, Chicago, IL, USA).Collector coverage and droplet density were analyzed in separate linear mixed models with seperationsmade at the p = 0.05 level.A one-way analysis of variance was conducted for droplet

Determination of Recovery
Exactly 0.05, 0.5, and 5.0 mg/kg of acetamiprid standard solution were added to blank cotton leaves.Three parallel tests were performed for each solution.Extraction and purification were conducted according to the method of 1.5.1, with the determination of recovery and standard deviation (Table 3).The results showed that the average recovery of acetamiprid in the cotton leaves was 90.8-101.5%,and the relative standard deviation was 3.7-6.9%,which met the residue analysis requirements [28].

Statistics and Data Processing
Data were statistically analyzed using the software SPSS (SPSS Inc., an IBM Company, Chicago, IL, USA).Collector coverage and droplet density were analyzed in separate linear mixed models with seperationsmade at the p = 0.05 level.A one-way analysis of variance was conducted for droplet deposition and drift in treatment 2 and treatment 3 to verify the significance effect of flight height on the experimental results (Table 4).The denominator degrees of freedom (df) was the protection from bias, achieved through the inclusion of the adjustment for the generalized linear mixed model.The significant effect of flight height on the deposition and drift of droplets is shown in Table 4: the flight height has a significant impact on droplet deposition (p = 0.022 < 0.05), the flight height has a highly significant impact on droplet drift (p = 0.0013 < 0.01).

Droplet Deposition
The uniformity of the spray-droplet distribution on targets is commonly described by the CV of the droplet density.The smaller the CV, the better the uniformity of the droplet deposition [29].The CV of droplet distribution at different flight heights of UAV is shown in Table 5.At the flight height of 1.5 m, the CVs of droplet density were 117.1%, 178.1%, and 85.8% on the upper, middle, and lower layer of the cotton canopy, respectively.At the flight height of 2 m, the CVs of droplet density were 79.4%, 50.3%, and 146.4%.The CV of upper and middle droplet density at the flight height of 1.5 m was significantly higher than 2 m.This may be a result of the reduced flight height, which generates a strong downward swirling airflow that causes the plants to sway substantially, and the droplet density causes a significant change of the cotton canopy [30].

Droplet Deposition Distribution
We studied the droplet deposition characteristics within the spray width sprayed by UAV (Figure 5).Flight height showed a significant effect on the droplet distribution sprayed by UAV.The distribution of droplets in the vertical direction in the test was represented by the droplet density.At the flight height of 1.5 m, the average coverage rates were 2.5%, 3.2%, and 1.9% for the upper, middle, and lower layers of the cotton canopy.At a flight height of 2 m, the average coverage rates were 4.9%, 5.5%, and 5.0%.The coverage rates at 2 m were higher than those at 1.5 m.The droplet distribution in the horizontal direction in the test was represented using the deposition.At the flight height of 1.5 m, the deposition of each sampling point in the deposition area was 0.018, 0.017, 0.015, 0.027, and 0.055 µL/cm 2 .The deposition was least in the spray center.Downwind, deposition increases with distance from the center of the spray width.A distance from the spray width center of 2 m showed the maximum deposition.At a flight height of 2 m, the deposition of each sampling point was higher than those at 1.5 m, the maximum deposition is more than the flight height of 1.5 m.This may be due to the increase of flight height and the weakening of the downward pressure wind field below the rotor.Part droplets have a small range of drift in the downwind direction due to the lateral wind field effects.The research on the wind field below the UAV rotor, such as in Chen et al., used a wireless wind speed sensor network measurement system for an unmanned helicopter to study the wind field distribution below the rotor.The experimental results show that the horizontal wind field and the vertical wind field below the rotor affect the distribution of droplets.Current research on wind field is not comprehensive enough and still requires further study [18].
Agronomy 2018, 8, x FOR PEER REVIEW 9 of 13 than the flight height of 1.5 m.This may be due to the increase of flight height and the weakening of the downward pressure wind field below the rotor.Part droplets have a small range of drift in the downwind direction due to the lateral wind field effects.The research on the wind field below the UAV rotor, such as in Chen et al., used a wireless wind speed sensor network measurement system for an unmanned helicopter to study the wind field distribution below the rotor.The experimental results show that the horizontal wind field and the v ertical wind field below the rotor affect the distribution of droplets.Current research on wind field is not comprehensive enough and still requires further study [18].

Droplet Drift
The data collected by the spray dr oplets at different flight heights of the UAVs were analyzed.The droplet drift in the test was expressed using the percentage of droplet drift in the horizontal direction of the drift zone, as shown in Figure 6.Flight height showed a significant effect on the droplet drift in the cotton canopy sprayed by UAV.When the flight height was 2 m, the drift percentage fluctuated from 15% to 30%, and the fluctuation range was large.When the flight height was 1.5 m, the drift percentage fluctuated from 5% to 12%.The average drift percentage (7.9%)at 1.5 m was much lower than that at 2 m (20%).This result could be attributed to the increased flight height, and the fact that the vertical wind field above the canopy of the cotton plant weakened, resulting in a large number of droplets drifting [26].

Droplet Drift
The data collected by the spray droplets at different flight heights of the UAVs were analyzed.The droplet drift in the test was expressed using the percentage of droplet drift in the horizontal direction of the drift zone, as shown in Figure 6.Flight height showed a significant effect on the droplet drift in the cotton canopy sprayed by UAV.When the flight height was 2 m, the drift percentage fluctuated from 15% to 30%, and the fluctuation range was large.When the flight height was 1.5 m, the drift percentage fluctuated from 5% to 12%.The average drift percentage (7.9%)at 1.5 m was much lower than that at 2 m (20%).This result could be attributed to the increased flight height, and the fact that the vertical wind field above the canopy of the cotton plant weakened, resulting in a large number of droplets drifting [26].
droplet drift in the cotton canopy sprayed by UAV.When the flight height was 2 m, the drift percentage fluctuated from 15% to 30%, and the fluctuation range was large.When the flight height was 1.5 m, the drift percentage fluctuated from 5% to 12%.The average drift percentage (7.9%)at 1.5 m was much lower than that at 2 m (20%).This result could be attributed to the increased flight height, and the fact that the vertical wind field above the canopy of the cotton plant weakened, resulting in a large number of droplets drifting [26].

Control Efficiency of Cotton Aphid and Spider Mite
To validate the superior cotton-pest control of pesticide application through the UAV sprayers, the aphid control effect by UAV sprayers was compared with that by boom sprayer.The control effect in the test was expressed using the decline rate of insect mouth, as shown in Figure 7.The control effect by boom sprayer for cotton aphids on the fifth day after the test was 90.0%, while the UAV sprayer control effect was 64.0%.The control effect by the UAV sprayer for cotton aphids was 26.0% lower than that by the boom sprayer.The control effect by the boom sprayer for spider mites on the fifth day after the test was 68.0%, while the UAV sprayer control effect was 61.3%.The control effects by UAV sprayer for spider mites was 6.7% lower than that by the boom sprayer.Qin et al.'s research on wheat pest control shows that the control efficacy was 92%-74% from 3 to 10 days after UAV spraying, and the control effect is good and the persistence period is long.In contrast, the prevention and control of cotton pests requires further research.

Control Efficiency of Cotton Aphid and Spider Mite
To validate the superior cotton-pest control of pesticide application through the UAV sprayers, the aphid control effect by UAV sprayers was compared with that by boom sprayer.The control effect in the test was expressed using the decline rate of insect mouth, as shown in Figure 7.The control effect by boom sprayer for cotton aphids on the fifth day after the test was 90.0%, while the UAV sprayer control effect was 64.0%.The control effect by the UAV sprayer for cotton aphids was 26.0% lower than that by the boom sprayer.The control effect by the boom sprayer for spider mites on the fifth day after the test was 68.0%, while the UAV sprayer control effect was 61.3%.The control effects by UAV sprayer for spider mites was 6.7% lower than that by the boom sprayer.Qin et al.'s research on wheat pest control shows that the control efficacy was 92%-74% from 3 to 10 days after UAV spraying, and the control effect is good and the persistence period is long.In contrast, the prevention and control of cotton pests requires further research.
(Cotton aphid control e ffe ct) (Spide r mite control e ffe ct) Figure 7.Control e ffe ct of the boom spraye r and UAV spraye r on cotton aphids and spide r mite s.

Attachment and Absorption of Pesticide on Cotton Leaves
To clarify the effects of defoliant adsorption on the attachment of cotton leaves, acetamiprid and spirodiclofen adsorption was studied.The result is shown in Table 6.After pesticide application through the boom sprayer, the cotton leaf attachment and adsorption of acetamiprid were concentrated in the middle and lower cotton leaves.After pesticide application through the UAV sprayer, the cotton leaf attachment and adsorption were concentrated in the upper and middle cotton leaves.The droplet penetrability by the boom sprayer was better than that by the UAV sprayer.After pesticide application through the boom sprayer, the cotton leaf attachment and adsorption of spirodiclofen on the upper, middle, and lower cotton leaves were 2.92, 0.67, and 1.10 mg/kg, respectively.By comparison, the cotton leaf attachment and adsorption through UAV spraying w ere 0.85, 0.46, and 0.77 mg/kg, respectively, which were slightly lower than that by the boom sprayer.

Attachment and Absorption of Pesticide on Cotton Leaves
To clarify the effects of defoliant adsorption on the attachment of cotton leaves, acetamiprid and spirodiclofen adsorption was studied.The result is shown in Table After pesticide application through the boom sprayer, the cotton leaf attachment and adsorption of acetamiprid were concentrated in the middle and lower cotton leaves.After pesticide application through the UAV sprayer, the cotton leaf attachment and adsorption were concentrated in the upper and middle cotton leaves.The droplet penetrability by the boom sprayer was better than that by the UAV sprayer.After pesticide application through the boom sprayer, the cotton leaf attachment and adsorption of spirodiclofen on the upper, middle, and lower cotton leaves were 2.92, 0.67, and 1.10 mg/kg, respectively.By comparison, the cotton leaf attachment and adsorption through UAV spraying were 0.85, 0.46, and 0.77 mg/kg, respectively, which were slightly lower than that by the boom sprayer.According to international oeko-tex standards, the residual amount of pesticides in cotton should not exceed 1.0 mg/kg.In this test, only the residual amount of spironolactone by UAV spraying complies with national standards, so the UAV application technology needs further research [31].

Conclusions
In summary, UAV was used for pesticide spraying to control aphids and spider mites in cotton in the flowering and boll-setting stages.The uniform and penetrative deposition and droplet drift law in cotton canopies at different flight heights are studied, analyzed, and compared with the control effect of cotton aphids and spider mite spray by a boom sprayer.Conclusions are drawn as follows.The flight height has a significant impact on droplet deposition and drift, the P-value representing droplet deposition and droplet drift are 0.022 and 0.0013, respectively.When the flight height is 2 m, the CV of the upper and middle droplet density is small and the droplet uniformity is good.At the flight height of 1.5 m, the deposition of each sampling point in the deposition area was 0.018, 0.017, 0.015, 0.027, and 0.055 µL/cm 2 ; the average coverage rate was 2.5%, 3.2%, and 1.9% for the upper, middle, and lower layers of the cotton canopy.This indicates that the working height is too low, which will result in a strong downward swirling airflow, and the cotton will sway greatly, causing a large change in the droplet deposition.These values are slightly inferior to those achieved through the 2 m spraying.At the flight height of 1.5 m, the percentage of droplet drift fluctuates from 5% to 15%.At the flight height of 2 m, the vertical wind field above the canopy of the cotton plant is weakened due to the increased flight height, resulting in several droplets drifting.The control effect by boom sprayer for cotton aphids and spider mites was 90.0%, 68.0%; the UAV sprayer control effect was 64.0%, 61.3%.The control effect by the UAV sprayer is slightly worse than that by the boom sprayer.After pesticide application through the boom sprayer, the cotton leaf attachment and adsorption of acetamiprid were concentrated in the middle and lower cotton leaves.The droplet penetrability by the boom sprayer was greater.The cotton leaf attachment and adsorption of spirodiclofen by UAV spraying on the upper, middle, and lower are slightly lower than that by the boom sprayer.
In recent years, cotton pests and diseases have occurred frequently, resulting in a significant decline in cotton production, and the development of plant protection machinery is vital in Xinjiang.UAVs application techniques are widely used because of their unique advantages in terms of their water-saving and medicine-saving characteristics as well as their high-operating efficiency, non-rolling of cotton sticks, and not dragging bolls during operation.The UAVs application technology must continue to optimize the operating methods and the selection of aviation-specific reagents to improve the spraying effects, improve the pesticide utilization rate, and improve the quality and efficiency of the cotton industry in Xinjiang.

Figure 2 .
Figure 2. Arrange me nt of krome kote cards and filte r pape r.

Figure 2 .
Figure 2. Arrangement of kromekote cards and filter paper.

13 Figure 3 .
Figure 3. Standard curve of allura re d solution.

Figure 3 .
Figure 3. Standard curve of allura red solution.
The sample was analyzed with an Agilent 7890B GC equipped with an HP-5 30 m capillary column with a film thickness of 0.32 mm × 0.25 µm (Agilent Technologies, Palo Alto, CA, USA).The initial temperature was set to 180 • C for 1 min; gradually increased to 230 • C at a rate of 30 • C/min and held at 230 • C for another 2 min; gradually increased to 260 • C at a rate of 25 • C/min and held at 260 • C for another 2 min and finally gradually increased to 280 • C at a rate of 15 • C/min and held at 280 • C for another 2 min.The injection, column and FID temperature was 270 • C, 260 • C, and 280

Figure 5 .
Figure 5. Distributions of drople t de position at diffe rent flight he ights.Note : Treatments 2 and 3 were low-he ight and low-volume spraying by the UAV at working he ights of 1.5 and 2 m above the canopy.The uppe r, middle , and lowe r laye rs of cotton are divide d according to Figure 2.

Figure 5 .
Figure 5. Distributions of droplet deposition at different flight heights.Note: Treatments 2 and 3 were low-height and low-volume spraying by the UAV at working heights of 1.5 and 2 m above the canopy.The upper, middle, and lower layers of cotton are divided according to Figure 2.

Figure 6 .
Figure 6.Distribution of droplet drift at different flying heights.Note: Treatments 2 and 3 were low-height and low-volume spraying by the UAV at working heights of 1.5 and 2 m above the canopy.

Agronomy 2018, 8 , 13 Figure 6 .
Figure 6.Distribution of drople t drift at diffe re nt flying he ights .Note : Tre atments 2 and 3 we re lowhe ight and low-volume spraying by the UAV at working he ights of 1.5 and 2 m above the canopy.

Figure 7 .
Figure 7.Control effect of the boom sprayer and UAV sprayer on cotton aphids and spider mites.

Table 1 .
Characte ristic parame te rs of UAV and boom spraye r.

Table 1 .
Characteristic parameters of UAV and boom sprayer.

Table 2 .
Treatment for insecticide dosage.

Table 3 .
Adde d re cove ry rate of ace tamiprid.

Table 3 .
Added recovery rate of acetamiprid.

Table 4 .
Analysis of variance of droplet deposition and drift.
Note: p means the significance level of the factor affecting the result, p < 0.01 (** represent factors has highly significant on test result), p < 0.05( * represent factors has significant impact on test result).

Table 5 .
Distribution of droplet density.Note: The droplet density in the table are mean.Data followed by different small letters are significantly different among different treatments at p < 0.05 level by Duncan's new multiple range test.

Table 6 .
Effect of cotton leaf attachment and absorption.: The data in the table are mean.Data followed by different small letters are significantly different among different treatments at p < 0.05 level by Duncan's new multiple range test. Note