The pepper defoliant (18% glufosinate ammonium soluble concentrate) was produced by Beijing Zhongnong Honglu Technology Development Co., Ltd., Beijing, China. The tested adjuvants were YS-20 (improved vegetable oil adjuvant, Anyang Quanfeng Biotechnology Co., Ltd., Anyang, China), Manniu (vegetable oil adjuvant, Qingdao Rishengyuan Crop Nutrition Co., Ltd., Qingdao, China), Puliwang (vegetable oil adjuvant, Oro Agri. International Co., Ltd., Palmela, Portugal), and AS-910N (improved vegetable oil adjuvant Momentive Performance Materials Inc., New York, USA). Allura Red (85%) was used as a droplet tracer (Zhejiang Gigagold Pigment Technology Co., Ltd., Wenzhou, China) and ethephon aqueous solutions (40%) were used as a ripening agent (Jiangsu Anpon Electrochemical Co., Ltd., Changzhou, China).

The aviation platform used was the T16 UAV (SZ DJI Technology Co., Ltd., Shenzhen, China). The UAV was equipped with RTK/GNSS precise positioning system, and its spraying system included a little water pump, pipeline, nozzles (8 XR11001VS, located directly below the rotor), and electronic control valve. T16 UAV has six rotors with a 16.0 L water tank and a payload of 15 kg. The flight height was 2.0 m and flight speed of 5.0 m/s with a spray width of 5 m and spraying volume of 15.0 L/hm².

There were five treatments in the experiment (Table 1). Treatments 1, 2, 3, and 4 were added with YS-20, Manniu, Puliwang, and AS-910N in the dosage of 225 g/hm². Treatment 5 was the control (CK) without adjuvant. In addition, 1,875 g/hm² of pepper defoliant, 300 g/hm² of Allura red, and 900 g/hm² of 40% ethephon aqueous solution were added to each treatment.

Reducing the surface tension of the pesticide solution can enhance the wetting performance and spreading ability of the spraying solution on the leaves. At 10 s, the surface tension of the YS-20, Manniu, Puliwang, AS910N, and CK was 45.83, 43.47, 42.57, 45.73, and 60.47 mN/m, respectively (Figure 2A). All four kinds of aerial spray adjuvants significantly reduced the surface tension of the pesticide solution to <46 mN/m. Compared with the surface tension of CK, Puliwang had the best effect on reducing the surface tension, which decreased by 29.6%. Liquid viscosity affects the atomization performance of the nozzle and also the number of satellite droplets, coalescence, viscosity dissipation in the collision process, and the spread of droplets on the leaves (Brenn and Kolobatic, 2006). As shown in Figure 2B, Puliwang and AS-910N could increase the viscosity to 1.37 and 1.34 mPa·s, respectively. The Manniu reduced the viscosity, while YS-20 had no effect on viscosity. The increase of viscosity is helpful to the deposition of droplets on leaves and avoids the bounce of droplets (Song et al., 2019). Pepper leaves are hydrophilic leaves. Increasing the viscosity of droplets, therefore, is conducive to the attachment of droplets on the leaves and improves the efficacy.

Adjuvants showed a significant effect on the contact angle of the droplet in the initial state (Figure 3). At 0 s, the contact angle without adjuvant was 83.20°, while that of YS-20, Manniu, Puliwang, and AS-910N were 60.85°, 50.22°, 60.26°, and 47.12°, respectively. The contact angle decreased rapidly within 10 s and gradually leveled off. After 5 s, the rate of contact angle slows down and tends to be stable. This indicated that the adjuvants could spread the spray solution more easily on pepper leaves, which is beneficial for the absorption of the defoliant. The adjuvant affected the contact angle by moderating the surface tension. In general, the contact angle of the leaf surface of the same crop will decrease with a decrease in surface tension (Lan et al., 2021). Xu et al. (2011) found that increasing viscosity and reducing surface tension were two main methods used to increase pesticide retention on superhydrophobic rice leaf surfaces. This was also consistent with our results, where after adding adjuvants the surface tension and contact angle of the droplets displayed the same trend. Our results showed that the surface tension and the contact angle of the pesticide solution on the pepper leaves were reduced, but the effect was different, which was based on the specific adjuvant used.

The spreading of fluids over solid substrates is of great importance to pesticide applications, including defoliants. The wetting and spreading of pesticides on the leaf surface are closely related to the combination of the leaf surface and the physicochemical properties of the pesticide solution. We found that adding aerial applications with adjuvants could increase the spreading ratio of defoliant droplets on the surface of pepper leaves (Figure 4). At 10 s, the spreading ratio of YS-20, Manniu, Puliwang, and AS-910N was 35.35, 34.54, 46.21, and 24.17%, respectively, which was significantly higher than that of CK (10.81%). This result was consistent with the analysis results of the contact angle. Different types of adjuvants can improve various aspects of spray dilution performance. Beacham et al. (2009) found that organosilicon adjuvants have a very prominent effect on improving the wetting of pesticide droplets on the leaf surface. However, when defoliants were used, ethephon, a strong acid ripening agent, needs to be added, which greatly destroys the stability of organosilicon adjuvants. Vegetable oil and modified vegetable oil adjuvants have been popular in recent years because of their wide tolerance. Zhou et al. (2017) found that modified seed oil slows down the evaporation rate of droplets on waxy leaves. Our experiments showed that four vegetable oil adjuvants could also effectively improve the spreading rate of defoliant droplets on pepper leaves and improve the defoliation effect.

Many factors, including adjuvants, pesticide formulations, and nozzles affect spray droplet size (Klein et al., 2009). The droplet size is one of the important indicators to evaluate the spraying quality when using UAVs. As shown in Table 2, the average droplet size of the treatments with adjuvants was significantly larger than that of those without adjuvants. The droplet sizes of the four adjuvants were also different. In the upper layer, the DV₁₀ and DV₅₀ of YS-20, Manniu, and Puliwang were larger than AS910N and CK, the DV₉₀ showed no difference. In the middle and lower layers, the DV₁₀ and DV₅₀ of Puliwang were larger than others, but not significant. On the ground, the DV₁₀ and DV₅₀ of the treatments with or without adjuvants had no difference. Overall, the DV₅₀ in the upper, middle, and lower layers, and ground with Puliwang was 402 ± 22.5, 377 ± 24, 365 ± 27.8, and 355 ± 28.8 μm, respectively, which was higher than other adjuvant augmented treatments. Although there is no specific droplet size range that is likely to drift under all conditions, droplets with diameters <100 μm are considered highly draftable (Nuyttens et al., 2014; Ferguson et al., 2016).

Matthews et al. reported that the optimum droplet size for herbicide spraying is 250 μm, while for fungicide, the particle size should be maintained at 50–150 μm (Matthews and Thomas, 2000). In this study, the droplet size in the treatment after adding Puliwang was between 350 and 450 μm, significantly higher than other treatments; the final defoliation rate was similarly the highest. It demonstrated that adjuvants could change droplet physicochemical properties, such as diameter and relative droplet span, thereby increasing the deposition amount on plant leaves and thus improving the utilization rate of pesticides (He et al., 2018). In addition, when the spray volume and droplet size are the same, the larger the droplet density, the higher the utilization rate of the chemical solution, and the better the control effect (Merritt, 1982). Combined with the results of contact angle and spreading rate, we found that adding spray adjuvants could increase the droplet size of the defoliant and reduce the risk of drift.

The droplet density of defoliants varied greatly among the layers of the pepper canopy (Figure 5). Overall, the droplet density of the upper layer of the pepper canopy was higher than that of other layers, which was due to the interception of the defoliant droplets by the upper layer with a larger leaf area in the later growth stage of pepper. In the upper and middle layers of the canopy, the average droplet densities of adjuvant-added defoliants were 27.31 and 8.11/cm² respectively, which were significantly higher than the CK (21.27 and 5.49/cm²), while in the lower layer and the ground, there was no significant difference. In addition, the droplet density in the upper layer with Manniu was the highest (28.18/cm²) among all the treatments, followed by AS-910N (28.01/cm²) and Puliwang (27.32/cm²), while the YS-20 (25.91/cm²) was significantly lower than others. The droplet density of the defoliant without adjuvants was significantly lower than the aerial application with adjuvants.

Meng et al. (2020) found that adding adjuvants could increase droplet coverage of the canopy, which would increase the probability of the droplets hitting the target, thereby improving its efficacy (Meng et al., 2020). As the droplets are intercepted by the upper layer of the pepper canopy, the coverage rate of the upper layer was significantly higher than other layers (Figures 6, 7). The presence (or absence), and the type of adjuvant caused a significant impact on droplet coverage. The average droplet coverage in the upper, middle, and lower layers, and ground (3.46, 1.26, 0.7, and 0.51%) of the crops treated with added adjuvant was significantly higher than CK (2.56, 0.76, 0.32, and 0.29%). When Puliwang was added, the droplet coverage of the upper, middle, and lower layers, and ground (4.44, 1.84, 1.19, and 0.76%) was significantly higher than other adjuvants. Manniu and YS-20 had the second highest coverage, and AS-910N had the least, with no significant difference from CK. In general, the defoliant droplet coverage rate for the aerial spray with adjuvants was significantly higher than that of the control (Figure 6). Previous research has shown that influenced by the wind field of the UAV rotor, the defoliant droplet coverage rate, particularly the range of spray width, varies considerably (Li et al., 2018). It should be noted that in the upper layer of the pepper crops, the droplet coverage rate was significantly different, and the adjuvant-added treatments were significantly better than the control without any adjuvant. In the middle and lower layers and the ground, there were no such differences. This may be due to the influence of drift and evaporation on the deposition of droplets in the upper layer.

The uniform distribution of droplets is expressed by the coefficient of variation (CV) of the same canopy droplet deposition. The smaller the coefficient of variation, the better the uniformity of droplet distribution (Chen et al., 2021). The field experiments were influenced by environmental conditions and the CV of droplet density and coverage rate were relatively large. The average droplet distribution uniformity of the defoliant treated with Manniu was the best (60.65%), followed by YS-20 and Puliwang (66.14 and 66.47% respectively). The results of AS-910N were the poorest (83.65%), even inferior to the CK (82.26%) (Table 3). However, most of the droplets were deposited in the upper layer due to interception, so the distribution uniformity of the upper layer was more representative. The best uniformity of droplet distribution in the upper layer was Manniu (33.60%).

The uniformity of droplet distribution was measured by the CV of the deposition in the same canopy layer of peppers (Zhan et al., 2022). According to the Chinese Civil Aviation Industry Standard, in the case of low-volume spray operation, the quality of the operation can only be guaranteed when the coefficient of variation of droplet distribution is <60%. The average variation coefficients of the whole plant droplet distribution (66.14, 60.65, 66.47, 83.65, and 82.26%) in the current study seem to be not standard. The planting density of the peppers in the experimental field reached 213,000 plants/hm² with an average height of about 0.88 m at the time of application. The interception effect of the upper layer was obvious, and its droplet variation coefficient (57.21, 33.60, 56.83, 60.97, and 64.79%) was more representative. Adding aerial spray adjuvants can reduce the coefficient of variation of droplet distribution which means improving the uniformity of droplet distribution, to meet the UAV operational standards.

The average deposition of droplets in the upper, middle, and lower layers and ground of the crops treated with adjuvants (1.15, 0.51, 0.25, 0.17 μg/cm²) was significantly higher than the CK (0.81, 0.25, 0.15, 0.07 μg/cm²) (Figure 8). In the upper layer, the droplet deposition amount of the Puliwang was the highest (1.19 μg/cm²). While in the middle layer, the YS-20 was the highest (0.72 μg/cm²). The droplet distribution penetration rates of YS-20, Manniu, Puliwang, and AS-910N were 15.51, 22.59, 27.11, and 20.97%, respectively, which were significantly higher than the CK (15.89%). The Puliwang showed a better effect in terms of penetration.

Some studies have suggested that droplets with smaller particle sizes are difficult to be intercepted by the upper layer and can penetrate better the middle and lower layers (Knoche, 1994; Wolf and Daggupati, 2009). Few other studies found that large particle-size droplets could not drift and evaporate easily and were more likely to reach the lower canopy layer (Derksen et al., 2008). In this study, we observed that the spraying penetration using Puliwang (27.11%) was better; it resulted in a larger droplet size and the DV50 reaching 402, 377, and 365 μm in the upper, middle, and lower layers. Therefore, in our study, penetration was better due to the larger droplet sizes.

The effective deposition rates (23.33, 21.08, 23.13, and 16.37%) of the four treatments with adjuvants were 5–15% higher than the CK (12.07%) with a significant difference (Table 4). In the upper layer of the pepper canopy, the effective deposition rate after adding Puliwang (39.3%) was the highest. In the middle layer, the YS-20 was the highest (23.2%). In the lower layer, there was no significant difference between the effective deposition rates of YS-20 and Puliwang (10.4 and 12.3%, respectively), while being significantly higher than the other treatments.

The effective deposition rate of droplets could also be remarkably improved by adjuvants, because of the larger droplet size, the improved atomization effect, and the reduced evaporation and drift (Lan et al., 2008; Sijs and Bonn, 2020). The results of this study were similar to previous studies. In addition, adding adjuvants during pesticide spraying can change the physicochemical properties, promote the absorption of target plants or insects, and the retention of the liquid (Wang et al., 2022), thereby improving the utilization rate of pesticides.

The addition (and absence) of adjuvants had a significant effect on the defoliation rate of pepper (Table 5). Leaf abscission began to form three days after the first spraying and the aerial spray with adjuvants had a considerable effect on the defoliation effect. Three days after spraying, the defoliation rate of crops sprayed with adjuvants was higher than that of CK, and Puliwang showed the best defoliation effect (64.04%). Between 6 and 12 days after spraying, Puliwang still showed the best defoliation rate, but there was no significant difference among the four adjuvants. Fifteen days after spraying, the defoliation rate of Puliwang treatment was 98.40%, and that of YS-20 and Manniu was more than 95%. However, the defoliation rate of AS-910N was only 89.07% and that of CK was only 79.92%. The above results showed that the addition of adjuvants to aerial applications could significantly improve the defoliation rate of pepper, and the results obtained with the use of Puliwang were the best.

The effects of adding adjuvants to aerial applications on the yield of pepper are shown in Table 6. The average yield of pepper using adjuvants (518.00 kg/666.7 m²) was significantly higher than that of the CK (446.85 kg/666.7 m²). The yield of adding Puliwang (540.19 kg/666.7 m²) was slightly lower than Manniu (543.41 kg/666.7 m²), but significantly higher than other treatments. Puliwang and Manniu could significantly improve the yield of peppers; their yield increase rate exceeded 20%.

To sum up, the aerial applications using adjuvants had varying degrees of effects on the physicochemical properties, droplet deposition, and defoliation rate of the pesticide solution (Table 7). The performance of adjuvants could be evaluated based on these effects as indicators. Pearson correlation analysis was used to study the relationship between these indicators (Figure 9). The results showed that surface tension was significantly positively correlated with contact angle (r = 0.987, p < 0.01), and significantly negatively correlated with spreading ratio and defoliation rate (r = −0.883 and −0.937, p < 0.05). This indicates that the addition of adjuvants could effectively reduce the surface tension, thereby promoting the spreading of the droplets. In addition, the spreading rate was significantly positively correlated with droplet coverage (r = 0.989, p < 0.01), deposition rate (r = 0.992, p < 0.05), and defoliation rate (r = 0.980, p < 0.05). The droplet coverage was also significantly positively correlated with deposition rate (r = 0.966, p < 0.01) and defoliation rate (r = 0.946, p < 0.05). It could be seen that the adjuvants improved the spreading ratio of the droplets, the coverage rate, and the deposition rate, so that the contact between the defoliant and the pepper leaves was increased, which finally enhanced the defoliation effect. Furthermore, although not significant, there was a positive correlation between droplet size and droplet distribution penetration (r = 0.494), which supports the previous observation. In addition, the dynamic viscosity had a certain effect on the droplet size (r = −0.335). Specifically, the higher the viscosity, the smaller the droplets produced by the UAV spray, which was consistent with previous studies (Jamalabadi et al., 2017).

Through the correlation analysis, we found that the correlation between the indicators caused obstacles to the comprehensive evaluation of different adjuvants. Therefore, principal component analysis (PCA) was used to comprehensively evaluate the adjuvants. We selected two principal components whose cumulative contribution rate of eigenvalue reached 85.16% (Figure 10A). The variance contribution rates of principal components 1 and 2 were 66.97 and 18.19% respectively, indicating that it could effectively reflect the original data in the auxiliary indicators. The loading plot for principal components was used to measure the contributions of the principal components. Specifically, a larger absolute value of the load means that the contribution of the corresponding principal component is larger (Karytsas and Choropanitis, 2017). Principal component 1 had a large to small load in terms of spreading ratio, contact angle, droplet coverage, surface tension, deposition rate, droplet density, penetration, and droplet size (Figure 10B). Principal component 2 had a large load in terms of uniformity and dynamic viscosity (Figure 10B). These results showed that except for uniformity and dynamic viscosity, other indicators could reflect the performance of the aerial application of adjuvants to a large extent, especially spreading ratio, contact angle, droplet coverage, surface tension, deposition rate, and droplet density.

Based on the mathematical model of PCA, we found that the comprehensive score for evaluating the performance of adjuvants (Table 8). Puliwang had the highest comprehensive score among the four adjuvants, followed by AS-910N, YS-20, and Manniu. The score of CK without additives was only 1.1806, far lower than the other four treatments. Therefore, it can be established that Puliwang had the best performance.