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Article

Effect of Vegetable Oil Adjuvant on Wetting, Drift, and Deposition of Pesticide Droplets from UAV Sprayers on Litchi Leaves

by
Bingjie Wang
1,
Ziqiong Geng
1,2,
Bo Pan
1,
Lei Jiang
1,* and
Yong Lin
1
1
Key Laboratory of Integrated Pest Management of Tropical Crops, Ministry of Agriculture and Rural Affairs, Environment and Plant Protection Institute, Chinese Academy of Tropical Agricultural Sciences, Haikou 571101, China
2
College of Plant Science and Technology, Huazhong Agricultural University, Wuhan 430070, China
*
Author to whom correspondence should be addressed.
Agronomy 2025, 15(2), 293; https://doi.org/10.3390/agronomy15020293
Submission received: 20 December 2024 / Revised: 23 January 2025 / Accepted: 23 January 2025 / Published: 24 January 2025
(This article belongs to the Section Precision and Digital Agriculture)
Browse Figures
Versions Notes

Abstract

:
The spatial transportation of pesticide spray droplets and their deposition and retention on plant leaf surfaces are critical factors contributing to pesticide loss. Adding adjuvants to pesticide solutions to improve their wettability and deposition behavior can enhance the targeted deposition efficiency of pesticides sprayed by unmanned aerial vehicle (UAV) sprayers. In this study, Maifei (MF), a prevalent vegetable oil adjuvant, was selected to analyze its effects on the physicochemical properties of water and 10% difenoconazole water-dispersible granules (D) and the wetting performance of droplets on litchi leaves. The changes in the drift and deposition of the spray solutions with or without MF were tested using a UAV sprayer, DJI T40. The results indicated that the addition of MF to water or D significantly decreased the surface tension (by 58.33% and 23.10%, respectively), wetting time (by 97.81% and 90.95%, respectively), and contact angle (by 40.95% to 70.75% for the adaxial and abaxial surfaces of litchi leaves), achieving the best effects at a 1% MF addition. Moreover, during the drift test, the addition of 1% MF to the solutions significantly reduced the cumulative drift rate (CDR) (by 48.10%). Finally, owing to the weakened spray drift risk and improved wettability of the droplets on litchi leaves with a 1% MF addition, the droplet deposition and penetration in the litchi canopy significantly improved, demonstrating an increased droplet density of 38.17% for the middle layers of the litchi and 15.75% for the lower layers, corresponding to increased coverage by 59.49% and 12.78%, respectively. Hence, MF can improve the interfacial properties of the spray solution on litchi leaves, reduce the drift risk, and promote deposition, thereby facilitating the efficient transfer and deposition of pesticide droplets from UAV sprayers.

1. Introduction

Litchi (Litchi chinensis Sonn.) is a subtropical fruit that is mainly planted in Asian countries, especially southern China [1]. The cultivated area of litchi in China accounts for 80% of the total global area [2]. Its attractive red skin, unique taste, and high nutritional value makes it extremely popular among consumers [3]. With its unique competitive advantages of early listing and high prices, the litchi industry has become an important landmark in tropical high-efficiency agriculture in Hainan, China. However, owing to the high temperature and humidity in tropical areas, there are serious occurrences of pests and diseases in litchi, which seriously affect its quality and yield. The average annual pesticide use is also higher than that in temperate areas [4]. Furthermore, traditional ground pesticide application methods, including manual knapsack sprayers and ground mechanical sprayers, encounter serious challenges that are regarded as inefficient, labor-intensive, and serious pesticide waste [5]. Moreover, spray operators are at a higher risk of pesticide exposure [6].
Using unmanned aerial vehicles (UAVs) as the application carrier achieves the efficiently targeted spraying of different kinds of crops, which is a key breakthrough in solving the problems of the unified prevention and control of crop pests and diseases in tropical regions, the reduction and efficiency improvement of pesticide use, and labor shortages [7]. The spraying of pesticides by UAVs belongs to the category of high concentration and low volume, which is significantly different from traditional large-volume and low-concentration spraying and puts forward higher requirements for the performance of spraying agents [8,9]. In particular, the canopies of litchi trees are large and thick, and the plants are tall, which increases the difficulty of the effective target sedimentation of pesticide droplets and the risk of drift during pesticide application by plant protection UAVs. In order to alleviate this problem, spray adjuvants have been added to conventional formulations to obtain ideal deposition and control effects [10,11].
Reducing the airborne drift of droplets and promoting the attachment and diffusion of droplets on the surface of target leaves are the main ways to promote the effective deposition of UAV spray droplets [12]. Spray adjuvants can improve the physicochemical properties of the liquid, reduce its surface tension and contact angle on crop leaves, and improve the wetting, adhesion, and deposition of the liquid on crop leaves, thereby increasing the pesticide utilization rate [13]. In China, numerous materials are recommended as “additives” for pesticide sprays to enhance their performance. These include surfactants, oils, organosilicons, polymers, and other macromolecules [14]. Meng et al. [15] discovered that the tank-mix adjuvant (NongJianFei) can reduce the contact angle and increase the droplet coverage on the citrus canopy. Wang et al. [16] found that when adjuvants are added during aerial spraying, both the cumulative drift rate (CDR) and the drift distance can be decreased. Zeeshan et al. [17] also reported that adding adjuvants when drones sprayed the acetamidine solutions could increase the droplet density and coverage in the cotton canopy.
The physicochemical properties of the liquid and the deposition characteristics of the droplets from UAV sprayers are crucial for improving the utilization rate of pesticides. Anthracnose is one of the most serious diseases on litchi trees [18], and so far, only 10% difenoconazole water-dispersible granules (WGs) are registered to control litchi anthracnose in China. Vegetable oils are commonly used as spray adjuvants in agriculture [19]. Vegetable oil adjuvants have a good affinity for plants and are characterized by biodegradability and low phytotoxicity [20,21]. Maifei is a commercial aerial-spraying adjuvant that enjoys a high level of popularity and wide application in China. There is a saying that “seven out of ten aerial-spraying adjuvants are Maifei”. Maifei belongs to the vegetable oil—based spray adjuvant category and is composed of vegetable oil or modified vegetable oil, an emulsifier, a wetting agent, a binder, and a pH regulator. It is suitable for aerial plant protection spraying or low-volume spraying. Therefore, 10% difenoconazole water-dispersible granules (WGs) and the vegetable oil adjuvant Maifei were used to study the effect of the vegetable oil adjuvant on the deposition of pesticide droplets on litchi, with the aim of improving the efficient deposition of UAV spray droplets on litchi trees, increasing pesticide utilization, reducing environmental pollution, and promoting the development of UAVs for plant protection in the prevention and treatment of fruit tree diseases and pests in tropical regions.

2. Materials and Methods

2.1. Materials and Treatments

A vegetable oil-based adjuvant (Maifei, Guangyuanyinong Chemical Co., Beijing, China) (MF) and 10% difenoconazole water dispersible granules (WGs) (Syngenta Crop Protection Co., Basel, Switzerland. Register number: PD20152176) were used. With reference to its recommended dose, 10% difenoconazole WGs were routinely diluted 100 times (1%, v/v) with deionized water (labeled as D). MF was diluted with deionized water and/or water diluent of difenoconazole (D) to obtain a series of concentrations (0.5%, 1.0%, and 2.0% v/v) of MF solution with and/or without 1% (v/v) of difenoconazole WGs (D). That is to say: control (deionized water), 0.5% MF, 1.0% MF, 2.0% MF, 1% D, 1% D + 0.5% MF, 1% D + 1.0% MF, 1% D + 2.0% MF.
Litchi leaves for the experiment were picked from Litchi chinensis “Feizixiao”. We selected the third and fourth leaves from the tops of the litchi trees. The experiments were completed within two hours of the leaves collection.

2.2. Surface Tension

According to Bao et al. [10], the dynamic surface tension measurements were performed using the bubble method (BP100, KRÜSS Co., Hamburg, Germany). The beaker containing the liquid was placed on the sample stage, and the sample position was raised such that the tip of the capillary tube was 1 cm away from the liquid surface. We started testing the change in the dynamic surface tension of the liquid within a time range of 10–10,000 ms.
The equilibrium surface tension was determined by the platinum-plate method with a horizontally placed automatic surface tension meter JK99F (Zhongchen Digital Technology Equipment Co., Shanghai, China). First, the surface tension meter was positioned on a stable workbench and calibrated to achieve balance. Before the test, weights were used to calibrate the surface tension meter, and the platinum plate was washed with water, burned red with an alcohol lamp, cooled naturally, hung on the hook, and adjusted to be level. The platinum plate was then adjusted horizontally. The beaker containing the liquid was placed on the carrier platform for automatic measurements. When the liquid touched the platinum plate, the surface tension was measured, and the measurement was repeated five times.

2.3. Wetting Time

The wetting time was tested using a standard circular canvas piece (p = 35 mm, Shanghai Nuo Cai Trade Co., Ltd., Shanghai, China), following the ISO 8022 [22]. The wetting time was determined by measuring the interval between the moment of immersion of the standard circular canvas piece and the moment when it began to sink, and the average value of the 10 tests was calculated for each sample.

2.4. Contact Angle

According to Bao et al. [10] and Zhang et al. [23], the contact angle measurements were performed using the sessile drop method, with a contact angle instrument KRÜSS DSA100S (KRÜSS Co., Hamburg, Germany), as shown in Figure 1. The litchi leaves were carefully cut into approximately 1.0 × 1.5 cm pieces without veins and attached to the slide with double-sided tape, without disturbing the leaf surface structure. Subsequently, they were placed on the loading platform. The height of the loading platform was adjusted such that litchi leaves appeared in the middle and lower parts of the computer window. We automatically dispensed 2 μL of liquid on the adaxial or abaxial surface of the leaves and captured the wetting morphology of the droplets on the leaves after the drops were stabilized for 30 s. The sampling interval was 1 s, and the sampling duration was 120 s. During the measurement, the ambient temperature was maintained at 25 ± 5 °C, and the relative humidity was 35–45%. The ambient temperature and relative humidity were controlled using an air-conditioning and humidification–dehumidification process, respectively. The obtained photos were processed using the DSA100s-ADVANCE(1.14.1.16701) program software to fit the droplet contour and obtain the contact angle and spreading diameter of the liquid on the leaves. Each treatment was repeated five times.

2.5. Maximum Liquid Retention

Litchi leaves of the same age were clipped and weighed using an electronic analytical balance (W1, g). The leaves were then held vertically using a clamp and immersed in the different treated liquids for 3–5 s. The leaves were quickly pulled out of the liquid, suspended vertically, and allowed to dry until no more droplets dripped. We weighed them again (W2, g) and used Leaf Area Meter AM300 (ADC Bioscientific, Hoddesdon, Hertfordshire, UK) to measure the leaf area (S, cm2). Then, according to Zhang [24], the maximum liquid retention (Rm, mg/cm2) of the leaves was calculated using the following formula: Rm (mg/cm2) = (W2 − W1) × 1000/S.

2.6. Droplet Spread Areas on Litchi Leaves

Maximum wetting area and evaporation time measurements were conducted using an ultra-deep microscope (VHX-5000, KEYENCE, Osaka, Japan). Fresh and healthy litchi leaves with different treated droplets were selected for wetting and evaporation tests. To avoid the main veins of the leaves, the leaves were cut into rectangles measuring 5.0 × 3.0 cm. We placed test droplets (0.3 μL) on both the adaxial and abaxial surfaces of the leaves. An ultra-deep microscope was used to record the entire droplet spreading process. When the droplet on the leaf surface stopped expanding, the maximum wetting area was measured, time was recorded, and photos were taken. In addition, the time at which the droplet completely evaporated was recorded, according to Precipito [25]. During the measurement, the ambient temperature was maintained at 25 ± 5 °C, and the relative humidity was 35–45%.

2.7. Spray Drift

The experiment was conducted at the Liangyuan experimental farm in Dacheng Town, Danzhou City, Hainan Province, following the ISO 22866 [26] and ISO 24253-1 [27] standards using DJI T40 UAV (DJI Agricultural, Shenzhen, China). Two treatments were set up: control and 1% MF (according to the results of laboratory tests comprehensively, the optimal MF addition amount is 1%). In this experiment, 0.1% fluorescein disodium salt (Sigma-Aldrich Trading Co., Ltd., Shanghai, China) was added as the fluorescent tracer. For real-time monitoring and recording of meteorological parameters, including the ambient temperature, humidity, wind direction, and speed, during the experiment, a portable automatic weather station (TH-TQX8Y, Tianhe Environmental Technology Co., Ltd., Weifang, China) was fixed by a tripod to avoid interference from the rotor wind of the UVA, a weather station 4.5 m above the ground vertically, approximately 35 m downwind distance from the operation zone. Once the wind speed was maintained below 3 m/s and the deviation of the constant wind direction remained at no more than ±30° for no less than 2 min, the drift experiment was performed. The UAV flew three flight paths for spraying, with the angle between the flight path direction and the wind direction of (90 + 30)°. The operation parameters were the same as before, with at least three repetitions. The meteorological parameters for the drift test are shown in Table 1.
For each treatment, three droplet sampling zones were arranged perpendicular to the flight paths. The sampling points in the deposition zone were located 0 m, −2.5 m, −5 m, −7.5 m, −10 m, −12.5 m, and −15 m downstream from the edge of the operation area. The sampling points in the drift zone were located 2 m, 4 m, 8 m, 12 m, 16 m, 20 m, 28 m, 36 m, 44 m, and 52 m downstream from the edge of the operation area and were spaced 10 m apart (with a tripod height of 2.5 m for the deposition zone, which is the average height of fruit trees, and a height of 1.6 m for the drift zone). PVC cards were horizontally fixed to a rigid plastic board at each sampling point and placed on a tripod, as shown in Figure 2.
After drift spraying was completed, the PVC cards at each sampling point were collected in self-sealing bags and labeled. The processing procedure and recovery rate analysis of the PVC cards followed the procedures outlined by Chen et al. [28]. Specifically, 20 mL of deionized water was added to the self-sealing bags containing the PVC cards, and the fluorescence value was measured using an enzyme label reader at λex 460 nm and λem 515 nm. Subsequently, based on the measured fluorescence value, the deposition amount in the working and drift areas was calculated according to ISO 22866 [26] and ISO 24253-1 [27]. In addition, the attenuation curve of the spray drift rate (y) along the downwind sampling distance (x) in the spray drift zone was plotted according to ISO 22866 [26]. The CDR and spray drift buffer zone distance (with a 90% CDR) (m) were calculated.

2.8. Spray Deposited on the Litchi Leaves

The agricultural UAV used for the field trials of spray deposition on litchi leaves was an electrically powered quadcopter (T40, DJI Agricultural, Shenzhen, China). DJI T40 UAV was used for application with the following operation parameters: flight height of 3.5 m, flight speed of 3 m/s, droplet size of 120 um, and liquid volume of 60 L/hm2. Two treatments were set up: control and 1% MF.
Sampling method: Three litchi trees of similar size and shape were randomly selected from the middle two flight paths of the UAV (the same three trees were selected for each test) for sampling. The upper, middle, and lower layers of the litchi trees were equipped with droplet test cards, as shown in Figure 3. Nine sampling points were set up in each layer of the litchi canopy, and 27 sampling points were set up for each litchi tree. The meteorological parameters for the spray deposition test are shown in Table 2. After the spray treatment of each test group was completed, droplet test cards were collected. Deposit Scan (1.2) software was used to analyze and process the data. Droplet deposition density and coverage of the droplet test cards were calculated according to the method described by Zhu et al. [29].

2.9. Data Processing

The experimental data were processed using the data processing system (SPSS 20.0), and the significance of differences between treatments was compared using the LSD method.

3. Results

3.1. Physicochemical Properties of Spray Dilutions

3.1.1. The Dynamic Surface Tension

The dynamic surface tension in the same treatment group solution with increasing bubble surface age is shown in Figure 4A. The fitting equations for the dynamic surface tension in the different treatment groups at surface ages of 10–100 ms are shown in Figure 4B. The results showed that the surface tension decreased with increasing surface-aging time. After adding MF to water, the surface tension decreased rapidly with increasing surface aging time (10–100 ms), and the k value increased from 0.010 to 0.065–0.145 (the larger the slope, the faster the surface tension decreased). Subsequently, the decreasing trend in the surface tension with an increase in the bubble surface age tended to be gentle until it was stable. Similarly, after adding MF to D, the surface tension decreased rapidly with increasing bubble surface age (10–100 ms), and the k value increased from 0.026 to approximately 0.147–0.168. With the addition of MF, the rate of decrease in the surface tension was faster than that in water. Subsequently, the decreasing trend of the surface tension with an increase in the bubble surface age tended to be gentle until it was stable.

3.1.2. The Equilibrium Surface Tension

The equilibrium surface tensions of the different treatment groups are shown in Figure 4C. After the addition of MF to control, the equilibrium surface tension reduced the equilibrium surface tension of solutions from 72 mN/m to around 29.69 mN/m (by 58.33%), and the equilibrium surface tension after the addition of 2.0% MF was significantly lower than that after the addition of 0.5% and 1.0% MF. The equilibrium surface tension of the 1.0% D group was 37.83 mN/m significantly lower than that of the control group. After the addition of MF to 1% D, the equilibrium surface tension significantly decreased from 37.83 mN/m to around 29.09 mN/m (by 23.10%). The surface tension of 1.0% D with 1.0% MF was significantly lower than those of 0.5% and 2.0% MF.

3.1.3. The Wetting Time

The wetting time indicates the wettability of the different treatments; the shorter the wetting time, the better the wettability. The wetting time in the control group was >800 s. After adding 0.5%, 1%, and 2% MF, the wetting time of solutions was significantly reduced to 33.56 s, 17.65 s, and 5.47 s, respectively. The wetting time of the 1.0% D group was 216.44 s, significantly shorter than that of the control group. After adding 0.5%, 1%, and 2% MF to 1.0% D, the wetting time was significantly reduced to 25.37 s, 19.57 s, and 2.91 s, respectively. The wetting time of 1.0% D with 2.0% MF was significantly lower than those of 0.5% and 1.0% MF (Figure 4D).

3.2. Effects of MF on the Spreading and Retention of Pesticide Droplets on the Litchi Leaves

3.2.1. The Contact Angles and Spreading Diameter

The changes in the contact angles at different additive concentrations and times on the adaxial and abaxial surfaces of the litchi leaves are shown in Figure 5. The contact angle of water on the adaxial surface of litchi leaves was maintained at 100~106°, and the contact angle on the abaxial surface of litchi leaves was maintained at 135~140°, indicating that the abaxial surface of litchi leaves have a hydrophobic structure and that litchi leaves are difficult to wet. The contact angle of 1% D on the adaxial surface of litchi leaves was maintained at 72~82°, and the contact angle on the abaxial surface of litchi leaves was maintained at 125~135°. After adding MF, the contact angles on both the adaxial (Figure 5A,C) and abaxial (Figure 5B,D) surfaces of the litchi leaves decreased significantly (p < 0.001), and the contact angles continued to decrease over time. The change in the contact angle of the droplets on the abaxial surface of litchi leaves was more significant. After adding MF, the spreading diameter on both the adaxial (Figure 5E) and abaxial (Figure 5F) surfaces of the litchi leaves increased significantly (p < 0.001), and the spreading diameter continued to increase over time. The experimental results indicate that the addition of MF to water and D enhanced the spreading of droplets on litchi leaves.
The contact angle of the droplets on the litchi leaves tended to stabilize at 30 s. The stable contact angles on the adaxial and abaxial surfaces of litchi leaves at different MF concentrations are shown in Figure 6A. After adding MF to the control and 1% D, the stable contact angles on both the adaxial and abaxial surfaces of the litchi leaves decreased significantly (p < 0.001). On the adaxial surface of the litchi leaves, there was no significant difference between the contact angles of 1.0% MF to the control and those of 0.5% MF and 2.0% MF to the control. However, the contact angle of 2.0% MF compared to that of the control was significantly lower than that of 0.5% MF. In addition, there was a significant difference between the contact angles of 1% D + 0.5% MF, 1% D + 1.0% MF, and 1% D + 2.0% MF on the adaxial and abaxial surfaces of the litchi leaves. The experimental results indicated that the solution reached a hydrophilic state after the addition of MF, exhibiting good wettability and enhancing the spreading of droplets on litchi leaves.

3.2.2. The Maximum Liquid Retention

Using the retention amounts of different solutions on litchi leaves as an indicator to evaluate the amount of droplet adhesion on the surface of litchi leaves, the maximum liquid retention on litchi leaves for the different treatments is shown in Figure 6B. The maximum liquid retention on litchi leaves was significantly increased from 0.98 mg/cm2 to around 3.45 mg/cm2 after the addition of MF to the control. The maximum liquid retention with 1.0% and 2.0% MF was significantly higher than that with 0.5% MF, and there was no significant difference in the maximum liquid retention between the 1.0% and 2.0% MF. The maximum liquid retention of 1% D on litchi leaves was significantly higher than that of the control group, and the maximum liquid retention on litchi leaves was significantly increased from 2.98 mg/cm2 to 3.95 mg/cm2 after the addition of MF to 1.0% D (p < 0.001). Similarly, the maximum liquid retentions with 1.0% and 2.0% MF in 1% D were significantly higher than those with 0.5% MF, and there was no significant difference in the maximum liquid retentions between 1.0% and 2.0% MF.

3.2.3. The Maximum Spreading Area and Evaporation Time

The state of the droplet at the time of maximum spread on the litchi leaf surface was considered the maximum spreading state for different treatments. The time required for the complete evaporation of droplets on litchi leaves is shown in Table 3. The results indicated that there was a significant difference in the evaporation time of water on the adaxial and abaxial surfaces of litchi leaves, with the evaporation time on the abaxial surface being approximately twice that on the adaxial surface. On the adaxial surface of the litchi leaves, the maximum spreading area of MF added to water was 3.8 times that of the control group. The evaporation time after the addition of 1.0% MF to water was approximately 1/1.2 of the control group; however, when 0.5% and 2.0% MF were added, there was no significant difference in the evaporation time of the droplets compared with the control. On the abaxial surface of the litchi leaves, the maximum spreading area was 4.8 times that of the control group, and the evaporation time was about 1/2.9 of the control group.
When MF was added to the 1% D group, the maximum spreading area on the adaxial and abaxial surfaces of litchi leaves was approximately 2.0 and 3.4 times that of the 1% D group, respectively. On the adaxial surface of litchi leaves, the evaporation time of the addition of 2.0% MF to 1% D was approximately 1/1.1 of that of the 1% D group. However, after the addition of 0.5% and 1% MF, there was no significant difference in the evaporation time of the droplets compared to 1% D alone. However, after adding the three concentrations of MF, there was no significant difference in the evaporation time of the droplets on the abaxial surface of litchi leaves compared with 1% D alone. These results indicate that the addition of MF promotes the spreading of droplets on litchi leaves. Based on the results, we speculated that the evaporation of the solution with added MF slowed down when the spreading area was the same.

3.3. Effects of MF on Droplet Drift and Deposition

3.3.1. Effects of MF on the Spray Drift

As shown in Figure 7, the attenuation curve of the spray drift rate (βdep%) along the downwind sampling distance in the spray drift zone satisfied the power function y = ax−b. The regression equation correlation coefficients R2 were 0.980 and 0.957, respectively. The downwind drift rates of the two treatment groups gradually decreased with increasing downwind sampling distance, and as the downwind sampling distance increased, the attenuation trend of the droplet drift rate gradually decreased. The cumulative drift rate and buffer zone (90% CDR) of the downwind drift area were calculated and are listed in Table 4. Specifically, the addition of 1% MF to deionized water reduced the CDR from 28.75% to 14.92% (by 48.10%). After adding 1% MF to the spray solution, the farthest drift distance was shortened from 20 m to 12 m, shortening the spray drift buffer zone distance (90% CDR) from 8.05 m to 5.68 m.

3.3.2. Deposition of Spray Droplets on Litchi Canopy

The field test results (Table 5) showed that there were significant differences in droplet density and coverage at different heights within the litchi canopy, with the distribution density and coverage of droplets on the upper layer of litchi being significantly higher than those in the middle and lower layers. The addition of MF significantly increased the distribution density of droplets from 35.87 drops/cm2 and 30.10 drops/cm2 to 49.56 drops/cm2 and 34.86 drops/cm2 in the middle and lower layers of the litchi, and coverage was significantly increased from 3.11% and 2.01% to 4.96% and 2.87%, significantly enhancing spray penetration. There was no significant change in both the deposition density and coverage of the upper layer of litchi. The DV50 was significantly increased from 148.05 to 161.63 in the lower layers of the litchi. There was no significant change in the DV50 of the upper and middle layers of litchi.

4. Discussion

The deposition and retention of pesticide spray droplets on hydrophobic plant leaf surfaces pose a significant challenge in agriculture. According to Law [30], the adaxial and abaxial surfaces of litchi leaves with water contact angles of 100~106° and 135~140°, respectively (Figure 6A), belong to difficult-to-wet leaves, which were rather unfavorable for pesticide droplet deposition. Once pesticide spray droplets arrive at plant leaves, their deposition is mainly influenced by the unmodifiable leaf surface characteristics as well as the physical properties of the spray solution [31]; therefore, improving the wettability of the spray solution would be a wise move. Wetting time indicates the wetting performance of the liquid. The shorter the wetting time, the better the wetting performance [32]. In this study, wetting times were significantly reduced after adding MF to water or 1.0% D (Figure 4D). These findings were consistent with the equilibrium surface tension (Figure 4C). Therefore, we speculated that MF could reduce the contact angle of pesticide droplets on litchi leaves owing to the decrease in surface tension and enhancement of wettability. The dynamic surface tension depending on the rate of migration of pesticide solution molecules from the bulk to the newly formed interface is another vital factor affecting the wettability of pesticide droplets on crop leaves; the faster the migration rate, the lower the dynamic surface tension. Adjuvants can reduce the dynamic surface tension of the pesticide solutions on wheat leaves that are difficult to wet [33]. In this study, we found that the addition of different concentrations of MF to both water and a 100-fold diluted benzoyl methide solution resulted in a rapid decrease in the surface tension with increasing bubble surface age (10–100 ms) (Figure 4B).
The contact angle, as a macroscopic parameter characterizing the gas–liquid–solid interface properties, can characterize not only the wettability of solid surfaces but also their roughness and heterogeneity [23]. The contact angle of a liquid on a solid surface is less than 90°, indicating that the liquid can wet the solid surface; the smaller the contact angle, the better the wettability. Therefore, reducing the contact angle improved the wettability of the liquid, making it easier for the liquid to spread onto the solid surface. Previous studies have shown that after the addition of adjuvants to water, the contact angle on rice leaves decreased from 130° to approximately 20° [23]. Typical tank-mix adjuvant concentrations (NongJianFei) can reduce the contact angle on citrus leaves [15]. Meng et al. [34] studied the effects of four adjuvants (Beidatong, Velezia Pro, Nongjianfei, and Leifeng) on the dynamic contact angle values of droplets on wheat leaf surfaces, and the results showed that all four adjuvant treatments significantly reduced the contact angle of the solution. In this study, the addition of MF significantly reduced the contact angles of spray solutions and increased the spreading diameter on both the adaxial and abaxial surfaces of the litchi leaves. Over time, the contact angles continued to decrease, and the change in contact angles on the abaxial surface of the litchi leaves was more significant (Figure 6A).
The dynamic characteristics of droplet evaporation are closely related to the physical properties of the interface and droplets [35,36]. The addition of spray adjuvants may inhibit or promote evaporation and spreading depending on the properties of the adjuvants. To comprehensively understand the interface characteristics of MF on litchi leaves, we tested the effects of MF on the spreading area and evaporation of droplets on litchi leaves and discovered that adding 0.5%, 1.0%, and 2.0% MF not only promoted the spreading of droplets on litchi leaves but also slowed down their evaporation on the leaves (Table 3). Previous studies reached a similar conclusion that adding Nimbus® adjuvant (0.5%, v/v) increased the wetting surface area of droplets on cucumber and sweet pepper leaves, and the evaporation time was inversely proportional to the wetting area [25]. The alcohol–ester mixture adjuvant Quanrun (QR) has good wetting and anti-evaporation properties on cotton leaves [19]. Gao et al. [37] found that the addition of adjuvants (Silwet L-77, 6501, JFC, and Greenwet 720) improves the diffusion of bifenthrin droplets on the surface of tea leaves. Moreover, on hydrophobic leaf surfaces, reducing the surface tension of the liquid through the addition of adjuvants can increase the spread of droplets on the leaf surface [38,39]. Our research reached the same conclusion that MF reduces surface tension and increases the spreading area of droplets on litchi leaves. Therefore, we speculated that MF promoted the spreading of droplets on litchi leaves owing to the decrease in surface tension.
On difficult-to-wet leaf surfaces, the addition of different adjuvants can significantly enhance the ability of the plant to retain droplets [33,38,40]. The primary purpose of increasing the maximum pesticide retention on plant leaves is to enhance the utilization rate and effectiveness of pesticides and reduce the loss of pesticides. Cao et al. [41] found the addition of Tween 80 increases the adhesion force of droplets on cabbage leaves. Similarly, in the present study, the maximum pesticide retention on litchi leaves significantly increased after the addition of MF (Figure 6B). The addition of adjuvants reduces the dynamic surface tension of the solution, increasing its retention on the leaf [33]. The use of adjuvants before spraying pesticides can disrupt the interfacial barriers on difficult-to-wet surfaces; when the surface tension of the solution and the static contact angle of the droplet are low, the retention of the droplet on the leaf surface is higher [39]. This study showed similar results: the surface tension of the solution and the static contact angle on the litchi leaves were negatively correlated with the maximum pesticide retention on litchi leaves.
The spatial transportation of pesticide droplets after spraying from UAV sprayers and their spreading and retention on plant leaf surfaces are critical factors contributing to pesticide loss. The laboratory test results showed that MF improved the spreading and retention of pesticide spray droplets on litchi leaves, and the best effects were from a 1% MF addition. The influence of MF on the spatial transportation of pesticide droplets from UAV sprayers was measured by a field spray drift test. In the process of pesticide application, the risk of spray drift can be reduced by adding tank-mix adjuvants to alter their physical and chemical properties [42]. Recent reports on the drift and deposition of pesticides applied by UAVs showed that 90% of the cumulative drift occurred within 9–30 m [43]. Wang et al. [16] conducted field experiments to evaluate the effects of the two most commonly used spray additives on the droplet drift of sprayed pesticides by plant protection UAVs and found that, after adding additives, the CDR and drift distance can be reduced. In this study, the addition of 1% MF to deionized water reduced the CDR from 28.75% to 14.92% (by 48.10%). After adding 1% MF to the spray solution, the farthest drift distance was shortened from 20 m to 12 m, shortening the spray drift buffer zone distance (90% CDR) from 8.05 m to 5.68 m (Table 4). This suggests that the addition of 1% MF to deionized water reduces the drift buffer, which can effectively mitigate the losses associated with spray drift.
Pesticides can prevent and control harmful organisms only when they are effectively deposited on plant leaves. In general, the lower the pesticide droplet drift, the more it is deposited on the target plants. To verify this phenomenon, we tested the effect of MF on the deposition of droplets from UAV sprayers on the litchi canopy and demonstrated that after adding MF, the distribution density and coverage of spray droplets on litchi trees increased significantly, effectively improving the deposition of spray droplets on the litchi canopy (Table 5). Some studies have found that the addition of spray adjuvants during drone spraying significantly increased the deposition of pesticides on rice [16,44]. Similarly, the addition of adjuvants increases the deposition coverage and deposition amount of the pesticide solution in the cotton canopy [14]. Meng et al. [15] found that the addition of a tank-mix adjuvant (NongJianFei) significantly increases the droplet coverage on the citrus canopy. Zeeshan et al. [17] found that adding adjuvants to acetamiprid formulations resulted in higher droplet density and coverage in the cotton canopy. Also, our study showed that the DV50 was significantly increased in the lower layers of the litchi after adding MF to water. This result may be attributed to the decreased liquid surface tension caused by the MF addition. Some studies have shown that the smaller the surface tension, the larger the droplet size [45]. The current study demonstrated that MF can not only reduce the drift of pesticide droplets during spatial movement to promote deposition but also enhance the spreading and wetting properties of droplets on litchi leaves, thereby reducing pesticide loss in two critical aspects and increasing pesticide utilization on litchi trees.

5. Conclusions

Currently, the most common types of spray additives used in China are vegetable oils and silicone additives. However, silicone additives have strong permeation effects that may lead to phytotoxicity. Vegetable oil adjuvants, derived from natural plants, are environmentally friendly and exhibit higher safety for perennial woody plants such as litchi trees.
Therefore, for difficult-to-wet litchi leaves, we found that vegetable oil spray adjuvants (MF) can significantly reduce the surface tension of the pesticide solution to improve its wettability on litchi leaves, promote spreading, and reduce evaporation, and according to the laboratory results comprehensively, the optimal MF addition amount is 1%. Additionally, the addition of vegetable oil spray adjuvants can significantly reduce the CDR and drift distance and improve the effective deposition of spray droplets in the litchi canopy when pesticides are applied using UAVs. This reduction in pesticide loss eliminates the need for repeated spraying, lowers operational costs and time, and improves the pesticide utilization rate. Using vegetable oil additives in the aerial spraying of litchi contributes to reducing environmental pollution from chemical pesticides and promotes sustainable agricultural development.
In conclusion, the application of vegetable oil additives in the aerial spraying of litchi not only enhances the pesticide deposition and utilization rate but also ensures the safe growth of litchi trees while mitigating environmental pollution risks and promoting the development of UAVs for plant protection in the prevention and treatment of fruit tree diseases and pests in tropical regions.

Author Contributions

Methodology, Z.G.; Data curation, B.P.; Writing—original draft, B.W.; Writing—review and editing, L.J.; Supervision, Y.L.; Funding acquisition, B.W. and L.J. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by the Hainan Province Natural Science Foundation of China (No. 324MS111), Central Public-interest Scientific Institution Basal Research Fund for Chinese Academy of Tropical Agricultural Sciences (No. 1630042022011), and HNARS-Litchi (HNARS-08).

Data Availability Statement

The authors confirm that the data supporting the findings of this study are available within the article.

Acknowledgments

We wish to thank the editor and the anonymous reviewers for their valuable comments.

Conflicts of Interest

The authors declare no conflicts of interest.

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Figure 1. Schematic diagram of contact angle measurement. Note: The blue represents the liquid droplet, and yellow represents the light source.
Figure 1. Schematic diagram of contact angle measurement. Note: The blue represents the liquid droplet, and yellow represents the light source.
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Figure 2. Layout of drift sampling points for field spray drift test.
Figure 2. Layout of drift sampling points for field spray drift test.
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Figure 3. Sampling schematic diagram of spray deposition test using DJI T40 UAV in litchi orchard.
Figure 3. Sampling schematic diagram of spray deposition test using DJI T40 UAV in litchi orchard.
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Figure 4. The dynamic surface tension (A,B), equilibrium surface tension (C), and wetting time (D) under different treatments. Note: The data in the figure are mean ± SD. Data followed by different lowercase letters are significantly different among different treatments at p < 0.05 level by LSD test.
Figure 4. The dynamic surface tension (A,B), equilibrium surface tension (C), and wetting time (D) under different treatments. Note: The data in the figure are mean ± SD. Data followed by different lowercase letters are significantly different among different treatments at p < 0.05 level by LSD test.
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Figure 5. The dynamic contact angles of different treatment solutions on the adaxial (A,C) and abaxial (B,D) surfaces of litchi leaves and spreading diameter of different treatment solutions on the adaxial (E) and abaxial (F) surfaces of litchi leaves.
Figure 5. The dynamic contact angles of different treatment solutions on the adaxial (A,C) and abaxial (B,D) surfaces of litchi leaves and spreading diameter of different treatment solutions on the adaxial (E) and abaxial (F) surfaces of litchi leaves.
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Figure 6. The stable contact angles of different treatment solutions on the adaxial and abaxial surfaces of litchi leaves at 30 s (A) and the maximum stable retention (B) of different treatment solutions on litchi leaves. Data followed by different letters are significantly different among different treatments at p < 0.05 level by LSD test. * and **, respectively, indicate a significant difference at p < 0.05 and p < 0.01.
Figure 6. The stable contact angles of different treatment solutions on the adaxial and abaxial surfaces of litchi leaves at 30 s (A) and the maximum stable retention (B) of different treatment solutions on litchi leaves. Data followed by different letters are significantly different among different treatments at p < 0.05 level by LSD test. * and **, respectively, indicate a significant difference at p < 0.05 and p < 0.01.
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Figure 7. Effects of MF on the drift of UAV spraying. Values are presented as the mean ± SD (n = 3). At the same downwind distance, data with different letters are significantly different at the p < 0.05 level by LSD test.
Figure 7. Effects of MF on the drift of UAV spraying. Values are presented as the mean ± SD (n = 3). At the same downwind distance, data with different letters are significantly different at the p < 0.05 level by LSD test.
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Table 1. Meteorological parameters for DJI T40 UAV field drift test.
Table 1. Meteorological parameters for DJI T40 UAV field drift test.
TreatmentsHumidity/%Temperature/°CWind Speed/m·s−1Wind Direction/°
Control77.25 ± 0.3727.07 ± 0.141.53 ± 0.6087.05 ± 17.08
1.0% MF70.30 ± 0.2530.18 ± 0.131.15 ± 0.37105.67 ± 6.00
Note: The data in the table are mean ± SD.
Table 2. Meteorological parameters for DJI T40 UAV field deposition test.
Table 2. Meteorological parameters for DJI T40 UAV field deposition test.
TreatmentsHumidity/%Temperature/°CWind Speed/m·s−1Wind Direction/°
Control57.24 ± 0.2930.85 ± 0.811.97 ± 0.4925.50 ± 1.20
1.0% MF54.02 ± 0.8832.19 ± 0.252.03 ± 0.6747.06 ± 3.08
Note: The data in the table are mean ± SD.
Table 3. Maximum spreading area and evaporation time of different treatment solutions on litchi leaves.
Table 3. Maximum spreading area and evaporation time of different treatment solutions on litchi leaves.
TreatmentsAdaxialAbaxial
Maximum Spreading Area (mm2)Evaporation Time (s)Maximum Spreading Area (mm2)Evaporation Time (s)
Control0.57 ± 0.04 d336.33 ± 20.55 a0.43 ± 0.01 d722.67 ± 11.02 a
0.5% MF2.18 ± 0.32 ab302.67 ± 23.12 a1.95 ± 0.44 ab255.67 ± 32.72 de
1.0% MF2.40 ± 0.11 a283.33 ± 8.08 bc2.07 ± 0.01 ab231.33 ± 12.22 e
2.0% MF2.16 ± 0.24 ab295.33 ± 43.32 abc2.19 ± 0.04 a227.00 ± 10.82 e
1.0% D1.06 ± 0.06 c313.00 ± 6.08 a0.46 ± 0.05 d314.33 ± 12.50 b
1.0% D + 0.5% MF1.82 ± 0.14 b322.37 ± 29.94 a1.87 ± 0.10 b299.67 ± 10.79 bc
1.0% D + 1.0% MF2.04 ± 0.05 b296.33 ± 3.22 abc1.65 ± 0.12 bc278.00 ± 11.36 cd
1.0% D + 2.0% MF2.20 ± 0.31 ab280.67 ± 39.31 bc1.85 ± 0.06 b287.00 ± 27.62 bcd
Note: The data in the table are mean ± SD. Data followed by different small letters are significantly different among different treatments at p < 0.05 level by LSD test.
Table 4. CDR and spray drift buffer zone distance.
Table 4. CDR and spray drift buffer zone distance.
TreatmentsCDR (%)Spray Drift Buffer Zone Distance (90% CDR) (m)
Water (Control)28.75 ± 2.34 a8.05 ± 0.70 a
1% MF14.92 ± 2.16 b5.68 ± 0.49 b
Note: The data in the table are mean ± SD. Data followed by different small letters are significantly different among different treatments at p < 0.05 level by LSD test.
Table 5. Droplet deposition density, coverage, and DV50 on litchi in different treatment groups.
Table 5. Droplet deposition density, coverage, and DV50 on litchi in different treatment groups.
TreatmentsDroplet Deposition Density/(Drops/cm2)Droplet Coverage Ratio/%DV50/μm
Upper LayerMiddle LayerLower LayerUpper LayerMiddle LayerLower LayerUpper LayerMiddle LayerLower Layer
Water
(Control)		50.52 ± 0.94 a35.87 ± 4.59 b30.10 ± 1.08 b4.50 ± 0.24 a3.11 ± 0.83 b2.01 ± 0.12 b165.50 ± 5.52 a159.73 ± 10.38 a148.05 ± 5.04 b
1% MF63.24 ± 10.25 a49.56 ± 2.65 a34.86 ± 0.63 a6.38 ± 1.95 a4.96 ± 0.35 a2.87 ± 0.05 a169.13 ± 5.76 a167.36 ± 4.38 a161.63 ± 5.60 a
Note: The data in the table are mean ± SD. Data followed by different small letters are significantly different among different treatments at p < 0.05 level by LSD test.
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MDPI and ACS Style

Wang, B.; Geng, Z.; Pan, B.; Jiang, L.; Lin, Y. Effect of Vegetable Oil Adjuvant on Wetting, Drift, and Deposition of Pesticide Droplets from UAV Sprayers on Litchi Leaves. Agronomy 2025, 15, 293. https://doi.org/10.3390/agronomy15020293

AMA Style

Wang B, Geng Z, Pan B, Jiang L, Lin Y. Effect of Vegetable Oil Adjuvant on Wetting, Drift, and Deposition of Pesticide Droplets from UAV Sprayers on Litchi Leaves. Agronomy. 2025; 15(2):293. https://doi.org/10.3390/agronomy15020293

Chicago/Turabian Style

Wang, Bingjie, Ziqiong Geng, Bo Pan, Lei Jiang, and Yong Lin. 2025. "Effect of Vegetable Oil Adjuvant on Wetting, Drift, and Deposition of Pesticide Droplets from UAV Sprayers on Litchi Leaves" Agronomy 15, no. 2: 293. https://doi.org/10.3390/agronomy15020293

APA Style

Wang, B., Geng, Z., Pan, B., Jiang, L., & Lin, Y. (2025). Effect of Vegetable Oil Adjuvant on Wetting, Drift, and Deposition of Pesticide Droplets from UAV Sprayers on Litchi Leaves. Agronomy, 15(2), 293. https://doi.org/10.3390/agronomy15020293

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