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Article

Efficacy of Two Tank-Mix Adjuvants to Control Mango Thrips Using a UAV Sprayer

by
Yuan Zhong
1,2,
Ye Jin
1,2,
Shaoqing Xu
1,2,
Xiangrui Liu
3 and
Jianli Song
1,2,4,*
1
College of Science, China Agricultural University, Beijing 100193, China
2
Centre for Chemicals Application Technology, China Agricultural University, Beijing 100193, China
3
EAvision (Hainan) Intelligent Breeding Equipment Co., Ltd., Sanya 572000, China
4
Sanya Institute of China Agricultural University, Sanya 572025, China
*
Author to whom correspondence should be addressed.
Agriculture 2023, 13(9), 1805; https://doi.org/10.3390/agriculture13091805
Submission received: 17 August 2023 / Revised: 10 September 2023 / Accepted: 11 September 2023 / Published: 13 September 2023
(This article belongs to the Section Agricultural Technology)
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Abstract

:
Thrips have become some of the most challenging pests to control in mango production due to their short developmental time, hidden locations and resistance to pesticides, in the tropical regions of China. To improve pesticides efficacy, the tank-mix adjuvants Qi Gong (QG) and the thrips attractant Lv Dian (LD) were added when using an unmanned aerial vehicle (UAV) to control thrips. The surface tension, contact angle on mango leaves, droplet size, spreading rate, and drying time of the two tank-mix adjuvants were determined. The effects of the two tank-mix adjuvants using a UAV sprayer on the droplet coverage and control efficacy against thrips on mango inflorescences were tested through field trials. The results showed that both QG and LD could reduce the surface tension of the liquid and the contact angle on mango leaves and could increase the spreading performance. The droplet coverage in the upper layer of the canopy was about 2% higher than that in the lower layer, and the coverage at the top of the panicle was 5% higher than that at the bottom. QG improved the deposition coverage of mango inflorescences by about 31.5%. The addition of QG increased the efficacy by 18.24% and 8.03%, respectively, at florescence and the young fruit stage. The addition of the LD increased the efficacy by 24.56% and 14.38%, respectively, at florescence and the young fruit stage. These test results can provide a scientific basis for the control of mango thrips with UAVs.

1. Introduction

Mango is the world’s fifth-most-produced fruit after citrus, bananas, grapes, and apples. China is the world’s second-largest producer of mango; the planting areas are mainly concentrated in Hainan, Guangxi, Sichuan, Yunnan, and other regions with superior natural conditions, and the planting area is greater than 300,000 hm2. The annual output is nearly 4 million tons [1], and this is the pillar industry for fruit farmers in tropical areas to increase their income [2]. Pest prevention and control are the parts of the mango production and management process that involve the greatest investment. Mango pests mainly include thrips, gall midges, red spiders, scale insects, and aphids; among these, thrips have the most frequent occurrence and cause the most serious damage, and their population is constantly increasing [3,4,5]. The main species of thrips that affect mango in China are Thrips hawaiiensis, Frankliniella intonsa, and Scirtothrips dorsalis [6]. Thrips can infest mango at the shoot, flower, and fruiting stages. Fruit infestation by thrips results in the formation of brown spots on the fruit skin, which reduce the commercial value of mangos and cause yield losses of up to 42% in severely affected orchards [7]. Thrips in the tropical regions of China have a developmental history of only 10–20 d, have a significant overlap of generations [8], are small and undetectable, and are hidden in the abaxial surfaces of the leaves, buds, petals, and calyxes [9,10], which makes it difficult for them to be affected by pesticides. Due to behavioral resistance, the reduction of osmosis, metabolic resistance related to various enzymes, target resistance related to the targets of insecticide action (such as acetylcholinesterase, the sodium ion channel, and acetylcholinesterase receptors) caused by heavy pesticide application, the resistance of thrips is gradually increasing, and the efficacy of control is gradually decreasing [11]. This seriously restricts the development of the mango industry [9,12].
In China, the control of mango thrips is dominated by the traditional application method of using a handheld spray gun. Due to the large number of thrips in the mango florescence and young fruit stages, pesticides are usually applied once every two days, thus consuming large amounts of chemical pesticides with a high frequency and low efficiency. At the same time, Chinese mangoes are mainly planted on hills and mountains, which makes it difficult to apply pesticides manually. In recent years, unmanned aerial vehicle (UAV) sprayers and low-altitude and low-volume aerial plant protection technologies have rapidly developed in China [13,14]; these have high efficiency, relative safety, good maneuverability, and other characteristics that can circumvent the shortcomings of manual application and ground-based machine spraying [15,16,17]. UAVs have been applied to many types of fruit trees, such as apples and oranges, and the scale of their application to mango trees is also gradually expanding [18,19,20,21,22]. Li et al. found that a UAV had better droplet deposition coverage in mango canopies than that of ground-based implements, especially for control on the abaxial surface of the leaves [23]. However, in actual production, thrips are prone to outbreaks during the flowering stage of mango. At this time, thrips have a high density and hide in the mango inflorescence, so the amount of pesticides deposited on the target site during application by plant protection drones is low, resulting in unsatisfactory control efficacy.
To improve the efficacy of the control of mango thrips, the following two aspects can be considered when applying pesticides: Firstly, more pesticides should be deposited at thrips’ hiding locations; secondly, the thrips should be lured out from their hiding locations for a greater probability of pesticide application. Tank-mix adjuvants are often used to improve the deposition and attachment of pesticides on plants’ surfaces, enhance the spread and penetration of the pesticides, and increase the efficacy. Meng et al. found that when a UAV sprayed citrus trees, the addition of a tank-mix adjuvant was able to increase the coverage of the droplets on the citrus leaves and improve the droplet penetration [24]. Fang et al. found that the addition of tank-mix adjuvants in a spraying operation using a UAV was able to increase the coverage of droplet deposition and improve the efficacy of the control of cotton thrips [25]. An attractant is a kind of green pest-control product that is developed based on the preferred food source of phytophagous pests or the volatiles of their host plants. Many studies have shown that benzene compounds and floral substances have an obvious attraction or synergistic effects on thrips and other pests [26,27,28]. Attractants are usually used in combination with trapping devices to monitor and trap thrips. Abdullah et al. showed that verbenone (Verbenone) can be used together with blue boards to improve the trapping effect [29]. Liu et al. combined the use of food attractants with Beauveria bassiana to establish a trapping–infection–spread system for thrips. Food attractants increase the attraction of thrips to fungal inoculation devices and facilitate the automated spread of fungal diseases among thrip populations [23]. There have been many studies on the extraction and trapping of food attractants but fewer studies on their efficacy against thrips in combination with chemical pesticides.
Qi Gong (QG) is a spreading adjuvant that can enhance the spread of the pesticides on the plant, so that more pesticides are deposited at thrips’ hiding locations. Lv Dian (LD) is a thrip feeding attractant, able to lure thrips out from their hiding locations. Therefore, in this study, QG and LD were added to pesticides for the control of mango thrips using a UAV sprayer to study the efficacy of tank-mix adjuvants.

2. Materials and Methods

2.1. Tank-Mix Adjuvant Performance Test

Two tank-mix adjuvants, QG and LD, were used in the test; specific information is shown in Table 1. Information on the tested adjuvants. Deionized aqueous solutions of the two adjuvants were prepared in the optimal concentrations according to the manufacturers’ instructions. Tank-mix adjuvants can change the wetting and spreading properties of a solution, so the dynamic contact angle, dynamic surface tension, spreading rate, drying time, and droplet size of the two solutions were tested to characterize the solutions’ performance. The tests were carried out at a temperature of 20 ± 0.2 °C and a relative humidity of 35% under ambient indoor conditions.

2.1.1. Determination of the Dynamic Contact Angles

The dynamic contact angles of the QG and LD solutions on mango leaves were determined with an Attension Theta instrument (Biolin Scientific, Stockholm, Sweden), and deionized water was used as a blank control. The sessile drop method was selected for the test, with the volume of liquid discharged from the syringe each time being set at 5 μL, and the dynamic contact angle of the droplets was determined within 10 s.

2.1.2. Determination of the Surface Tension

The dynamic surface tension of the QG and LD solutions was measured with a BPA-2P instrument (Sinterface Inc., Washington Crossing, PA, USA), and deionized water was used as a blank control. The maximum bubble pressure difference method was used to measure the dynamic surface tension [30], the maximum bubble lifetime was set to 20 s, and the duration of the experiment was set to 20 min.

2.1.3. Determination of the Spreading Rate and Drying Time

Ponceau 4R (Shanghai Dye Research Institute, Shanghai, China ) with a concentration of 5 g/L was added to the QG and LD solution, and 5 μL of the mixed solution was dropped onto the surface of a PVC card; then, the time from dropping to complete drying was recorded. After the droplets were completely dry, the PVC card was scanned, and the Deposit Scan software was utilized to calculate the area of the droplet. The spread rate was defined as the ratio of the spread area of the additive solution to the spread area of the deionized water.

2.1.4. Determination of the Droplet Size

The QG and LD solutions and water were used as spraying liquids, and atomization was carried out with the centrifugal nozzle (CCMS-L20000) of an EA-30X plant protection UAV (Eavision Technologies Co., Ltd., Suzhou, China). The spray droplet size was tested with a DP-02 Spray Particle Size Analyzer (Omec, Zhuhai, China). The nozzle was fixed horizontally 1.5 m above the spray particle size analyzer, and the spray pressure was adjusted to the same flow rate as that used during actual fly-control operations in the field. In order to better determine the droplet size and simulate the application of the UAV in the field, a wind field was added above the centrifugal nozzle, and the wind speed reached at the test point was 2.8 m/s.

2.2. Droplet Deposition on Mango Inflorescence and Efficacy of Thrip Control through Application by a UAV in the Field

2.2.1. Experimental Design

Field trials were conducted on 30 January 2022, during the florescence stage of mango, and on 9 February 2023, during the young fruit stage of mango, in Huanhe Village, Tianya District, Sanya City, Hainan Province (109°17′11″ E, 18°20′49″ N). The row spacing of the mango trees in the test plot was 5 m, the plant spacing was 3 m, the plant height was 2.25 m, and the crown diameter was 3.55 m. Three treatment groups were set up for the two tests, and the area of each treatment plot was 40 m × 30 m. The two tests were conducted according to the growth demand of the crops and in combination with the local farmer’s pesticide regimen. The combinations of the pesticides and the dosage for each treatment group are shown in the Table 2 and Table 3. The EA-30X UAV was used to apply the pesticides, and the flight parameters were set according to the crop and plot characteristics: the flight height was 3.5 m, the flight speed was 2 m/s, the spraying volume was 120 L/ha, and the spray swath was 5 m.

2.2.2. Droplet Deposition

On 9 February 2023, when the UAV applied the pesticides, ponceau 4R was added to the pesticides at a concentration of 5 g/L. High-definition photographic paper was used to collect the droplets. Three mango trees in each treatment group with similar growth were randomly selected on which to lay out the photographic paper; the layout of the samples is shown in Figure 1. Since mango trees mainly hang their fruits in the middle and lower parts of the canopy, the canopy was divided into two layers—the upper sample was labeled with U, and the lower sample was labeled with L. The main inflorescences of mango are generally concentrated in the outer part of the canopy, so four inflorescences were evenly selected in the outer part of the canopy in each layer. Mango flowers belong to the panicle, so each inflorescence was chosen so that it would have one branch at the top and four evenly distributed branches at the bottom; the photographic paper was rolled around the branches and fixed with a paper clip. The sample of the top branch in each inflorescence was labeled with 0, and the samples of the bottom branches were, respectively, labeled with 1, 2, 3, and 4.

2.2.3. Control Efficacy

Effectiveness of the Attractant in Trapping Thrips with Blue Sticky Boards

An experiment was carried out on 1 February 2023, during mango florescence. In it, a treatment group and a control group were set up, each plot area was 500 square meters, and a 30 m wide buffer zone was set up between the two plots. Five mango trees with similar growth were evenly selected in each plot, and 20 × 25 cm blue PVC sticky boards (Shandong Minsheng Agricultural Technology Co., Ltd., Heze, China) with a wavelength of 465 nm were hung on the outside of each tree. In the treatment group, 1 mL of 0.200% LD thrip attractant was added to the center of the blue boards, and in the control group, 1 mL of water was added to the center of the blue boards. The number of thrips captured by the blue boards was counted after 4 h.

Efficacy of Thrip Control in the Field

Before 1 and 3 days after the application of the pesticides, three mango trees with similar growth were randomly selected in each treatment group and the control group, and one flower inflorescence was selected from each mango tree in each of the four directions: east, west, south, and north. The number of thrips was counted by using the patting method, in which each flower inflorescence was gently patted by hand five times to cause the thrips to fall onto a white plate, and the number of thrips on the white plate was quickly counted. The rate of population reduction (P, %) and the control efficacy (Ec, %) were calculated according to Equations (1) and (2), respectively.
P / % = N C K − N t N C K × 100
E c / % = P t − P C K 100 − P C K × 100
Where NCK represents the number of live pests before pesticide application; Nt represents the number of live pests after pesticides application; Pt represents the rate of population reduction at the treatment area; and PCK represents the rate of population reduction at the control area.

2.3. Meteorological Conditions

The test temperature, humidity, and wind speed were measured by using a Yigu YG-XB portable weather station. The tests were conducted at 20:00 local time. During the test period on 30 January 2022, the ambient temperature was 24 ± 1.4 °C, the relative humidity was 79 ± 2.6%, and the ambient wind speed was 1.0 ± 0.3 m/s; during the test period on 9 February 2023, the ambient temperature was 27 ± 1.8 °C, the relative humidity was 74 ± 1.9%, and the ambient wind speed was 0.5 ± 0.1 m/s.

2.4. Data Processing and Analysis

The photographic paper was scanned and processed in Deposit Scan, an image-processing-based droplet analysis system, and the droplet coverage was exported by the system. The data were analyzed by using SPSS 20.0. Significant differences between treatments at the level of 0.05 were analyzed by using Tukey’s method and one-way ANOVA.

3. Results

3.1. Tank-Mix Adjuvant Performance Test

3.1.1. Effect on the Dynamic Contact Angle

The contact angle of the solution on the crop surface is one of the most important parameters for measuring wetting performance. The smaller the contact angle, the easier it is for the droplets to spread and adhere to the crop surface. The dynamic contact angles of the QG solution, LD solution, and water on the mango leaves in 10 s are shown in Figure 2. The contact angle of the water was always higher than that of the two kinds of tank-mix adjuvants in 10 s, and it stayed at about 86°. After adding the tank-mix adjuvants, the contact angle at 0 s was significantly reduced, and the contact angle gradually decreased after 0 s. The contact angle stabilized at about 8 s; that of the LD solution stabilized at about 60°, and that of the QG solution stabilized at about 33°. The QG solution had a more significant effect on the reduction in the contact angle. The attractant LD also improved the properties of the solution to a certain degree and reduced the contact angle of the solution.

3.1.2. Effect on the Dynamic Surface Tension

Surface tension is one of the most important physicochemical properties of a solution. The lower the surface tension of the solution, the more easily it is wet and spread on plant leaves. The dynamic surface tensions of the QG solution, LD solution, and water is shown in Figure 3. The surface tension of the water was around 72 mN/m; in comparison with water, both QG and LD were able to reduce the surface tension of the solution. The 0.067% QG solution had a greater effect on the surface tension. The starting surface tension was about 57 mN/m; it decreased and tended to balance in a shorter period of time. It decreased, reached equilibrium in a shorter time, and stabilized at about 33 mN/m, with a decrease of 42.11%. The 0.200% LD solution had a starting surface tension of about 70 mN/m. It took a longer time for it to decrease to the steady state, and it finally stabilized at about 54 mN/m, with a decrease of 22.86%.

3.1.3. Effect on the Spreading Rate and Drying Time

The physicochemical properties of the solution changed, thus affecting its spreading area and drying time. Figure 4 and Table 4 show the results of the experiment with the QG solution, LD solution, and water on a PVC card. After adding the tank-mix adjuvants, the area of spread increased; that of the 0.067% QG solution was the largest, and its spreading rate was 5.76. The spreading rates of the two kinds of tank-mix adjuvants ranked as follows: 0.067% QG > 0.200% LD. The tank-mix adjuvants were able to reduce the contact angle of the solution and the surface tension, improve the physicochemical properties of the solution, and enhance the spreading performance. Solutions with a high spreading rate tend to have a short drying time; the drying times were ranked as follows: 0.067% QG < 0.200% LD. The drying time of 5 μL of deionized water was 44 min, while the drying time of 5 μL of the QG solution was only 15 min; thus, the drying speed was improved by two times.

3.1.4. Effect on Droplet Size

Tank-mix adjuvants can change the physicochemical properties of a liquid, which may result in a change in the droplet size when spraying. The droplet sizes of water, the 0.067% QG solution, and the 0.200% LD solution after atomization by the CCMS-L20000 centrifugal spray nozzle used in the EA-30X UAV are shown in Table 5. The tank-mix adjuvants did not have much of an influence on the droplet size. The RS value is a measure of the uniformity of droplets; the smaller the RS value is, the more uniform the atomization is. The RS values of the 0.200% LD and 0.067% QG solutions were smaller than that of water, which indicated that the addition of these two tank-mix adjuvants was able to make the atomization more uniform to a certain extent; the RS value of the 0.067% QG solution was the smallest.
R S = D V 90 − D V 10 D V 50
Where RS represents relative span; DV90 represents the diameter which 90% of the volume in the spray is less than or equal to; DV50 represents the median for a volume distribution; DV10 represents the diameter which 10% of the volume in the spray is less than or equal to.

3.2. Droplet Deposition on Mango Inflorescence

When no tank-mix adjuvants were added, the coverage of droplet deposition on mango inflorescences after application by the UAV was measured, as shown in Figure 5. The coverage of the upper inflorescences was greater than that of the lower inflorescences at each paper sampling point, and the overall coverage rate of the upper layer was about 2% higher than that of the lower layer. The coverage at position 0 at the top of both the upper and lower inflorescences was significantly higher than that at positions 1, 2, 3, and 4 at the bottom of the inflorescence, and there were no significant differences in the coverage at positions 1, 2, 3, and 4. In the upper part of the canopy, the coverage at the top of the inflorescence was about 6% higher than that at the bottom. In the lower part of the canopy, the coverage at the top of the inflorescence was about 4.5% higher than that at the bottom. When the UAV applied the pesticides, the airflow was from the top to the bottom, the upper canopy intercepted some of the droplets, and the coverage rate of the upper layer was greater than that of the lower layer. The airflow was stronger at the top of the mango panicle, the droplets were more directional, and the droplets were deposited more at the top of the inflorescence, resulting in less coverage being deposited at the bottom of the inflorescence than at the top.
The coverage of mango inflorescences at each paper sampling point after the addition of the two tank-mix adjuvants (QG and LD) by the UAV is shown in Figure 6 and Figure 7. The addition of the QG tank-mix adjuvant increased the coverage of each sample point by 15–51%. The coverage of more than 50% of the sampling points was significantly improved in comparison with that of the control group. According to the 95% confidence interval, the confidence intervals of droplet coverage were all increased after the addition of the QG tank-mix adjuvant, indicating that the coverage could be improved by adding QG to most inflorescences (Table S1). QG is a hyperbranched polyether-modified organosilicon stellar polymer; in the performance test, QG was able to significantly reduce the contact angle and surface tension of the solution, and it had a strong spreading performance. Therefore, the addition of QG was able to increase the spread and attachment of pesticide droplets on the inflorescences, reduce the movement of droplets, such as through shattering, bouncing, and rolling, and effectively increase the coverage when applied by the UAV. The average coverage of the upper and lower layers of the mango canopy increased by about 35% and 28%, respectively, after the addition of the QG tank-mix adjuvant. Without the addition of the tank-mix adjuvant, the coverage of the lower layer accounted for about 45% of the overall coverage of the canopy, and after the addition of QG, the percentage of the coverage of the lower layer changed to 43%. The flow field of the UAV was from top to bottom, so the upper canopy intercepted most of the droplets; the droplets were more likely to attach to the surface of the inflorescences after the addition of QG, and the increase in the coverage of the upper layer was greater than that of the lower layer of the canopy. However, after the addition of the QG, the percentage of droplet coverage of the lower layer only changed from 45% to 43%, and the penetration of the droplets was not reduced. The proportion of the droplets that would have been lost may have been reduced by the addition of the QG, which increased the deposition coverage in the lower layer.
After the LD attractant was added to the pesticides, there were no significant differences in the coverage at each sample site with respect to the control group. The main components of the LD attractant were plant essential oils, plant volatiles, and pheromones, which were mainly used to lure thrips. Although in the performance test, the LD solution showed the property of being able to reduce the contact angle and surface tension of the solution, compared with QG, the improvement in the properties of the LD solution was smaller. Therefore, LD had no effect on the deposition coverage.

3.3. Control Efficacy in the Field

3.3.1. Effectiveness of Trapping Thrips with Drops of Attractant on Blue Sticky Boards

After dropping the LD solution on the blue board, the number of thrips attracted by it was about 43% higher than the number of thrips attracted by the blue board that was dropped with water (Figure 8). This indicated that the LD attractant was able to enhance the efficacy of the blue board in trapping thrips. The LD attractant contained plant extracts, pheromones, etc., which are capable of luring a variety of thrips, and it can be used as a luring agent in combination with sticky blue or yellow boards to attract more thrips and enhance the trapping effect.

3.3.2. Efficacy of the Control of Thrips in the Field

The thrip population base was larger during the florescence of mango. Cowpea, maize, and other crops were at the end of their growth stage during this period, and thrips were constantly migrating from other crops to the mangoes, making thrip control more difficult. The efficacy of the pesticides was only 40.65% without the tank-mix adjuvants during the florescence, and the duration of efficacy was very short—not more than 1 day. After adding the QG tank-mix adjuvant, the surface tension and contact angle of the solution decreased, the coverage on the surface of mango flower inflorescence increased, and the efficacy increased from 40.65% to 58.89%, which was a relative increase of nearly 45%. With the addition of LD, the thrip control efficacy reached 65.21%, which was significantly higher than that of the treatment groups without the tank-mix adjuvant and with only QG, and the control efficacy was relatively increased by about 60% compared with that of the treatment without the tank-mix adjuvant. When sprayed onto the surface of mango plants in a mixture with pesticides, LD, as a thrip attractant, could quickly attract thrips hidden in the inflorescence and other hidden locations to come into contact with the pesticides and, thus, significantly improve the control efficacy. Due to the rapid growth of the number of insects during the florescence, it was basically ineffective 3 days after the application of the pesticides.
At the young fruit stage, the fruit diameter was between 0.3 and 1.2 cm. Although the number of thrips was reduced at this time compared with that during florescence, the thrips hid in the residual inflorescences and could not be easily exposed to pesticides, and the surfaces of the fruits were irreversibly raised after the damage, which seriously affected the quality of the fruits. From the data in the Table 6, it can be seen that after the addition of the QG and LD tank-mix adjuvants, the efficacy increased at 1 d and 3 d after dosing, which was the same trend as that during florescence. At 1 d post-pesticide, compared with the treatment without tank-mix adjuvants, the relative increase in efficacy after adding QG was about 12%, and the relative increase in efficacy after adding LD was about 22%. At 3 d post-pesticide, the efficacy decreased; the efficacy of the QG treatment was no longer significant in comparison with that of the treatment without tank-mix adjuvants, and the efficacy of the LD treatment was still significantly higher than that of the QG treatment and the treatment without tank-mix adjuvants in the three days after the addition of the pesticides.
The tank-mix adjuvants QG and LD at the recommended concentrations increased the efficacy of thrip control in both the florescence and young fruit stages of mango. LD had a better synergistic effect than that of QG, and it had a longer duration of efficacy.

4. Discussion

Tank-mix adjuvants can improve the physicochemical properties of liquids so that the pesticides can be better deposited and attached to plant surfaces. In addition, they can synergize with pesticides to achieve the effects of reducing the quantity and increasing the efficiency [31,32,33]. QG and LD were able to reduce the surface tension and contact angle of liquids, thus increasing the coverage and the efficacy of the pesticides. Compared with that of water, the spreading areas of the QG and LD solutions on PVC cards increased by 5.76 times and 3.24 times, respectively, while the drying times were shortened by 66% and 33%, respectively. For systemic pesticides, after adding spreading tank-mix adjuvants, the spreading area is enlarged to increase the absorption channel, which helps plants absorb the pesticides. At the same time, the drying time becomes shorter, and the quantity of active ingredients of the pesticides that are absorbed by the capillary pores on the surfaces of the leaves is reduced, leading to a decrease in the efficacy of the pesticides. However, for non-systemic pesticides, the addition of spreading tank-mix adjuvants expands the spreading area, which facilitates exposure to the pesticides. The shorter the drying time, the smaller the chance of droplets being blown from the surfaces of the plant leaves, which may increase the efficacy of the pesticides to a certain extent [34]. The insecticides used in this study were mainly poisonous to the touch and to the stomach, so the spreading area became larger and the efficacy increased after the addition of the tank-mix adjuvants.
The test results for the droplet size in this study were inconsistent with the results of previous studies, most of which showed that tank-mix adjuvants changed the droplet size after changing the nature of the liquid [35,36]. Wang et al. investigated the droplet size and drift index of seven different types of tank-mix adjuvants with the LU120-01 nozzle, and the silicone tank-mix adjuvants used in the test were able to increase the droplet size and reduce the drift index [37]. Lin et al. tested the droplet sizes of various tank-mix adjuvants with the hydraulic SX nozzle and IDK nozzle. For the SX110-015 nozzle, the addition of Mai Fei, DS10870, and betatron (three sprayed tank-mix adjuvants), the DV50 significantly increased, and the increases ranged from 5.6% to 14.1%. For the IDK120-015 nozzle, the addition of sprayed tank-mix adjuvants caused all DV50 values to decrease, with decreases ranging from 9.5% to 26.2% [38]. In this study, which used a CCMS-L20000 centrifugal nozzle, QG and LD, two kinds of tank-mix adjuvants, the size of the atomized droplets did not change, which may be because with the CCMS-L20000 centrifugal atomization nozzle, after the atomization, the droplet size was very fine; even if the solution’s surface tension was reduced, the droplet size could not be reduced further. However, after adding the tank-mix adjuvants, the RS value of the droplets was reduced, and the atomization was more uniform. If other types of nozzles are used with the QG and LD tank-mix adjuvants, the droplet size of the solution may also be changed. The droplet size is influenced by multiple factors, such as the properties of the solution, the structure of the nozzle, and the atomization method. For actual production processes in the field, we not only need to understand the impacts of tank-mix adjuvants on the nature of the solutions, but also need to make clear that the choice of the structure of the nozzle and the atomization mode have an impact on the distribution of droplet sizes.
Some studies on insect attractants have shown that attractants can increase the number of trapped pests and can be used to better monitor their population dynamics [26,39]. In this study, it was found that the LD attractant was also able to reduce the surface tension and the contact angle and to improve the properties of the liquid, but it did not increase the coverage of the pesticides on mango inflorescences. Through the blue board lure test, it was found that LD was able to increase the number of insects trapped on the blue boards, indicating that the main way to increase the efficacy of LD was to lure thrips out and expose them to pesticides. In addition, it should be noted when using this attractant that it does not have a luring effect on all thrips, and the population structure of thrips changes during the different growing periods of mango. Blue boards should be used to conduct validation tests before applying this attractant. Currently, there are few studies on the methods of using attractants. LD is able to attract thrips away from their hiding places, so whether spraying LD attractants first and then spraying pesticides after an interval of time can increase the efficacy to a greater extent needs to be followed up with further research.
In this study, when two tank-mix adjuvants were added to the normal dose of pesticides at the florescence and young fruit stages of mango, the effectiveness of thrip control was significantly higher at the young fruit stage than at florescence. This may have been due to the larger base of the thrip population and its rapid growth at florescence, which was during a period of migratory outbursts of thrips. When the thrip population was at a low level, the added tank-mix adjuvants were able to achieve a higher level of efficacy, and it would be possible to consider reducing the amount of pesticides applied. In a study of the control of wheat aphids with a UAV, Meng et al. found that the addition of appropriate tank-mix adjuvants was able to reduce the dosage of imidacloprid by 20% [40]. In the control of cowpea thrips, Wang et al. found that in the case of a 10–30% reduction in the dosage, the addition of tank-mix adjuvants was still able to achieve a higher efficacy. Their research also revealed that the addition of tank-mix adjuvants not only increased the deposition of thiamethoxam on cowpeas, but also facilitated the elimination of pesticide residues [41].
In this study, two different types of tank-mix adjuvants (QG and LD) were added for the determination of their efficacy in the field, and they played a synergistic role through different mechanisms of action. Further studies are necessary to determine whether the two tank-mix adjuvants can play a synergistic role when they are mixed and blended in pesticides simultaneously.

5. Conclusions

Based on the findings and discussion, the following conclusions were drawn.
  • The use of both the adjuvant QG and the thrip attractant LD in a spray was able to reduce the surface tension and contact angle of the liquid and enhance its spreading performance; QG had a more significant effect on the properties of the liquid. After atomization with a CCMS-L20000 centrifugal spray nozzle, the two tank-mix adjuvants had no effect on the droplet size.
  • When a UAV was used to control mango thrips, the coverage at the top of the mango panicle was higher than that at the bottom of the panicle, and the coverage rate in the upper layer of the canopy was greater than that in the lower layer. The addition of the QG tank-mix adjuvant was able to significantly improve the droplet deposition coverage of mango inflorescences.
  • As a thrip attractant that was added dropwise to blue boards, LD enhanced their attractive effect.
  • When a UAV was used at the florescence and young fruit stages of mango, the control efficacy was increased through the addition of the two tank-mix adjuvants (QG and LD), of which LD had the more pronounced synergistic effect.
Therefore, when controlling thrips in the field, we can add spreading adjuvants or attractants to the pesticides as a way to increase the control effect.

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/agriculture13091805/s1, Table S1: Effect of tank-mix adjuvants on the depositional coverage of mango inflorescences.

Author Contributions

Conceptualization, J.S.; Data curation, Y.Z. and S.X.; Funding acquisition, J.S.; Investigation, Y.Z., Y.J. and S.X.; Methodology, J.S.; Project administration, Y.Z. and Y.J.; Resources, J.S.; Software, Y.J. and X.L.; Writing—original draft, Y.Z.; Writing—review & editing, Y.Z., Y.J. and J.S. All authors have read and agreed to the published version of the manuscript.

Funding

This research was supported by the Sanya Yazhou Bay Science and Technology City (Grant No. SYND-2022-23).

Institutional Review Board Statement

Not applicable.

Data Availability Statement

The data that support this study are available from the corresponding author upon reasonable request.

Conflicts of Interest

The authors declare no conflict of interest.

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Agriculture 13 01805 g001
Figure 1. Schematic diagram of how the mango trees were sampled with photographic paper: (a) front view, (b) top view, and (c) physical view of the paper samples of the inflorescence.
Figure 1. Schematic diagram of how the mango trees were sampled with photographic paper: (a) front view, (b) top view, and (c) physical view of the paper samples of the inflorescence.
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Figure 2. Effect on the dynamic contact angle.
Figure 2. Effect on the dynamic contact angle.
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Figure 3. Effect on the dynamic surface tension.
Figure 3. Effect on the dynamic surface tension.
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Figure 4. Spreading on a PVC card.
Figure 4. Spreading on a PVC card.
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Figure 5. Coverage of the droplet deposition at each sampling point for mango without tank-mix adjuvants. Different lowercase letters indicate significant differences that exist at the level of 0.05 for the same paper sampling site according to the Tukey test.
Figure 5. Coverage of the droplet deposition at each sampling point for mango without tank-mix adjuvants. Different lowercase letters indicate significant differences that exist at the level of 0.05 for the same paper sampling site according to the Tukey test.
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Figure 6. Coverage of the upper mango canopy inflorescences.
Figure 6. Coverage of the upper mango canopy inflorescences.
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Figure 7. Coverage of the lower mango canopy inflorescences.
Figure 7. Coverage of the lower mango canopy inflorescences.
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Figure 8. Number of thrips trapped with the blue boards.
Figure 8. Number of thrips trapped with the blue boards.
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Table 1. Information on the tested adjuvants.
Table 1. Information on the tested adjuvants.
Tank-Mix AdjuvantManufacturerTypeBasesFunctionApplication Concentration
QGGuilin Jiqi Biochemical Co.SurfactantHyperbranched polyether-modified silicone star polymersEnhanced spreading, rapid penetration, strong adhesion, and resistance to rainfall0.067%
LDShanghai Agro Agricultural Technology Co.Thrip attractantPlant essential oils, plant volatiles, pheromonesAttracting thrips0.200%
Table 2. Experimental treatments at florescence stage.
Table 2. Experimental treatments at florescence stage.
TreatmentPesticides and AdjuvantDosages/g·hm−2
Treatment 15% Emamectin benzoate·Chlorfenapyr EW400
36% Oligochitosan·Bifenthrin·Clothianidin SC400
Treatment 25% Emamectin benzoate·Chlorfenapyr EW400
36% Oligochitosan·Bifenthrin·Clothianidin SC400
Adjuvant QG80
Treatment 35% Emamectin benzoate·Chlorfenapyr EW400
36% Oligochitosan·Bifenthrin·Clothianidin SC400
Adjuvant LD240
Blank controlWater
Table 3. Experimental treatments at young fruit stage.
Table 3. Experimental treatments at young fruit stage.
TreatmentPesticides and AdjuvantDosages/g·hm−2
Treatment 110% Deltamethrin SC800
5% Emamectin benzoate ME1200
Treatment 210% Deltamethrin SC800
5% Emamectin benzoate ME1200
Adjuvant QG80
Treatment 310% Deltamethrin SC800
5% Emamectin benzoate ME1200
Adjuvant LD240
Blank controlWater
Table 4. Drying time and spreading rate on PVC cards.
Table 4. Drying time and spreading rate on PVC cards.
Water0.067% QG0.200% LD
Drying time (min)43.95 a14.95 c29.42 b
Spreading rate—5.76 b3.24 a
Note: Different lowercase letters indicate significant differences that exist at the level of 0.05 for the same paper sampling site according to the Tukey test.
Table 5. Effect of the tank-mix adjuvants on the droplet particle size.
Table 5. Effect of the tank-mix adjuvants on the droplet particle size.
DV10/μmDV50/μmDV90/μmRS
Water41.14 a66.63 a91.13 a0.75 a
0.067%QG43.13 c66.44 a87.97 c0.67 c
0.200%LD42.56 b66.82 a89.45 b0.70 b
Note: Different lowercase letters indicate significant differences that exist at the level of 0.05 for the same paper sampling site according to the Tukey test.
Table 6. Efficacy of treatments against thrips in the field.
Table 6. Efficacy of treatments against thrips in the field.
TimeTreatmentsPest Population (Head)1 d Post-Pesticide3 d Post-Pesticide
Pest Population (Head)Control Efficacy (%)Pest Population (Head)Control Efficacy (%)
FlorescencePesticide Combination 143156340.65 ± 3.56 f--
Pesticide Combination 1 + 0.067%QG44040358.89 ± 0.72 d--
Pesticide Combination 1 + 0.200%LD44533765.21 ± 1.18 c--
Water418920---
Young Fruit StagePesticide Combination 21268565.29 ± 0.31 c9164.76 ± 0.46 c
Pesticide Combination 2 + 0.067%QG1065573.32 ± 1.51 b6968.13 ± 1.96 c
Pesticide Combination 2 + 0.200%LD903679.67 ± 1.01 a4675.16 ± 1.24 b
Water119232-244-
Note: Pesticide combination 1 was 5% Emamectin benzoate·Chlorfenapyr EW 400 g·hm−2 + 36% Oligochitosan·Bifenthrin·Clothianidin SC 400 g·hm−2; pesticide combination 2 was 10% Deltamethrin SC 800 g·hm−2 + 5% Emamectin benzoate ME 1200 g·hm−2. The number of thrips spiked 3 days after the florescence due to their migration and was not comparable. Different lowercase letters indicate significant differences that exist at the level of 0.05 for the same paper sampling site according to the Tukey test.
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MDPI and ACS Style

Zhong, Y.; Jin, Y.; Xu, S.; Liu, X.; Song, J. Efficacy of Two Tank-Mix Adjuvants to Control Mango Thrips Using a UAV Sprayer. Agriculture 2023, 13, 1805. https://doi.org/10.3390/agriculture13091805

AMA Style

Zhong Y, Jin Y, Xu S, Liu X, Song J. Efficacy of Two Tank-Mix Adjuvants to Control Mango Thrips Using a UAV Sprayer. Agriculture. 2023; 13(9):1805. https://doi.org/10.3390/agriculture13091805

Chicago/Turabian Style

Zhong, Yuan, Ye Jin, Shaoqing Xu, Xiangrui Liu, and Jianli Song. 2023. "Efficacy of Two Tank-Mix Adjuvants to Control Mango Thrips Using a UAV Sprayer" Agriculture 13, no. 9: 1805. https://doi.org/10.3390/agriculture13091805

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