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 hm
2. 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.
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 DV
50 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 DV
50 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.