Application of Ag-SiO2 Nanocomposites in the Control of Citrus Pests and Diseases

Abstract

The present invention belongs to the technical field of the application of nanomaterials, and particularly relates to the application of Ag-SiO2 nanocomposites in the prevention and control of citrus pests and diseases. The technical problem to be solved by the present invention is to provide a new option for the prevention and control of citrus pests and diseases. In order to solve the technical problem, the present invention optimizes the synthesis method of Ag-SiO2 nanocomposites; and through experiments, it is verified that the Ag-SiO2 nanocomposites have high toxicity to Diaphorina citri and Xanthomonas citri subsp. citri, can be used as an effective insecticide for Diaphorina citri, and can also be used as a new type of fungicide for the prevention and control of citrus canker, providing a new option for the prevention and control of citrus pests and diseases.

Description

Technical Field

[0001] The present invention belongs to the technical field of the application of nanomaterials, and particularly relates to the application of Ag-SiO2 nanocomposites in the prevention and control of citrus diseases and pests. Background Art

[0002] As the world's largest category of fruits, citrus has a wide cultivation area and a high economic status in southern China. However, the frequent occurrence of its diseases and pests seriously endangers the growth of citrus, resulting in a decline in fruit quality and yield. Asian citrus psyllid, citrus leafminer, citrus fruit fly, huanglongbing, citrus canker, anthracnose, sooty mold, etc. are common diseases and pests in citrus production, occurring frequently. The Asian citrus psyllid Diaphorina citi belongs to the Hemiptera order, Liviidae family, and is the main pest during the new shoot stage of citrus in China. Adults and nymphs gather on new shoots, young leaves and tender shoots to grow, reproduce and suck, causing the young buds of the plant to wither, deform and twist. Moreover, the white sticky honeydew excreted by nymphs can induce sooty mold on branches and leaves, affecting the photosynthesis of plants. The Asian citrus psyllid is also an important vector insect of the huanglongbing pathogen. After citrus plants are infected with huanglongbing, they show symptoms such as withering of young branches and shoots, slow growth, mottled yellowing of leaves, deterioration of fruit quality and even fruit drop. Since huanglongbing cannot be cured after infection, preventing and controlling the Asian citrus psyllid is the key measure to block the spread of huanglongbing in the field.

[0003] Citrus canker is a bacterial quarantine disease caused by Xanthomonas citrisubsp. citri (Xcc). It mainly damages the leaves, branches and fruits during the new shoot emergence period and young fruit period of citrus. Corky raised ulcer necrosis spots are produced at the diseased parts. After being infected, the quality of citrus fruits is damaged with disease spots at least; at worst, fruit cracking and fruit drop occur, seriously affecting the economic benefits in citrus production. This disease spreads rapidly and is difficult to control. Currently, the prevention and control of citrus diseases and pests in production mostly rely on chemical agents, but the abuse of chemical pesticides is likely to cause environmental pollution. At the same time, due to the long-term single use of drugs in many areas, the Asian citrus psyllid has developed significant resistance to organophosphates, pyrethroids, neonicotinoids, carbamates and other agents. Inorganic fungicides such as copper agents and agricultural antibiotics used to control citrus canker can also cause phytotoxicity to crops. Therefore, studying new strategies for the prevention and control of citrus diseases and pests and developing and applying new green agents have become new development directions.

[0004] Silver nanoparticles (AgNPs) are one of the most promising nanoparticles in the fields of biomedicine, agricultural research, etc. (Molnar and Bodai et al., 2018). AgNPs have a high specific surface area, porous structure, and strong catalytic and bactericidal properties. The particle size of AgNPs is less than 100 nm, generally between 25 and 50 nm. Within a certain range, the smaller the particle size of AgNPs and the higher the valence state, the stronger the bactericidal property. Summary of the Invention

[0005] The technical problem to be solved by the present invention is to provide a new option for the prevention and control of citrus pests and diseases.

[0006] The technical solution of the present invention is the application of Ag-SiO2 nanocomposite in the prevention and control of citrus pests and diseases.

[0007] Further, the citrus pests and diseases are citrus psyllids or citrus canker.

[0008] Specifically, the citrus psyllids are adults, older nymphs or younger nymphs.

[0009] The present invention also provides a method for controlling citrus psyllids, which includes the following steps: spraying Ag-SiO2 nanocomposite solution on citrus psyllids.

[0010] Especially, the citrus psyllids are adults, older nymphs or younger nymphs.

[0011] Specifically, the concentration of the Ag-SiO2 nanocomposite is 1 - 16 mg / mL.

[0012] According to the 95% confidence interval of the concentration causing 50% adult death after 48 h of treatment, when the citrus psyllids are adults, the preferred concentration of the Ag-SiO2 nanocomposite is 5.9 - 15.3 mg / mL.

[0013] According to the 95% confidence interval of the concentration causing 75% older nymph death after 48 h of treatment, when the citrus psyllids are older nymphs, the preferred concentration of the Ag-SiO2 nanocomposite is 4.8 - 12.9 mg / mL.

[0014] According to the 95% confidence interval of the concentration causing 75% younger nymph death after 48 h of treatment, when the citrus psyllids are younger nymphs, the preferred concentration of the Ag-SiO2 nanocomposite is 2.8 - 5.3 mg / mL.

[0015] The present invention also provides a method for controlling citrus canker, which includes the following steps: using the above product for controlling citrus canker to treat citrus canker bacteria.

[0016] Specifically, the treatment is to spray citrus trees with an Ag-SiO2 nanocomposite solution.

[0017] Specifically, the concentration of the Ag-SiO2 nanocomposite is 100-1600 μg / mL.

[0018] Preferably, the concentration of the Ag-SiO2 nanocomposite is 800-1600 μg / mL.

[0019] The present invention also provides a product for controlling Diaphorina citri, comprising an Ag-SiO2 nanocomposite.

[0020] Specifically, the concentration of the Ag-SiO2 nanocomposite is 1-16 mg / mL.

[0021] According to the 95% confidence interval of the concentration causing 50% adult death after 48 h of treatment, when the Diaphorina citri is an adult, the preferred concentration of the Ag-SiO2 nanocomposite is 5.9-15.3 mg / mL.

[0022] According to the 95% confidence interval of the concentration causing 75% death of older nymphs after 48 h of treatment, when the Diaphorina citri is an older nymph, the preferred concentration of the Ag-SiO2 nanocomposite is 4.8-12.9 mg / mL.

[0023] According to the 95% confidence interval of the concentration causing 75% death of younger nymphs after 48 h of treatment, when the Diaphorina citri is a younger nymph, the preferred concentration of the Ag-SiO2 nanocomposite is 2.8-5.3 mg / mL.

[0024] The present invention also provides a product for controlling citrus canker, comprising an Ag-SiO2 nanocomposite.

[0025] Specifically, the concentration of the Ag-SiO2 nanocomposite is 100-1600 μg / mL.

[0026] Preferably, the concentration of the Ag-SiO2 nanocomposite is 800-1600 μg / mL.

[0027] The present invention also provides a preparation method of the Ag-SiO2 nanocomposite, comprising the following steps:

[0028] a. Add NH·H2O and H2O to absolute ethanol under stirring to make the pH of the solution 9-12; the main function of NH·H2O and H2O is to adjust the pH;

[0029] b. Dropwise add tetraethyl orthosilicate under stirring and continue stirring for 6 h; the mass ratio of tetraethyl orthosilicate to the solution is 1:20-30;

[0030] c. While stirring, dropwise add a mixed solution of absolute ethanol and propyltrimethoxysilane, and continue stirring for 12 h; centrifuge, and wash the precipitate with alcohol and water successively to obtain SiO2 powder; the mass ratio of absolute ethanol to propyltrimethoxysilane is 5:1; the function of propyltrimethoxysilane is to modify the mercapto groups on the surface of SiO2 submicron spheres, and only a trace amount needs to be added;

[0031] d. Mix the prepared SiO2 powder with H2O and disperse it by ultrasonic treatment; then dropwise add silver nitrate solution, mix well, and stir at 80 °C for 3 h. The color of the solution gradually changes from white to brown; centrifuge; after washing with water, store it in water for later use.

[0032] Preferably, in step a, the pH of the solution is 11.

[0033] Among them, in step d, the dosage ratio of SiO2 powder to silver nitrate solution is: for every 0.5 g of SiO2 powder, 10 mL of 0.1 mol / L silver nitrate solution is added dropwise.

[0034] Specifically, in the above method, centrifugation is carried out at 12000 rpm for 5 min.

[0035] The present invention also provides the Ag-SiO2 nanocomposite material obtained by the above preparation method.

[0036] Advantages of the present invention: The present invention explores a new way for the prevention and control of citrus pests and diseases by analyzing the prevention and control effects of Ag-SiO2 nanocomposite materials on Diaphorina citri and citrus canker. The Ag-SiO2 nanocomposite material is synthesized by optimizing conditions, and the toxicity effect of different concentrations of Ag-SiO2 nanocomposite material treatment on Diaphorina citri and the antibacterial activity against citrus canker bacteria are measured, and the morphology and behavior of Diaphorina citri after Ag-SiO2 nanocomposite material treatment are observed. The LC 50 values of Ag-SiO2 nanocomposite material on the nymphs, older nymphs and adults of Diaphorina citri within 72 h are 1.3949, 1.965 and 5.4648 mg / mL respectively. With the increase of the concentration of Ag-SiO2 nanocomposite material, the mortality rate of Diaphorina citri also gradually increases, showing a dose-dependent phenomenon; with the increase of the treatment time of the medicament, the number of dead Diaphorina citri increases, and the LC 50 becomes lower. By observing and comparing the morphology and behavior of the treated Diaphorina citri with the control group, it is found that the treated Diaphorina citri shows varying degrees of body atrophy, which indicates that the physical damage caused by Ag-SiO2 nanocomposite material to Diaphorina citri and the resulting water loss of its body are one of the reasons for insect death. The EC 50The value is 1.1853 mg / mL, and it has the strongest antibacterial activity at a concentration of 1600 μg / mL. The Ag-SiO2 nanocomposite obtained by the optimized method of the present invention has high toxicity to Diaphorina citri and Xanthomonas citri subsp. citri, and can be used as an effective insecticide against Diaphorina citri and also as a new type of fungicide for the prevention and control of citrus canker, providing a new option for the prevention and control of citrus pests and diseases. At the same time, it also expands the application scope of the Ag-SiO2 nanocomposite. As a new type of nano-pesticide, the Ag-SiO2 nanocomposite will play a greater application value in the field of citrus pest and disease control. Brief Description of the Drawings

[0037] Figure 1 The corrected mortality rates of different concentrations of Ag-SiO2 nanocomposite on different instars of Diaphorina citri; a, adult group of Diaphorina citri; b, high-instar nymph group of Diaphorina citri; c, low-instar nymph group of Diaphorina citri.

[0038] Figure 2 The external morphology of Diaphorina citri nymphs after 24 hours of treatment; a, the back of the nymphs in the Ag-SiO2 nanocomposite treatment group; b, the back of the nymphs in the clear water control group; c, the ventral side of the nymphs in the Ag-SiO2 nanocomposite treatment group; d, the ventral side of the nymphs in the clear water control group.

[0039] Figure 3 Scanning electron microscope comparison of Diaphorina citri after treatment with Ag-SiO2 nanocomposite; a, compound eyes of adults (untreated); b, compound eyes of adults (treated for 48 hours); c, forewings of adults (untreated); d, forewings of adults (treated for 48 hours); e, back of nymphs (untreated); f, back of nymphs (treated for 48 hours); g, ventral side of nymphs (untreated); h, ventral side of nymphs (treated for 48 hours).

[0040] Figure 4 The antibacterial effects of Ag-SiO2 nanocomposite on Xanthomonas citri subsp. citri at different concentrations (n = 3); a, 1600 μg / mL; b, 800 μg / mL; c, 400 μg / mL; d, 200 μg / mL; e, 100 μg / mL; f, sterile water control.

[0041] Figure 5 Schematic diagram of the formation process of Ag-SiO2 nanocomposite.

[0042] Figure 6 Scanning electron microscope images of Ag-SiO2 nanocomposite at different magnifications (a) scale bar is 500 nm, (b) scale bar is 300 nm.

[0043] Figure 7, Transmission electron microscope images of Ag-SiO₂ nanocomposites at different magnifications. (a) Scale bar is 200 nm, (b) scale bar is 100 nm, (c) scale bar is 50 nm. Detailed implementation manners

[0044] AgNPs have high biocompatibility in organisms and have less negative impact on aspects such as cell viability and immune function during growth and development. Therefore, compared with traditional chemical agents, AgNPs are safer. When nano silver microparticles encounter moisture, they are prone to aggregation and lose their antibacterial effect. Silica microparticles (SiO₂ MP) have porous loading capacity, strong adsorption, and certain insecticidal activity themselves. Using SiO₂ MP to load AgNPs may have the dual effect of killing pests and diseases. The inventor found through experiments that the Ag-SiO₂ nanocomposite has a control effect on citrus pests and diseases.

[0045] In order to further improve the control effect of the Ag-SiO₂ nanocomposite on citrus pests and diseases, the inventor improved the impregnation adsorption method for synthesizing the Ag-SiO₂ nanocomposite. Different synthesis processes and variables during the synthesis of the Ag-SiO₂ nanocomposite, such as the mass of ammonia water, the concentration of silver nitrate, reaction temperature and time, etc., will affect the physicochemical properties of the Ag-SiO₂ nanocomposite. And the physicochemical properties such as the particle size, shape, and aggregation degree of the Ag-SiO₂ nanocomposite directly affect its toxicity, thereby affecting the stability of the virulence determination and antibacterial activity results of the Ag-SiO₂ nanocomposite. In-depth research was carried out to clarify the relationship between the toxicity of the Ag-SiO₂ nanocomposite and various factors, and to optimize the synthesis process.

[0046] The synthesis mechanism of this method is as follows: a: Hydrolyze TEOS to prepare SiO₂ submicron spheres with mercapto groups on the surface, b: Modify the mercapto groups on the surface of the SiO₂ submicron spheres through MPTMS, c: Adsorb and fix Ag + on the SiO₂ submicron spheres, and two mercapto groups condense to form -S-S- bonds, d: Reduce Ag + so that monolayer silver atoms are adsorbed on the surface of the SiO₂ submicron spheres and grow into AgNPs.

[0047] The inventor optimized the following contents:

[0048] Dispersion method of SiO₂ submicron spheres: In traditional synthesis, methods such as shaking and stirring are used to disperse SiO₂ submicron spheres in water, with uneven dispersion and relatively large particles. After optimization, an ultrasonic cell disruptor is used for ultrasonic dispersion for 10 min, and the solution texture is uniform without particle suspension.

[0049] Method of adding silver nitrate solution: In the traditional synthesis method, silver nitrate solution was directly mixed with silica solution. However, when the tiny silver nanoparticles encounter moisture, they are prone to aggregation and lose their antibacterial effect. After optimization, it is selected to be added dropwise at a speed of about 1 drop / second under stirring conditions, reducing the aggregation of silver ions and increasing the adsorption efficiency.

[0050] Reaction time and reaction temperature: In the traditional synthesis, the reaction was carried out at 50 - 100 °C until the solution changed color. After repeated experiments, it was found that the solution had high stability and excellent insecticidal and antibacterial activities when stirred at 80 °C for 3 h.

[0051] The toxicity effect of Ag-SiO2 nanocomposite on Diaphorina citri and its antibacterial activity against Xanthomonas citri subsp. citri were determined and analyzed. The morphology and behavior of Diaphorina citri after treatment with Ag-SiO2 nanocomposite were observed, and its action mechanism was explored, aiming to provide a basis for further research and development of Ag-SiO2 nanocomposite as a new physical control technology.

[0052] The sources of the reagents and materials used in the following examples are as follows:

[0053] 1. Test agents

[0054] Propyltrimethoxysilane (98.0%, Shanghai Aladdin Biochemical Technology Co., Ltd.); ammonia water (analytical pure, Sinopharm Chemical Reagent Co., Ltd.); tetraethyl orthosilicate (98.0%, Shanghai Aladdin Biochemical Technology Co., Ltd.); anhydrous ethanol (analytical pure, Tianjin Fuyu Fine Chemical Co., Ltd.); silver nitrate (0.1N, Shanghai Macklin Biochemical Technology Co., Ltd.), etc.

[0055] 2. Test insects

[0056] Diaphorina citri was collected from Murraya paniculata plants on the campus of South China Agricultural University and continuously reared on Murraya paniculata plants in the Insect Rearing Room of the Engineering Research Center for Biocontrol of the Ministry of Education, South China Agricultural University. The temperature in the insect rearing room was controlled at 26 ± 1 °C, the relative humidity was 60% ± 5%, and the photoperiod was 14L:10D.

[0057] 3. Test culture media

[0058] NA bacteria-containing culture medium: The bacteria-containing culture medium was prepared by the method of pre-adding bacterial liquid and pouring plates. Approximately 10 mL / L of bacterial liquid was injected into the NA plate culture medium cooled to about 50 °C, mixed evenly and then poured into plates, about 20 mL / plate.

[0059] 4. Test strains

[0060] Xanthomonas citri subsp. citri was isolated from the diseased leaves of citrus canker on the campus of South China Agricultural University. After verifying its pathogenicity by Koch's postulates, it was preserved in a 4 °C refrigerator with glycerol and activated in an NA plate when used.

[0061] Preparation of Ag-SiO2 Nanocomposite in Example 1

[0062] Add 60 mL of absolute ethanol into a 100 mL round-bottom flask, then add 3 mL of NH·H2O and 1 mL of H2O, and stir at 500 rpm for 15 minutes at room temperature. Dropwise add 2.3 mL of tetraethyl orthosilicate (TEOS), and continue stirring at a constant temperature for 6 h. Dropwise add a mixed solution of 500 μL of absolute ethanol and 100 μL of propyltrimethoxysilane (MPTMS) into the round-bottom flask and continue stirring for 12 h. Take out the solution, centrifuge at 12000 rpm for 5 minutes, and wash the precipitate twice with alcohol and twice with water to obtain SiO2 powder, which is stored for later use. Mix the prepared SiO2 powder with 10 mL of H2O, disperse it by ultrasonic treatment with an ultrasonic cell disruptor for 10 min, then dropwise add 10 mL of 0.1 mol / L silver nitrate solution, mix well, and stir at 80 °C for 3 h. The color of the solution gradually changes from white to brown ( Figure 5 ). Take out the solution, centrifuge at 12000 rpm for 5 minutes, wash the precipitate three times with water, and store it in water for later use.

[0063] From the scanning electron microscope ( Figure 6 ) and transmission electron microscope ( Figure 7 ) of the Ag-SiO2 nanocomposite, the size and shape of the prepared Ag-SiO2 nanocomposite can be observed. It can be clearly seen from the figure that a large number of Ag nanoparticles are uniformly loaded on the surface of the silica submicron spheres.

[0064] Toxicity Determination of Ag-SiO2 Nanocomposite against Diaphorina citri in Example 2

[0065] Gradiently dilute the Ag-SiO2 nanocomposite with water into five concentrations of 1, 2, 4, 8, and 16 mg / mL. Place 15 adult Diaphorina citri, late instar nymphs (instars 3-5) or early instar nymphs (instars 1-2) in a petri dish respectively, and seal it with a mesh bag and a rubber band. Make a small hole at the bottom of a transparent plastic cup, place a 8-10 cm long twig of Murraya paniculata with leaves, pass the end of the twig through the small hole, wrap the end of the twig with water-retaining cotton, and put it into a centrifuge tube to keep the twig fresh. Spray 5 times towards the petri dish with a spray bottle to ensure that the test insects evenly contact the liquid medicine, transfer the treated Diaphorina citri to the plastic cup, and seal it with a mesh bag. Each treatment is repeated 3 times, and the water treatment is used as a blank control. Observe and record the number of dead Diaphorina citri at 24, 48, and 72 h, and calculate the mortality rate and corrected mortality rate. The determination criterion for dead Diaphorina citri is that there is no response when gently touching the insect body.

[0066] Statistical analysis was performed using the data processing software IBM SPSS Statistics 22 and Excel (Zhang Zhixiang, Xu Hanhong, Cheng Dongmei, 2002; Chen Bin and Zheng Yu, 2019). With the logarithm of the concentration of the test agent as the x-axis and the probit value of the mortality rate (inhibitory rate) as the y-axis, the virulence regression equation y = ax + b was calculated based on the linear relationship between the two, and the correlation coefficient and LC 50 (EC 50 ) values were calculated. Duncan's test was used for significant analysis of differences between groups. GraphPad Prism 8 was used to create statistical charts.

[0067]

[0068]

[0069] At 24, 48, and 72 h after treatment with the Ag-SiO2 nanocomposite, a stereomicroscope was used to observe the morphological characteristics of freshly dead citrus psyllids (adults, older nymphs, younger nymphs) in each group (15 in each group), as well as the behaviors of the remaining surviving citrus psyllids.

[0070] Adult and nymph citrus psyllids treated with the Ag-SiO2 nanocomposite within 30 s and 48 h after treatment were fixed overnight with 2.5% glutaraldehyde, washed 3 times with 0.1 mol / L phosphate buffer, 15 - 30 min each time, dehydrated with gradient ethanol solutions (30%, 50%, 70%, 90%, 100% respectively), and then sequentially de-ethanolized in tert-butanol - ethanol gradient solutions with volume ratios of 1:3, 1:1, 3:1, 1:0; the samples were placed in a -50°C vacuum freeze dryer for freeze drying for 12 h. After drying, the samples were divided into different orientations and pasted on the sample stage according to the order, the placed samples were sputter-coated with gold, and finally observed under a scanning electron microscope.

[0071] Within 72 h of treatment with different concentrations of the Ag-SiO2 nanocomposite, as the effective concentration of the Ag-SiO2 nanocomposite increased, the cumulative mortality rate of citrus psyllids gradually increased, showing a dose-dependent phenomenon ( Figure 1 ). The corrected mortality rate of adults reached up to 66.67%. At 24 h of treatment, the mortality rate of adults was relatively stable. After 48 h of treatment, the mortality rate began to increase rapidly, and after 72 h of treatment, the mortality rate increased slowly ( Figure 1 a); the mortality rate of nymphs was more linearly correlated with the treatment time at the experimental concentrations ( Figure 1 b). Among them, the mortality rate of younger nymphs at a concentration of 16 mg / mL reached 100% at 24 h of treatment ( Figure 1 c), indicating that the Ag-SiO2 nanocomposite has a stronger killing effect on younger nymphs of citrus psyllids.

[0072] With the increase of the medicament treatment time, the number of dead Diaphorina citri increases, and the LC 50 value decreases (Table 1). Among them, the toxicity of the Ag-SiO2 nanocomposite to adult D. citri, older nymphs and younger nymphs is the strongest after 72 h of treatment, and the LC 50 values are 5.4648, 1.965 and 1.3949 mg / mL, respectively. The younger nymphs of D. citri are the most sensitive to the Ag-SiO2 nanocomposite. The LC 50 of the younger nymphs is 1.4 times that of the older nymphs and 3.9 times that of the adults after 72 h of treatment, indicating that the toxicity of the Ag-SiO2 nanocomposite to the nymphs of D. citri is higher than that to the adults.

[0073] Table 1 Toxicity regression analysis of Ag-SiO2 nanocomposite to adult and nymph of D. citri

[0074]

[0075] The cases of 8 mg / mL and 16 mg / mL in the nymph group are the most similar, indicating that the toxicity of 8 mg / mL to insects is already very high. In the concentration range of 1-8 mg / mL, the lower the concentration, the fewer the number of dead insects and the better the insect vitality. When the older nymphs and younger nymphs of D. citri are treated with 8 mg / mL Ag-SiO2 nanocomposite, it can be observed that the activity of the nymphs in the treatment group weakens at 24 h, and they are easy to fall from the branches. The fallen nymphs die in a short time. The dead nymphs lose water significantly, and the body color becomes darker and the body shrinks with the increase of time. There are significant differences under the stereomicroscope and scanning electron microscope ( Figure 2 , Figure 3 ). It can be seen that the observation at 8 mg / mL concentration is the most representative. Therefore, Figure 2 and Figure 3 only show the situation at 8 mg / mL concentration. The insect mortality rates are different after treatment with each concentration, but the situations of the dead insects are similar.

[0076] The treatment of the adult group was similar to that of the nymph group, and the observation at a concentration of 8 mg / mL was the most representative. The specific conditions for each concentration were as follows: at a concentration of 1 mg / mL, 4 - 6 adults were prostrate; at a concentration of 2 mg / mL, 5 - 7 adults were prostrate; at a concentration of 4 mg / mL, 6 - 8 adults were prostrate; at a concentration of 16 mg / mL, 8 - 12 adults were prostrate. The lower the concentration, the fewer the prostrate insects and the lighter the corresponding symptoms. When using 8 mg / mL Ag-SiO2 nanocomposite to treat adult citrus psyllids for 24 h, 7 - 10 adult insects could be observed prostrate at the bottom of the plastic cup. The prostrate adults had difficulty moving. When gently touching with a writing brush, it could be observed that the feet of the adults struggled violently, and the wings flapped slightly but could not fly to the branches for feeding; at 48 h, most of the citrus psyllids at the bottom of the cup were dead, and a small number flew to the branches and survived. The bodies of the surviving adults made a 45° angle with the leaves and mostly docked on the back of the leaves or the stems for feeding; at 72 h, the number of newly added dead was small, and the mortality rate basically no longer increased after 72 h. The bodies of the adults in the Ag-SiO2 nanocomposite treatment group were slightly shrunk under the stereomicroscope, and the color of the feet was darker, showing no obvious difference compared with the adults in the clear water control group; under the electron microscope, a large number of flocculent adhesion impurities could be seen on the surface of the compound eyes of the adults after 48 h of Ag-SiO2 nanocomposite treatment, while the surface of the compound eyes of the untreated adults was relatively smooth( Figure 3 a and 5b); there were a large number of obvious scratches on the surface of the forewings of the adults after 48 h of treatment, while the surface of the forewings of the untreated adults was relatively smooth with few scratches( Figure 3 c and 5d).

[0077] Example 2 Determination of the antibacterial activity of Ag-SiO2 nanocomposite against Xanthomonas citri subsp. citri

[0078] The test was carried out by the plate antibacterial circle method. Three equilateral triangle holes with a diameter of 9 mm were punched in the bacteria-containing culture medium with a sterilized puncher, and the moisture in the holes was dried. 50 μL of Ag-SiO2 nanocomposites with different concentrations (100, 200, 400, 800, and 1600 μg / mL) were added to the holes respectively, and each concentration was repeated 3 times. Sterile water was used as a control. After culturing at a constant temperature of 28 °C for 48 h, the diameter of the antibacterial circle was measured by the cross method, and the average value of the antibacterial circle under different concentrations of the medicament was calculated.

[0079]

[0080] The results showed that the treatment with Ag-SiO2 nanocomposite had an inhibitory effect on Xanthomonas citri subsp. citri and inhibited the growth of Xanthomonas citri subsp. citri in a dose-dependent manner( Figure 4 ). When the concentration was 1600 μg / mL, the inhibitory effect of the AgNPs solution was the strongest. Using probit analysis, the virulence regression equation of the Ag-SiO2 nanocomposite was estimated to be y = 0.910x + 2.202, EC 50It was 1185.302 μg / mL, and the correlation coefficient was 0.984 (Table 2).

[0081] Table 2 Indoor virulence determination of Ag-SiO2 nanocomposite against Xanthomonas citri pv. citri (n = 3)

[0082]

Claims

1. A method for controlling Diaphorina citri, characterized in that: The method includes the following steps: spraying citrus psylla with an Ag-SiO₂ nanocomposite solution; the citrus psylla being an adult, an older nymph or a younger nymph; when the citrus psylla is an adult, the concentration of the Ag-SiO₂ nanocomposite is 5.9 - 15.3 mg / mL; when the citrus psylla is an older nymph, the concentration of the Ag-SiO₂ nanocomposite is 4.8 - 12.9 mg / mL; when the citrus psylla is a younger nymph, the concentration of the Ag-SiO₂ nanocomposite is 2.8 - 5.3 mg / mL. The preparation method of the Ag-SiO₂ nanocomposite includes the following steps: a. Adding NH₃·H₂O and H₂O to absolute ethanol under stirring to make the pH of the solution 9 - 12. b. Dropwise adding tetraethyl orthosilicate under stirring and continuing to stir for 6 h; the mass ratio of tetraethyl orthosilicate to the solution is 1:20 - 30. c. Dropwise adding a mixed solution of absolute ethanol and propyltrimethoxysilane under stirring and continuing to stir for 12 h. Centrifuging, washing the precipitate with alcohol and water successively to obtain SiO₂ powder; the mass ratio of absolute ethanol to propyltrimethoxysilane is 5:

1. d. Mixing the prepared SiO₂ powder with H₂O and dispersing it by ultrasound; then dropwise adding silver nitrate solution, mixing well and stirring at 80 °C for 3 h, the color of the solution gradually changes from white to brown; centrifuging; washing with water and storing it in water for standby.

2. The method according to claim 1, wherein: It has one of the following characteristics: (1) In step a, the pH of the solution is 11. (2) In step d, the dosage ratio of SiO₂ powder to silver nitrate solution is: dropping 10 mL of 0.1 mol / L silver nitrate for every 0.5 g of SiO₂ powder. (3) In the above method, centrifuging is carried out at 12000 rpm for 5 min.

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