CN117866855A - Serratia marcescens SMKY2308 strain and application thereof - Google Patents

Abstract

The invention discloses a Serratia marcescens SMKY2308 strain and application thereof, relating to the technical field of microorganismsSerratia marcescens) The preservation name is Serratia marcescens SMKY2308; the microbial strain is preserved in the China general microbiological culture Collection center (China Committee for culture Collection), and the preserving unit address is North Chen Xili No.1 and No. 3 in the Chaoyang area of Beijing city; preservation date: 2023, 9, 15; preservation number: CGMCC No.28466. The Serratia marcescens SMKY2308 is used for preventing and controlling rice leaf rollers which are major crop pestsCnaphalocrocis medinalisThe biocontrol fungi of the larvae of the Lepidoptera borer family have strong pathogenicity to the larvae of cnaphalocrocis medinalis, are safe to the environment and do not existThe pesticide resistance of pests is easy to generate, and the pesticide is used for biologically controlling cnaphalocrocis medinalis.

Description

Serratia marcescens SMKY2308 strain and application thereof

Technical Field

The invention relates to the technical field of microorganisms, in particular to a Serratia marcescens SMKY2308 strain and application thereof.

Background

Rice leaf rollerCnaphalocrocis medinalisBelongs to Lepidoptera (Lepidoptera), the family of borers (Pyralidae), commonly called rice leaf rollers and leaf rollers, is an important agricultural pest with migratory habit and is widely distributed at home and abroad. The main rice planting areas from south to north in China are distributed, especially in the middle and downstream of Yangtze river and in the south China rice area, and are widely distributed abroad in Asia such as Indonesia, philippines, pakistan, korea, japan, etc. The rice leaf rollers can be harmful to corn Zea mays and wheat besides riceTriticum aestivumBarleyHordeum vulgareAnd sugar caneSaccharum officinarumThe crops can also harm barnyard grassEchhinochloa crusgalliRhizoma Dioscoreae SeptemlobaeSetaria viridisGrass weeds are identified. The first hatched larva of cnaphalocrocis medinalis is fed with rice heart leaves, leaf and bud are rolled on the rice leaves after 2 years old, and rice spike bags are drilled into the rice in the late booting stage of the rice to feed, so that damage is caused to the rice spikes. The rice leaves damaged by cnaphalocrocis medinalis are white stripe, which affects photosynthesis of the rice leaves and transportation of moisture and nutrient substances, and affects development and maturation of the rice, thereby reducing the yield of the rice. The rice leaf rollers seriously affect the rice yield, cause great loss to agricultural economy,therefore, importance must be attached to the prevention and control of cnaphalocrocis medinalis, and the agricultural development of China is ensured.

At present, the prevention and control of cnaphalocrocis medinalis is mainly performed by adopting prevention and control measures mainly based on pesticide control, and excessive use of chemical pesticides not only causes drug resistance of pests, but also causes the problems of pesticide residues, environmental pollution and the like. How to fundamentally solve the problems is still required to be explored continuously. The microbial pesticide can avoid the problems, and has the advantages of high efficiency, low toxicity, no residue, high specificity, good lasting effect and the like, so that the research of the microbial pesticide becomes the current hot research field of biological prevention, and the prevention and control of the cnaphalocrocis medinalis by utilizing the microbial pesticide is the current development trend.

However, the basis of biopesticides is to obtain highly pathogenic biocontrol strains. The entomopathogenic bacteria have strong natural popularity, no environmental pollution, safety to human, livestock and plants, little or no resistance to pests, and meet the requirements of biological control and environmental protection, thus becoming an important measure for biological control of pests. Therefore, the research on biocontrol bacteria of the cnaphalocrocis medinalis guenee provides basis for biocontrol of the cnaphalocrocis medinalis guenee and also provides guarantee for reducing drug resistance and environmental pollution caused by chemical control.

Serratia marcescens @ sSerratia marcescens) A bacterium which produces bright red pigment is found in 1819 by Italian pharmacist and named 1823, and is classified into Monomonas (Halomonas) and named LINGsingle bacteriaMonas prodigiosus) The bacteria were then ascribed to Bacillus (Bacillus), designated LING BI. Many scholars have bioassay of the biocontrol potential of Serratia marcescens strains against pests and control of Asian diaphorina citri by different Serratia marcescens strainsDiaphorina citri(Kuwayama) nymphs, huang Jing Small locustsOedaleus infernalisSaussure, brown planthopperNilaparvata lugens(Stal), beet armywormSpodoptera exigua(Hubner)。

At present, it is reported that dead black-wing termites naturally infect from natural conditionsOdontoternes formosanusSerratia marcescens obtained by separating from potato tuber moth, and also from SongmoSerratia marcescens is found, but Serratia marcescens which naturally infects insects under natural conditions is not reported yet.

Disclosure of Invention

The invention aims to provide Serratia marcescens SMKY2308 and application thereof, and aims to overcome the defect that the conventional Serratia marcescens cannot parasitic cnaphalocrocis medinalis under natural conditions, prevent and treat cnaphalocrocis medinalis by a biotechnology means, and avoid or reduce the problems of drug resistance and environmental pollution caused by unreasonable use of chemical pesticides.

In order to solve the problems in the prior art, the invention provides the following technical scheme: the Serratia marcescens SMKY2308 strain is Serratia marcescensSerratia marcescensSMKY2308, accession number Serratia marcescensSerratia marcescensSMKY2308; the microbial strain is preserved in the China general microbiological culture Collection center (China Committee for culture Collection), and the preserving unit address is North Chen Xili No.1 and No. 3 in the Chaoyang area of Beijing city; preservation date: 2023, 9, 15; preservation number: CGMCC No.28466.

Further, the nucleotide sequence of 16S rDNA of Serratia marcescens SMKY2308 strain is shown in SEQ ID No. 1.

Further, serratia marcescens SMKY2308 strain is a gram negative bacterium, the cell shape is short rod shape, the size is 1-1.3 mu m multiplied by 0.7-1.0 mu m, and the bacterial strain SMKY2308 is found to be circular in colony, neat in edge, orange-yellow, smooth in surface, moist, slightly convex in the center and flat in the periphery through LB plate streak purification culture.

The invention separates and obtains serratia marcescens SMKY2308 wild strain from morbid rice leaf roller larva, the wild strain is tiebased on rice leaf roller larva and rejuvenated to obtain serratia marcescens, and the strain is separated and cultured according to the conventional method to obtain pure serratia marcescens with strong pathogenicity. On LB culture medium, the bacterial colony of Serratia marcescens SMKY2308 strain is single bacterial colony convex, the center is opaque, the surface is smooth, the edge is regular, and the strain is sticky.

The Serratia marcescens SMKY2308 bacterial agent prepared by the Serratia marcescens SMKY2308 bacterial strain.

Further, the active ingredients are at least one of the following (a), (b) and (c):

(a) A fermentation broth primary extract of Serratia marcescens SMKY2308 strain;

(b) Ultrasonic lysis supernatant of the obtained Serratia marcescens SMKY2308 strain cells;

(c) The obtained Serratia marcescens SMKY2308 strain cells were subjected to ultrasonic lysis and precipitation.

The preparation method of the Serratia marcescens SMKY2308 microbial inoculum is characterized by comprising the following steps:

(1) Separating from field naturally infected rice leaf roller larva to obtain Serratia marcescens SMKY2308 strain, and culturing with LB culture medium;

(2) Then purifying on LB culture medium to obtain purified Serratia marcescens SMKY2308 microbial inoculum;

(3) The method adopts Serratia marcescens SMKY2308 strain suspension to measure the lethal ability of the rice leaf roller larvae, and the mortality rate of the rice leaf roller larvae is 100%. At a high concentration of 1.06X10 ⁹ cfu/mL、1.06×10 ¹⁰ The cumulative mortality of cfu/mL bacterial suspension to rice leaf roller 1-instar larvae is 100%, and the cumulative mortality of 4-instar larvae is 93.33% and 100% respectively.

The invention discloses application of Serratia marcescens SMKY2308 strain in preparing biocontrol agents for rice leaf roller Holotrichia scrobiculata larvae.

The beneficial effects are that: in the invention, on the day 29 of 8 months in 2013, when the rice leaf roller Cnaphalocrocis medinalis is infected by serratia marcescens and causes epidemic diseases of the rice leaf roller population, a large number of rice leaf roller larvae are infected and die in Kaolin city and Yingcun paddy field investigation in red river of Yunnan province. Therefore, the Serratia marcescens SMKY2308 strain is an insect pathogenic bacterium which can be infected with parasitic cnaphalocrocis medinalis and has strong popularity under natural conditions for the first time in China at present, and has great development and application prospects in preventing and controlling cnaphalocrocis medinalis.

Compared with the prior art, the invention has the following advantages: (1) The invention is a strain separated from the morbid rice leaf roller larva for the first time, and has the advantages of simple culture, high growth speed and rapid propagation. Secondly, the serratia marcescens SMKY2308 strain obtained by separating and purifying the morbid rice leaf roller larvae has high pathogenic speed and strong lethal ability on the rice leaf roller larvae.

(2) At 1.06X10 ³ cfu/ mL、1.06×10 ⁴ cfu/ mL、1.06×10 ⁵ cfu/ mL、1.06×10 ⁶ cfu/ mL、1.06×10 ⁷ cfu/ mL、1.06×10 ⁸ At cfu/mL, the cumulative mortality of rice leaf roller larvae was 26.67, 70.00, 52.22, 63.33, 75.56, 88.89 and 100%, respectively. At a high concentration of 1.06X10 ⁹ cfu/mL、1.06×10 ¹⁰ At cfu/mL, the cumulative mortality of the rice leaf roller 1-instar larvae reaches 100%, and the cumulative mortality of the 4-instar larvae is 93.33% and 100% respectively. Therefore, the serratia marcescens SMKY2308 strain has strong pathogenicity on rice leaf roller larvae, has higher pathogenicity than reported serratia marcescens separated from the intestinal tracts of the rice leaf roller larvae, and can be used for preventing and controlling the rice leaf roller.

Drawings

FIG. 1 shows the symptoms of Serratia marcescens SMKY2308 of the invention naturally infecting lethal cnaphalocrocis medinalis larvae.

FIG. 2 is a colony morphology of Serratia marcescens SMKY2308 according to the present invention.

FIG. 3 is a diagram showing the bacterial status of Serratia marcescens SMKY2308 according to the present invention.

Detailed Description

The present invention will be described in further detail with reference to the following examples in order to make the objects, technical solutions and advantages of the present invention more apparent. It should be understood that the specific embodiments described herein are for purposes of illustration only and are not intended to limit the scope of the invention.

Examples

The Serratia marcescens SMKY2308 strain is Serratia marcescens Serratia marcescens SMKY2308, and the preservation name is Serratia marcescens Serratia marcescens SMKY2308; the microbial strain is preserved in the China general microbiological culture Collection center (China Committee for culture Collection), and the preserving unit address is North Chen Xili No.1 and No. 3 in the Chaoyang area of Beijing city; preservation date: 2023, 9, 15; preservation number: CGMCC No.28466.

The nucleotide sequence of 16S rDNA of Serratia marcescens SMKY2308 strain SEQ ID No.1 is shown below. CGGCTTACCATGCAAGTCGAGCGGTAGCACAGGGGAGCTTGCTCCCTGGGTGACGAGCGGCGGACGGGTGAGTAATGTCTGGGAAACTGCCTGATGGAGGGGGATAACTACTGGAAACGGTAGCTAATACCGCATAACGTCGCAAGACCAAAGAGGGGGACCTTCGGGCCTCTTGCCATCAGATGTGCCCAGATGGGATTAGCTAGTAGGTGGGGTAATGGCTCACCTAGGCGACGATCCCTAGCTGGTCTGAGAGGATGACCAGCCACACTGGAACTGAGACACGGTCCAGACTCCTACGGGAGGCAGCAGTGGGGAATATTGCACAATGGGCGCAAGCCTGATGCAGCCATGCCGCGTGTGTGAAGAAGGCCTTCGGGTTGTAAAGCACTTTCAGCGAGGAGGAAGGTGGTGAACTTAATACGTTCATCAATTGACGTTACTCGCAGAAGAAGCACCGGCTAACTCCGTGCCAGCAGCCGCGGTAATACGGAGGGTGCAAGCGTTAATCGGAATTACTGGGCGTAAAGCGCACGCAGGCGGTTTGTTAAGTCAGATGTGAAATCCCCGGGCTCAACCTGGGAACTGCATTTGAAACTGGCAAGCTAGAGTCTCGTAGAGGGGGGTAGAATTCCAGGTGTAGCGGTGAAATGCGTAGAGATCTGGAGGAATACCGGTGGCGAAGGCGGCCCCCTGGACGAAGACTGACGCTCAGGTGCGAAAGCGTGGGGAGCAAACAGGATTAGATACCCTGGTAGTCCACGCTGTAAACGATGTCGATTTGGAGGTTGTGCCCTTGAGGCGTGGCTTCCGGAGCTAACGCGTTAAATCGACCGCCTGGGGAGTACGGCCGCAAGGTTAAAACTCAAATGAATTGACGGGGGCCCGCACAAGCGGTGGAGCATGTGGTTTAATTCGATGCAACGCGAAGAACCTTACCTACTTCTTGACATCCAGAGAACTTTCCAGAGATGGATTGGTGCCTTCGGGAACTCTGAGACAGGTGCTGCATGGCTGTCGTCAGCTCGTGTTGTGGAAATGTTGGGTTAAGTCCCG

The serratia marcescens SMKY2308 strain is a gram negative bacterium, the cell shape is short rod shape, the size is 1-1.3 mu m multiplied by 0.7-1.0 mu m, and the serratia marcescens SMKY2308 strain is found to be circular, regular in edge, orange yellow, smooth in surface, moist, slightly convex in the center and flat in the periphery through the streak purification culture of an LB plate.

The invention separates and obtains serratia marcescens SMKY2308 wild strain from morbid rice leaf roller larva, the wild strain is tiebased on rice leaf roller larva and rejuvenated to obtain serratia marcescens, and the strain is separated and cultured according to the conventional method to obtain pure serratia marcescens with strong pathogenicity. On LB culture medium, the bacterial colony of Serratia marcescens SMKY2308 strain is single bacterial colony convex, the center is opaque, the surface is smooth, the edge is regular, and the strain is sticky.

The Serratia marcescens SMKY2308 bacterial agent prepared by the Serratia marcescens SMKY2308 bacterial strain. The active ingredients are at least one of the following (a), (b) and (c):

(a) A fermentation broth primary extract of Serratia marcescens SMKY2308 strain;

(b) Ultrasonic lysis supernatant of the obtained Serratia marcescens SMKY2308 strain cells;

(c) The obtained Serratia marcescens SMKY2308 strain cells were subjected to ultrasonic lysis and precipitation.

The preparation method of the Serratia marcescens SMKY2308 microbial inoculum is characterized by comprising the following steps:

(1) Separating from field naturally infected rice leaf roller larva to obtain Serratia marcescens SMKY2308 strain, and culturing with LB culture medium;

(2) Then purifying on LB culture medium to obtain purified Serratia marcescens SMKY2308 microbial inoculum;

(3) The method adopts Serratia marcescens SMKY2308 strain suspension to measure the lethal ability of the rice leaf roller larvae, and the mortality rate of the rice leaf roller larvae is 100%. At a high concentration of 1.06X10 ⁹ cfu/mL、1.06×10 ¹⁰ The cumulative mortality of cfu/mL bacterial suspension to rice leaf roller 1-instar larvae is 100%, and the cumulative mortality of 4-instar larvae is 93.33% and 100% respectively.

The Serratia marcescens SMKY2308 strain of the invention is used for preparing cnaphalocrocis medinalisCnaphalocrocis medinalisApplication of larva biocontrol agent is provided.

Test example 1

Isolation and identification of pathogenic bacteria

1.1 Materials and methods

1.1.1 Material

When the rice leaf rollers are investigated in Kaifeng city and Yingcun paddy field of Honghezhou of Yunnan province, the rice leaf rollers infected by Serratia marcescens and causing rice leaf roller population epidemic disease are foundCnaphalocrocis medinalis。

LB solid medium: tryptone 10 g, yeast extract 5 g, sodium chloride 10 g, agar 15-20 g, pH 7.0 and water 1L.

Aseptic operating conditions: all vessels and appliances are subjected to high-temperature sterilization (121 ℃ for 30 min), inoculation and other operations are performed in an ultra-clean workbench.

Culture conditions: culturing in a 28 deg.C illumination (12L: 12D) incubator, transferring to test tube NA solid slant culture medium after colony formation, culturing for 2-3 days, and transferring to 4 deg.C refrigerator for storage.

Isolation and purification of pathogenic bacteria: selecting 1 patient, sterilizing the body surface with 70% alcohol, absorbing proper amount of body fluid in a sterile centrifuge tube, adding small amount of physiological saline, stirring with a gun head to homogenize, and continuously adding physiological saline to constant volume of 1 mL to obtain a sample stock solution for later use. 100 mu L of the prepared suspension is evenly coated on an LB solid culture medium, after 48-h continuous culture is carried out in a climatic chamber at 28 ℃, colonies with the same color as the morbid insects are picked up and continuously streaked on a new LB solid culture medium until pure cultures are obtained, and then whether the bacteria are the same or not is determined by tabletting and microscopic observation.

And (5) morphological identification of pathogenic bacteria. According to the form, the identification of the pathogenic bacteria type is carried out by combining a molecular technology, the shape, the color, the cell size and other culture properties of each strain are observed and recorded, and the strain is primarily identified by combining the related content in bacterial taxonomy.

The molecular identification of pathogenic bacteria adopts a freeze-thawing method to extract bacterial DNA, and the specific operation steps are as follows: single colonies were picked from the purified LB plates into sterile centrifuge tubes of 1.5. 1.5 mL, respectively, and mixed well by vortexing with 0.5. 0.5 mL sterile distilled water. Freezing the mixed bacterial suspension in an ice box filled with liquid nitrogen for 10 min, fixing the centrifuge tube on a foam float plate for 5 min in boiling water bath, and finally centrifuging at a high speed of 12000 r/min for 2 min, wherein the supernatant is used as a PCR template. Bacterial universal primer 27F:5'-AGAGTTTGATCCTGGCTCAG-3' and 1492R:5'-CGGTTACCTTGTTACGACTT-3' primers 27F, 14992R 1 [ mu ] L, template DNA 1 [ mu ] L, 2X Taq PCR Mastermix 12.5.5 [ mu ] L, ddH O9.5 [ mu ] L in the amplification system. The PCR instrument was programmed to heat denature at 94℃for 5 min, denature at 94℃for 1 min, anneal at 53℃for 1 min, extend at 72℃for 2 min,32 cycles, and extend at 72℃for 10 min. The PCR product was detected by 1.0% agarose gel electrophoresis, and the qualified PCR product was sent to Shanghai Bioengineering Co., ltd for two-way sequencing.

1.2 Results

Separating and obtaining a wild strain from a rice leaf roller larva naturally infected by bacteria, culturing the wild strain on an LB (LB) culture medium, inoculating the wild strain back to the rice leaf roller larva for rejuvenation to obtain a 1 strain, and separating and obtaining a purified strain, namely Serratia marcescens SMKY2308.

On LB culture medium, serratia marcescens SMKY2308 bacterial colony is red, round, convex on surface, moist and glossy. The strain was cultured on LB solid agar medium at 28℃and the colony color gradually increased until it became red as the culture time was prolonged.

And (3) according to the field infection symptoms, collecting and observing the indoor separation of the morbid insects, and carrying out strain identification according to the related content combined with bacterial taxonomy, determining the serratia marcescens SMKY2308.

Test example 2

Indoor toxicity determination of Serratia marcescens SMKY2308 on rice leaf roller larvae

2.1 materials and methods

2.1.1 test insect source

Rice leaf rollerCnaphalocrocis medinalis) Larvae were fed from the laboratory at 28.+ -.1 ℃ and 70.+ -.5% humidity in a 12L/12D photoperiod light incubator with ginkgo leaves. Healthy and consistent 3-day-old rice leaf roller larvae are obtained as test insects.

2.1.2 preparation of the fermentation broth with bacteria

The activated Serratia marcescens SMKY2308 strain was inoculated into a conical flask containing 150 mL NA broth and placed in a shaker (28 ℃,180 r/min) for shaking culture 24 h. Preparation of 1.06X10 with sterile LB liquid Medium ¹⁰ cfu/mL of the fermentation stock solution with bacteria. Sequentially diluting the original fermentation broth to a concentration of 1.06X10 ⁹ 、1.06×10 ⁸ 、1.06×10 ⁷ 、1.06×10 ⁶ 、1.06×10 ⁵ 、1.06×10 ⁴ cfu/mL, sterile LB liquid medium was used as a blank.

2.1.3 Toxicity determination

Healthy 3-day-old larvae of cnaphalocrocis medinalis with consistent age and growth conditions were selected for testing. Soaking rice leaf in prepared bacterial fermentation liquid by leaf soaking method for 5 s, taking out, naturally air drying at room temperature, placing in a sterilizing culture dish with diameter of 15 cm (a layer of absorbent paper is filled in the culture dish), picking leaf roller larva of rice leaf roller on the surface of leaf (20 heads per dish), covering with preservative film with small holes, and culturing at room temperature for feeding. Each strain treatment and control was set up with 3 replicates of 30 test insects each. The leaf was immersed in the sterile LB liquid medium as a Control (CK). After the treatment, the observation was continued for 10 days. The death number of the test rice leaf roller larvae is recorded every day, the death number of Serratia marcescens SMKY2308 appears on the surfaces of the larvae, and the average death rate is calculated. The data were subjected to linear regression statistical analysis using DPS 14.0 software.

3.2 Results

The results show that: serratia marcescens SMKY2308 strain has higher toxicity to rice leaf roller larvae, and is 1.06 multiplied by 10 ³ cfu/ mL、1.06×10 ⁴ cfu/ mL、1.06×10 ⁵ cfu/ mL、1.06×10 ⁶ cfu/ mL、1.06×10 ⁷ cfu/ mL、1.06×10 ⁸ At cfu/mL, the cumulative mortality of rice leaf roller larvae was 26.67, 70.00, 52.22, 63.33, 75.56, 88.89 and 100%, respectively. At a high concentration of 1.06X10 ⁹ cfu/mL、1.06×10 ¹⁰ At cfu/mL, the cumulative mortality of the rice leaf roller 1-instar larvae reaches 100%, and the cumulative mortality of the 4-instar larvae is 93.33% and 100% respectively. Therefore, the serratia marcescens SMKY2308 has strong pathogenic effect on cnaphalocrocis medinalis, and has the potential of being developed into a microbial pesticide for controlling cnaphalocrocis medinalis.

The strain is serratia marcescens SMKY2308 which is obtained by first separating and purifying rice leaf roller larvae, CGMCC No.28466, and can grow rapidly on LB culture medium at the temperature of 28 ℃ and relative humidity of more than 70%. The bacterial suspension has good pathogenicity to the rice leaf roller larvae, and can be widely used for preventing and controlling the rice leaf roller larvae. Meanwhile, the strain has wide sources of culture raw materials, low price and simple culture method, is easy to produce in large quantity, has great development and application potential, is used for preventing and controlling cnaphalocrocis medinalis by using the serratia marcescens SMKY2308, is typical biological prevention and control, can avoid the problems of drug resistance, environmental pollution and the like caused by chemical pesticides, and provides basis for green prevention and control of cnaphalocrocis medinalis larvae.

The foregoing has shown and described the basic principles, principal features and advantages of the invention. It will be understood by those skilled in the art that the present invention is not limited to the foregoing embodiments, which have been described in the foregoing embodiments and description merely illustrates the principles of the invention, and various changes and modifications may be made therein without departing from the spirit and scope of the invention, the scope of which is defined in the appended claims, specification and their equivalents.

Claims (7)

1. A serratia marcescens SMKY2308 strain, characterized in that: the Serratia marcescens SMKY2308 strain is Serratia marcescens Serratia marcescens SMKY2308, and the preservation name is Serratia marcescens Serratia marcescens SMKY2308; the microbial strain is preserved in the China general microbiological culture Collection center (China Committee for culture Collection), and the preserving unit address is North Chen Xili No.1 and No. 3 in the Chaoyang area of Beijing city; preservation date: 2023, 9, 15; preservation number: CGMCC No.28466.

2. The serratia marcescens SMKY2308 strain according to claim 1, characterized in that: the nucleotide sequence of the 16S rDNA of the Serratia marcescens SMKY2308 strain is shown in SEQ ID No. 1.

3. The serratia marcescens SMKY2308 strain according to claim 1, characterized in that: the Serratia marcescens SMKY2308 strain is a gram negative bacterium, the cell shape is short rod-shaped, the size is 1-1.3 mu m multiplied by 0.7-1.0 mu m, and the bacterial strain SMKY2308 is found to be circular in colony, regular in edge, orange-yellow, smooth in surface, moist, slightly convex in the center and flat in the periphery through LB flat streak purification culture.

4. The Serratia marcescens SMKY2308 strain according to claim 1.

5. The Serratia marcescens SMKY2308 microbial agent according to claim 4, wherein the active ingredient is at least one of the following (a) (b) (c):

(a) A fermentation broth primary extract of Serratia marcescens SMKY2308 strain of claim 1;

(b) An sonicated supernatant of Serratia marcescens SMKY2308 strain cells obtained according to claim 1;

(c) Ultrasonic lysis pellet of Serratia marcescens SMKY2308 strain cells obtained according to claim 1.

6. The preparation method of Serratia marcescens SMKY2308 microbial inoculum according to claim 4, which is characterized by comprising the following steps:

(1) Separating from field naturally infected rice leaf roller larva to obtain Serratia marcescens SMKY2308 strain, and culturing with LB culture medium;

(2) Then purifying on LB culture medium to obtain purified Serratia marcescens SMKY2308 microbial inoculum;

(3) The method adopts Serratia marcescens SMKY2308 strain suspension to measure the lethal ability of the rice leaf roller larvae, and the mortality rate of the rice leaf roller larvae is 100%.

7. The use of Serratia marcescens SMKY2308 strain according to claim 1 for preparing biocontrol agents for cnaphalocrocis medinalis larvae.