Screening Of Biocontrol Fungi And Preliminary Study On The Mechanism And Application Of Biocontrol Fungi BS-1 Of Megalurothrips Usitatus(Bagnall)

Megalurothrips usitatus[Bagnall],belonging to Thysanoptera subfamily,Thripina(Stephens)Priesner,Megalurothrips.M.usitatus is a very important pest in agriculture and forestry because of its small body,strong reproduction ability,hidden in flowers and difficult to control.M.usitatus are widely distributed in tropical and subtropical countries and regions near the equator.It has a wide range of hosts,most of which are legumes.In recent years,M.usitatus has become a major pest of cowpea[vigna unguiculata(L.)walp.]in Hainan Province,which seriously affects cowpea quality.At present,the general control for M.usitatus mainly for chemical control,supplemented by some physical measures.Long term chemical control measures based on pesticides will bring resistance,pesticide residues and environmental pollution.And biological control,In recent years,more and more attention has been paid to the advantages of low production cost,no pollution to the environment and the safety of human and livestock.Therefore,the study of biological control measures for M.usitatus has a good application prospect and great significance.The main results of this paper are as follows:1.Six cities and counties of cowpea production area in Hainan Province were selected to collect common thrips and 19 fungi belonging to 5 genus were isolated and identified as Isaria sp.Penicillium sp.Aspergullus sp.Fusarium sp.And Cladaxporism sp.By testing the pathogenicity of M.usitatus,a strain with good pathogenicity was named BS-1 and identified as isaria javanica.The LC50 of BS-1 to the second instar nymphs was 1.4 × 106 spores·mL-1 for 24 h,and the LC50 for 72 h to adult was 6.92 × 106spores·mL-1,LCso for 96h was 6.32 × 1O5 spores·mL-1.The LT50 of second instar nymphs and female adults of M.usitatus were 17.43 h and 58.30 h,respectively.2.Through the combination of optical microscope,SEM and TEM,the infection process of biocontrol fungus BS-1 on the second instar nymphs and adults of M.usitatus was photographed in this paper.The results showed that the infection structure was observed in BS-1 after 6h contact with the second instar nymphs,and the infection of the second instar nymphs was completed in 24 hours.The infection structure was observed in BS-1 after 8h contact with the adults and the infection was completed within 72 hours.3.During the BS-1 infection,the contents of reactive oxygen species(ROS),the activities of superoxide dismutase(SOD),catalase(CAT),peroxidase(POD)and malondialdehyde(MDA)in M.usitatus were studied.BS-1 treatment group had the highest ROS content of 4193.66 FL·mg prot-1(control group:1392.37 FL·mg prot-1)and the highest activity of T-SOD was 0.34 U·mg prot-1(control group:0.16 U·mg prot-1).The highest activity of Mn-SOD was 0.252 U·mg prot-s(control group:0.067 U·mg prot-1).The activity of CAT was 0.243 U·g pros-1(control group:0.196 U·g pros-1).The activity of POD was 0.013 U·mg prot-1(control group:0.009 U·mg prot-1).The highest MDA content was 1.11 μmol·g prot-1(control group:0.27 μmol·g prot-1).4.High throughput sequencing was used to compare the intestinal microbial diversity between infected and uninfected thrips.The results showed that BS-1 infection would upset the dynamic balance of microbial environment in the digestive tract of common thrips.The number and species of probiotics decreased rapidly,the contents of pathogenic bacteria and conditioned pathogens increased sharply,the composition of microbial community in thrips changed from complex and diverse to simple,and the richness and diversity decreased sharply.It is considered that BS-1 can reduce the immunity of common thrips by destroying the diversity of microbial species in the digestive tract of common thrips and enhance their infection efficiency and virulence.5.In this paper,the feasibility of the application of biocontrol fungus BS-1 was preliminarily explored.The effect of temperature on the germination of conidia was tested.The results showed that the optimum temperature of the germination of BS-1 conidia at the 25℃.20℃ and 30℃ was no obvious effect on the germination speed and germination rate of BS-1 conidium,and the germination speed of conidia would be slowed down slightly at 35℃.But there is no obvious effect on the germination rate.The BS-1 conidium can be successfully made into freeze-dried powder and microcapsule,and the success rate of the resuscitation of the freeze-dried powder 93.33%.The LT60 of Microcapsules after the resuscitation to M.usitatus was 62.63h.Small molecular volatile compound Lin and BS-1 will improve the virulence to the M.usitatus.BS-1 has certain toxicity to the Orius sauteri.The LT50 was 74.22h.Combination of these two biological control measures is not recommended.