Screening Of A High Virulent Isolate Of Beauveria Bassiana And The Studies On Histopathology Of Infected Apriona Germari Larvae

In this study,a high virulent isolate Beauveria bassiana Bb00 to Apriona germari larvae was screened from the isolates collected from the soil using the "Tenebrio molitor Bait Method".To this isolate,the basal biological characteristics,the pathogenicity against A.germari and factors affecting the pathogenicity,infection process and defense response of the host to the pathogen,the pathological changes of A.germari larvae infected by B.bassiana were studied.The purposes of this study were to establish the pathogenesis of B.bassiana to the A.germari and interaction between B.bassiana and A.germari,to evaluate the potential of the fungus as a biocontrol agent for A.germari,which can provide the basis for the application of the fungus in the integrated control of A.germari.The results showed that:1.The screening of the high virulent isolateNine isolates of B.bassiana were isolated from different soil using the "Tenebrio molitor Bait Method".The isolation rate was 45%.The pathogenicity of different strains to A.germari larvae was tested.The result showed that Bb00 was the most virulent one.The correct mortality was up to 100%and the infection rate was 86.3%.LTs0 was 4.13d,and LC₅₀on the A.germari larvae was determined to be 3.05×10⁶/mL.2.The study of biological characteristics of Bb00The culture medium,temperature and humidity were the most important dominant factors of the growth and pathogenicity to B.bassiana The most suitable media for the mycelia growth and conidia production of Bb00 were PPDA and Czapek,respectively.For the nutrient growth and the conidia germination,the optimal temperature and humidity were 25℃and 100%,respectively.At 25℃and RH 100%,the pathogenicity reached the highest.3.The study of infection process to the pathogenThere were some changes in color and morphology on the Apriona germari larvae infected with Beauveria bassiana Bb00.Penetration process of B.bassiana includes adherence,germination,appressoria and penetration.Through the scanning electron microscope we can observe most conidia can adhere to the cuticle after 8h.The conidia on the cuticle of A.germari larvae germinated 12h after inoculation.Most tissues and organs were infected and disintegrated with the heavy multiplication of the mycelium in body cavity until the larvae were dead.There are immune response in A.germari larvae body but the defense responses were just temporarily inhibition for the pathogen but could not clear them completely.4.The Symptom and Histopathological Study of Apriona germari Larvae Infected by Beauveria bassiana Bb00Paraffin sections showed the spores of B.bassian attached to the cuticle of A.germari larvae after 24h.After 48h,the conidium germinated and penetrated through the cuticle, the endocuticle were disintegrated.The mycelium multiplied and invaded the fat body, muscle,trachea tissue,etc nearby the point of penetration after the germ tubes into the body cavity.Most tissues and organs were infected and disintegrated with the multiplication of the mycelium,by 4d,infected larvae were dead.The hyphae emerged through the cuticle and formed hyphal layer on the surface of the dead insect 5d postinoculation and conidium emerged.