Virulence And Pathology Of Beauveria Spp., Metarhizium Spp., And Isaria Spp. Against Two Different Insect Species

Biological Control Agents have gathered considerable attention by scientists worldwide and are popular as comparatively safe,efficacious and posing minimal risks and can be applied innundatively.Entomopathogen's pivotal role in regulation of insect population in nature elucidates them as the earliest insect pest control agents.Among all entomopathogens,fungi possess significant importance vis-a-vis bacteria and viruses as it enters though insect cuticle unlike needs to be orally ingested in case of others.It is well known that complex interactions occur between entomopathogenic fungi and their host insect where pathogenic fungi mannerly follows the sequential steps of interaction/infection in order to kill the insect.Till date many attempts have been made for understanding meticulous details underlying physiological and biochemical mechanisms involved in infection but not all of the questions been answered yet.The study conducted elucidates the efficacy and virulence of three different entomopathogenic fungus?EPFs?genera Beuveria spp.,Isaria spp.,and Metarhizium spp.as an alternative management for insect pests using Mythimna separata and Tenebrio molitor as insect models.Selected host's defense enzyme activities were analyzed during the infection to understand the interaction of the fungi and insects during the infection.Bioassay results in general,suggested that the strains used were pathogenic with varying level of virulence to both tested insect hosts and did not possess host specific requirements.In the experiment with T.molitor as a host insect,Qin-21?B.bassiana?and YYC-091?M.robertsii?were among the most virulent isolates of all in order with LD₅₀ of 9.12×10⁴ and 3.4×10⁵ conidia/g and Mean Survival Time?MST?of 3.44±0.04 and 4.33±0.07 days at 2×10⁸ conidia/g for respective treatments,whereas BSH-03?B.asiatica?was the least virulent of all with LD₅₀ of 4.1×10⁷ conidia/g.Surprisingly,isolates of Isaria spp.outcompeted Beauveria and Metarhizium spp for their virulence in M.separata as host insect.ILDS-03 and ILDS-01 isolates of I.cateniannulata were the most virulent isolates in order with LD₅₀ of 1.24×10⁰⁶ and 1.71×10⁰⁶ conidia/ml respectively and mean survival time of4.73±0.16 and 3.64±0.12 days at 2×10⁰⁸ conidia/ml dose of infection respectively.We then studied the changes in activity profiles of anti-oxidative enzymes:peroxidases?PODs?,catalase?CATs?,superoxide dismutases?SODs?and detoxification enzymes:glutathionhe S-transferase?GSTs?and carboxylesterase?CarE?and alkaline phosphatase?AKP?in both host insects.peroxidase?POD?and carboxylesterase?CarE?enzyme activity were inhibited throughout the infection period?POD,P<0.01?by the most virulent isolate Qin-21,suggesting the selective suppression of defense related enzymes for the effective pathogenicity.Further,the highly virulent strains of Isaria spp.ILDS-03 showed consistent reduced enzymatic profiles for SOD,CAT,CarE throughout the time period from the 6-hpi but for POD,AKP reduced activity started from 6 hpi.Similarly,for ILDS-1 the second most virulent isolates continuously reduced the activity of GST and CarE.Whereas,Metarhizium isolates with less virulence showed consistent elevated enzymatic activities of GST and CarE complying to the effect of virulence in the host defense responsive enzymes.Scanning electron microscopy revealed that Qin-21 and YYC-091 have comparatively faster germination rate within 12 hpi followed by vigorous extension of germ tube for later one within 24 hpi.The infection process of ILDS-03 isolates studied through transmission electron microscope in different time points after infection showed effective penetration of the insect's cuticle and epidermis at 12 hpi followed by proliferation in body cavity at 48 hpi,which extend towards epidermis at 72 hpi and mycosis has been observed at 96 hpi.The study showed that different EPF can have varying level of virulence against host species,and the virulence differences were related to downregulation of selective enzymes of the host insects.It gives fundamental knowledge to understand the interaction of entomopathogenic fungi and their host insects and a valuable mark to select high virulence strain of entomopathogenic fungi.