Study On The Virulence And Mechanism Of Several Entomogenous Fungi Against Pieris Rapae

In this study,4 strains of Metarhizium anisopliae and 5 strains of Beauveria bassiana were collected,as the preliminary screening,the biological characteristics of these nine strains,including growth rate,sporulation,time of initial sporulation and spore gemination rate,and conducting virulence were tested on 4 instar P.rapae by spraying methods,the experimental data was simulated using time-dose-mortality model to reveal the trends of tested insect mortality rate along with spore dose and treat time,in order to screen out the high virulent strains against P.rapae.At the same time,the activities of protective enzymes of P.rapae infected by the high virulent strains M.anisopliae Ma60 were assayed in order to define the defense mechanisms of P.rapae to M.anisopliae,the work will provide scientific basis for biologic control of P.rapae.The results are as follows:1.By the result of determinating biological characteristics of 4 M.anisopliae strains and 5 B.bassiana strains known,in respect of growth rate,sporlation,time of sporulation initial and spore gemination rate are significant different between the different strains.The basic biological characteristics of Ma60,Mf82,Bbr17 and Bbr12 strains are superior to the other 5 strains;on 15 days,the colony diameters are 73.22 mm,70.95 mm,71.44 mm and70.49 mm,respectively;the initial sporulation time is relatively short;the sporulations are5.30×108conidia/cm2,5.52×108conidia/cm2,5.65×108conidia/cm2 and 5.18×108conidia/cm2,respectively;the spore germination rates in 24 th are higher than 95%.2.The cumulative mortality rate of nine strains against P.rapae were measured,the result showed that: after vaccinating the strains Ma60,Mf82,Bbr17 and Bbr12,The cumulative mortality probability of P.rapae was 96.67%,83.33%,90% and 86.67%respectively,but the other five strains had been not reached 80% at fourteen days.Based on the test of biological index and fatality rate,Ma60?Mf82?Bbr12 and Bbr17 strains were screened out with excellent quality and higher fatality rate,and determinated the toxicity of P.rapae.The results showed that: in four strains,the strongest pathogenicity is Ma60 strain,the infection rate and corrected mortality rate were 84.1% and 96.4%,the half death time is shortest,only 2 days.The Bbr17 strain is less than Ma60,the infection rate and corrected mortality rate were 75.6% and 88.9%,LT50 is 2.85 days.On the basis of filtering out Ma60 and Bbr17 strains,by spraying conidial suspension of 103-107 conidia/mL serial concentrations of the fungi onto P.rapae,the lethal speed of Ma60 strain is obviously faster than Bbr17 strain,so Ma60 strain was selected for further analysis.A time-dose-mortality model was used to analyze the bioassay data,the estimated LC50 values of P.rapae on days 5 after treatment was 2.18?104 conidia/mL,while that of LC90 was 2.43?107 conidia/mL.The median lethal time of M.anisopliae to the P.rapae differed at various concentrations.Values of LT50 tested was shortened with concentration.In the range of 1×105–1×108conidia /mL,the LT50 values was 4.2–2.0 days.The results showed the potential of M.anisopliae for use in biological control of P.rapae.3.The method of spectrophotometry was used to assay the protein content and activities of protective enzymes of P.rapae infected by M.anisopliae,aiming at defining the defense mechanisms of P.rapae against M.anisopliae.The results indicated that,the total protein content of the infected P.rapae was obviously lower than that of the control not infected,this result showed that M.anisopliae interfered with the metabolism of protein of P.rapae;the SOD,POD and CAT activity of the infected P.rapae was increased at the tital infected stage,while dropped at the later infected stage,this result reflected that the defense system of P.rapae was enhanced at initial period,but dropped at the later period.The highest activites of SOD,POD and CAT respectively occurred at 36 hours,24 hours and 48 hours after inoculation,this result suggested that POD played important role in defending the invasion of M.anisopliae,and then SOD functions after invasion,while the last effective protective enzymy was CAT.