Studies On Control Of Pieris Rapae With Metarhizium (VM07 Isolate)

Pieris rapae were the major Cruciferae Vegetables pests. The strategy of controllingthis pest species has mainly relied on chemical insecticides. Insecticides have detrimentalimpact on the natural enemies of Pieris rapae as well as on environment. In general,natural parasitoids and predators in cabbage fields are more sensitive to most chemicalinsecticides than the Pieris rapae. Therefore, interest in ecologically sound controlmeasures is increasing. The objective of this study was to study application ofentomopathogenic fungus Metarhizium against cabbage Pieris rapae. The results are asfollows:1, A highly virulent strain (VM07) was isolated from Lepidopteran pests larvae(tussock moth). According to the cultural and morphological characters, the isolate shouldbe identified as M.flavoviride Gams & Rozsypal var.minus Rombach. The isolate wastested against third instar larvae of Pieris rapae, Prodenia litura, Plutella xylostella,Argyrogramma agnata. Bioassay results showed that Pieris rapae was most susceptible tothe isolate infection, mortality of 100% and 92% was achieved 6 days after treatment with5×10⁸conidia/ml and 1×10⁸conidia/ml of the isolate, respectively. Prodenia litura wasthe second susceptible, the respective two dose conidia suspension of the isolate causemortality of 93% and 85%. Plutella xylostella and Argyrogramma agnata were leastsusceptible, the high dose achieved 68% and 70% of kill at the end of the bioassay, the lowdose could achieved 45% and 52% of kill at the end of the bioassay.2, The biological characteristics of the isolate were investigated by agar plate andglass slide methods. The results indicated that the optimal condition for mycelial growthand sporulation was about 25℃, pH6, PDA+ powder of Pieris rapae, carbon source from maltose or lactose, fluorescent light for 12h photoperiod. The optimum condition forgermination ofconidia was about 25℃, pH6-7, high humidity (RH≥96%).3, The effects of temperature on the virulence of the isolate against Pieris rapae werestudied in laboratory. The results indicated that the optimal temperature for the fungus tocontrol Pieris rapae was about 25℃. In addition, The effects of age on the virulence of theisolate against Pieris rapae were studied in laboratory too. There were significantdifferences in susceptibility among different age of Pieris rapae to fungal infection at 25℃tested. Younger group were generally more susceptible than older group. The effects ofMetarhizium isolate VM07, infected in the second instar larvae of Pieris rapae, on thenutrition metabolism of this insect pest were investigated in the laboratory. The resultsindicated that there were significant difference in the nutrition metabolism betweentreatment and CK. Treatment were lower than comparison significant on the nutritionalphysiology value (food consumption,weight,AD,ECI,ECD).4, Metarhizium strain VM07 showed the high pathogenicity. Under laboratoryconditions, the LD₅₀ value of strain VM07 on the 6th day after treatment of third instarlarvae was 1.5×10⁷ conidia/ml, and the mortality rate of larvae reached 93% at 6 days aftertreatment with a conidial dose of 1×10⁸ conidia/ml concentration. The results weredemonstrated in detail by time-dose-mortality modeling method. Hosmer-Lemoshow test(x²=12.26, p≥0.05) showed that the theoretic value well fitted to the observed date, andt-test(P＜0.01) indicated that the parameters ((?)_j&(?)_j) of the model reached a significantlevel. The effects of dose((?))of VM07 isolate is 1.1199.The results of a field study oflarvae in cabbage field indicated that the survival rate of larvae decreased withMetarhizium strain VM07. The highly virulent isolate (VM07) may provide a potentialmicrobial control agent of Pieris rapae of vegetable crops.