Determination of genes encoding crystal proteins of Bacillus thuringiensis Berliner isolated from silkworm environment and the insecticidal activity against Helicoverpa armigera (Hubner)
1998
Ratchanee Siriyan
Litters of silkworm including larval feces and dust from silkworm insectaries were collected from Burirum, Sisaket, Mukdahan, Khon Kaen, Sakon Nakhon, Amnat Charoen, Udon Thani, Roiet, Ubon Ratchathani, Surin, Loei, Nong Khai and Nakhon Ratchasima. Fourty-five out of 217 samples were found to contain B. thuringiensis. From a total of 289 colonies of spore-forming bacteria recovered, 161 were identified as B. thuringiensis. All 161 isolates of B. thuringiensis were screned for their insecticidal activity against the American bollworm, Helicoverpa armigera. The preliminary test using sporecrystal suspension showed two levels of mortality, higher than 70 percent and lower than 50 percent. Twenty-three isolates that demonstrated their toxicity higher than 70 percent were selected for final screening test using solubilized crystal proteins at the concentrations of 50, 10, 2 and 0.4 micro g/ml of diet. B. thuringiensis isolates No 154, 118 and 156 were considered toxic to H. armigera, with LC50 values of 6.09, 7.43 and 9.16 micro g/ml, respectively. Polymerase chain reaction (PCR) was used for the identification of genes encoding the crystal proteins of the 23 selected isolates. The PCR product profiles indicated that 22 isolates of B. thuringiensis contained fragments of genes cryIAa, cryIAb and cryIAc with the expected sizes of 724 bp, 238 bp and 487 bp, respectively. The only one exception B. thuringiensis isolate No 154 gave PCR products of 238 bp and 487 bp which were cryIAb and cryIAc and the extra fragments of 414 bp and 883 bp representing cryID and cryIE, respectively. This study introduces some promising B. thuringiensis isolates collected from silkworm environment to be further developed as the modern and effective control agent for the bollworm.
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