Evaluating the virulence of Bacillus thuringiensis strains isolated from host and different habitats on diamondback moth, Plutella xylostella (Lep.: Plutellidae)

Regarding the importance of Bacillus thuringiensisisolates in microbial control, we attempted to collect native isolates and study their virulence on diamondback moth, Plutella xylostella. More than 148 samples were collected from Guilan, Mazandaran, West Azarbaijan and Alborz Provinces. Experimental samples including soil samples from forest, fruit gardens, agricultural fields, diseased and dead larvae, were transferred to laboratory in sterile plastic containers. The soil samples, after sieving, were processed by acetate sodium selective method at two concentrations 0.25 and 0.35 M. Insect cadaver samples were surface sterilized and dissected and body contents were placed on Nutrient agar with sterile loop. Microscopic observations, Catalase test and Gram staining were carried out for all samples. For evaluating virulence of B. thuringiensis isolates, a cabbage leaf dip method with 106 cell/ml concentration of various Bt isolates was applied. Larval mortality was recorded 72 hours after treatment. According to the results, Bt isolates varied based on morphological characteristics. All isolates were Gram-positive and catalase positive. Spores and crystals were observed in 155 Bt isolates obtained from soil and insect host. In addition, this research introduced ten strains that are highly virulent to P. xylostella and would be valuable as the potential biocontrol agents for controlling lepidopteran pests.