Isolation of local isolates of Bacillus thuringiensis from forest soils of Golestan province and evaluation of their toxicity on larva of Helicoverpa armigera (Lepidoptera: Noctouidae)

Bacillus thuringiensis (Bt) is a Gram-positive, sporulate, soil-borne, aerobic and rod-shape bacteria which is the most important microbial control agent for agricultural pests such as Lepidoptera, Coleoptera and Diptera. Effective control of these pests is based on finding and applying the native isolates of each country or region. The first step as the basic for research on Bt, is isolation of native strains and keeping of them as a gene bank of this bacteria. In this research, 24 samples of forest soils in different regions of Golestan province were screened using the method of selective Sodium acetate and 60 numbers isolates of obtained Bt. Initial soil samples were cultured on LBA medium culture and the initial selection of colonies were performed based on morphological shapes of Bt colonies on medium culture and final selection based on microscopical identification of their crystal proteins. In microscopical study, the most of isolates were formed bipyramidal (56/6 %). Then, Bioassay of these native strains on the third age of Helicoverpa armiger larvae was studied. The results of Bioassay evaluation showed that these isolates have significant toxicity effect on these larvae and showed about 55-80 percent mortality. Generally, the results of this research showed that the forest soils are a suitable source for finding effective isolates of Bacillus thuringiensis.