Pathogenic effects of Bacillus thuringiensis native Iranian soil strains on Egyptian cotton leafworm Spodoptera littoralis Z

The Egyptian cotton leafworm is a polyphagous pest with a broad range of hosts causing annually significant damage to agricultural crops. It has been found that no all strain of Bacillus thuringiesis are effective on this pest and small numbers of the isolates are effective due to the contents of their crystalline proteins. The aim of the present study was to obtain native isolates of Bt that were capable of controlling and investigate the effect of growth medium of Bt on the insecticidal ability of selected isolates. Hence, the pathogenicity of 118 native isolates of B. thuringiesis cultivated in R2NB medium were determined on five–day–old larvae of Spodptera.littoralis at 27 °C on artificial diet. The mortality rate of 118 native isolates of Bt with a concentration of 103 spore ml–1 illustrated that the highest mortality rates belong to four isolates GON–9, QM–2, GN–13 and QM–1 with 93.33, 70, 46.67 and 43.43, respectively. In addition, GN–12, EN–2, GON–7, CHI–2, AGI–7, AGI–3 and AGI–2 isolates were ineffective on S. littoralis larvae. Then, the effective isolates grown in nutrient broth and R2NB medium were evaluated on mentioned pest. The results showed that the four selected isolates were more lethal on R2NB medium than NB on larvae of Egyptian cotton leafworm and their differences were significant. Also, the content of cry genes particularly cry 2Ab in effective isolates were examined and traced. The outcomes of polymerase chain reaction demonstrated QM–1, QM–2 and GON–9 have cry 2Ab.