Insecticidal activity of selected actinomycete strains against the Egyptian cotton leaf worm Spodoptera littoralis (Lepidoptera: Noctuidae).

Actinomycete strains isolated from soil samples of Saudi Arabia and Egypt were used in the present study to investigate the biological activity of their secondary metabolites on the cotton leaf worm Spodoptera littoralis. Secondary metabolites of 41 strains were given to the newly molted last instar larvae through the food plant (Castor leaves). Many actinomycete strains (58% of all) caused larval mortality ranging from 10-60%. Treatments with strains no. 7, 36, 41 resulted in significant prolongation of larval life (10.0 +/- 1.26, 11.5 +/- 0.7 and 10.6 +/- 1.5 days Vs 9.2 +/- 0.4 of controls). Pupation was affected by the actinomycete treatments (60.1% of all). Success of pupation ranged only from 20 to 80%, depending on the efficiency of the tested secondary metabolites. In contrast, the pupal duration had not been significantly affected but 19.5% of actinomycete strains halted the pupation, because different percentages of pupal deformities were counted (100% pupal deformation after treatment with secondary metabolites of actinomycete strains no. 36). In addition, considerable lethal effect of some actinomycetes were observed on pupae (60, 42.9, 40, 50 and 66.6% pupal mortalities caused by secondary metabolites of strains: 2, 9, 16, 18 and 26, respectively). Secondary metabolites of 34.1% of all actinomycete strains blocked the adult emergence. Depending on the available data, Streptomyces and Streptoverticillum were found the most potent actinomycetes affecting the biological and physiological criteria of the present insect species.