The present work was performed to study the synergistic effect of this new natural formulation on the biological aspects of the 2nd and 4th instar larvae of Spodoptera littoralis and Spodoptera frugiperda. Results showed that the mortality rate of 2nd instar larvae was higher than that of 4th instar larvae of S. littoralis. While S. frugiperda did not feed on castor bean leaves treated with a mixture containing neem oil ("azadirachtin") and fruit seeds combined with Bacillus thuringiensis Berliner (Bt), 500 ppm of the new formulation induced 50% mortality of 2nd instar larvae of S. frugiperda after 48 hrs., while 100% mortality was induced after treating the same instar with 3000 ppm. This new formulation could be used, within integrated pest management programs, for field control of S. littoralis and S. frugiperda.