In this investigation the toxicity effect of eight transconjugants, four strains of Bacillus thuringiensis (Bt) and Serratia marcescence on larval body weight of Egyptian cotton leaf worm, Spodoptera littoralis were evaluated. S. marcescens produced a huge amount of chitinase in short time because of complete hydrolysis of colloidal chitin in agar medium. This enables this strain to be used in this study as a donor strain in conjugation with Bt strains. Bt transconjugants were resulted from four matings between S. marcescens as a donor strain and four recipient strains of Bt. Crystals and endospores were isolated as an insecticidal protein highly specific to S. littoralis. Almost bioinsecticides including parental strains and their recombinants significantly reduced larval weight daily if compared with untreated larvae. Some recombinants such as, Tr1, Tr2, Tr3, Tr4, Tr7 and Tr8 induced significant decrease in larval weight at the concentration of 2 ×105 ppm if compared with the mid parents after four days of treatments. In addition, older larvae tolerated Bt better than neonates. Although, the tolerated larvae still survive, feeding and consequently increasing in their weight but still less than in control. The results appeared that the surviving larvae after eight days of Bt treatments are relatively tolerant to bioinsecticides. After nine days of treatments, most bioinsecticides caused insignificantly reduction in larval weight in relation to that feeding on untreated diet. A time - dependent increase in larval weight was observed along with increase of larval period but it was still less than in control group. Bioinsecticide strains, as well as, bioinsecticide concentrations and the interaction between both of them showed significant effect on larval weight at most time intervals of larval age. Thus, a combination between chitinase from S. marcescens and Cry protein genes from Bt would result in effective insect control.

Keywords: Bacillus thuringiensis, conjugation, larval body weight, recombinants, Spodoptera littoralis, Serratia marcescens.