Field investigation revealed that emulsified oils of Cotton seed, Sesame and Soybean enhanced the activity and persistence of the bio-product, Spinosad 24SC, Dipel 2x 6.4 Wp and Protecto 9.4 Wp against the 2 nd and 4 th instar larvae of the cotton leafworm, S. littoralis (Boisd).Physico-chemical properties (Suspensibility, Viscosity and pH value) of tested biocides either alone or in combination with vegetable oils were also determined under field dilution rates (200 Lw/fed).The obtained results clearly showed that ,all the candidate oils were compatible with the tested bioinsecticides, they gave homoganic suspension and increased their viscosity and pH value. Sesame oil scored the highest increase in both viscosity and pH value. The data proved that all the tested oils increased both initial kill and residual activity of candidate biocides. The most evaluated promising synergism, occurred when Sesame oil at 0.3% mixed with full recommended doses of Spinosad, Dipe l2x and Protecto, it showed a remarkable effect in increasing both initial and residual effect after time intervals i.e. zero, 3,7,9 and 14 days post application. It caused 100% larval mortality at the initial time (zero-day) against both 2 nd and 4 th instar larvae of S. littoralis, as compared with biocides alone. Also, residual activity was increased from 49.9, 30.15 and 24.34% to 95.05, 85.13 and 77.25%, respectively against 2 nd instar and from 30.15, 12.77and 12.5% to 79.75, 49.15, and 40.4% against 4 th instar larvae of S. littoralis after 14 days post application. In addition, that all the treated larvae were biologically affected by the three tested biocides alone or in combination with the three vegetable oils. The effect varied according to the larval instar, tested biocide and added oil. Therefore, the treatment of larvae were resulted in decrease the pupation and adult emergence %,pupal weight , adult longevity, fecundity, eggs hatchability and increased the larval and pupal duration and induced the larval,pupal, adult malformations% and shifted the sex ratios% of males and females ,as compared to that of control. The synergistic effect may be due to the addition of vegetable oil that could increase the viscosity of bio-insecticides spray solution. This would increase the deposit on the treated plant leaves, reduce the drift and increase the persistence and improving the target pest control.Also, addition of vegetable oils inhibits respiration and thus, in turn may synergize the toxicity of Spinosad that can act on the nervous system.Also, to change pH of the gut, being more alkaline and thus enhancing the endotoxin breakdown and release of toxic fragments of Dipel 2x and Protecto.