this work aims to study the impact of an organophosphorous insecticides (profenofos) and I.G.R (Pyriproxyfen) on the whitefly and its parasitoid Encarsia Formosa Gohan. Experiments were achieved in field simulators containing cotton plants at Rothamsted (England). Data revealed that: with no insecticide application and in the absence of parasitoids, B. tabaci numbers began to increase from day 21 after B. tabaci release, when the offspring of the initial numbers began to emerge as adults resulting in a shrp increase in population size. By day 51, the population was so large (>20,000 individuals).