Cassava (Manihot esculenta Crantz) is a food crop with tuberous roots that plays an important role in feeding populations. The need to control cassava pests led to testing the effect of the biopesticide NECO 50 EC on Phenacoccus manihoti in a plot of the city of Grand-Lahou. The effectiveness of the biopesticide NECO 50 EC was tested on larvae and adults of P. manihoti, in comparison with a conventional insecticide K-OPTIMAL 35 EC. Dilutions gave 5 respective concentrations of NECO 50 EC: 8.33 g/l; 4.54g/l; 3.12g/l; 2.38 g/l and 1.92 g/l, and a concentration of 0.093 g/l for the control insecticide. Spraying of cassava plants infested with mealybugs was undertaken and observations were made 24 hours, 48 hours and 72 hours after treatment. On the larvae, the highest rate (92.07 ± 0%) is obtained at a concentration of 8.33 g/l for NECO and 98.51% for the control insecticide. In adults for NECO the highest rate (62.50 ± 0%) is obtained at the concentration of 8.33 g/l and 96.17% for the control insecticide. The biopesticide NECO 50 EC could be used as an alternative to the excessive use of synthetic insecticides to reduce the damage of the pest P. manihoti.