Parthenium hysterophorus L. (Asteraceae) commonly known as Parthenium weed, is a highly invasive weed that is considered as environmental, medical, and agricultural hazard. The objectives of this study was to test the impact of Zygogramma bicolorata Pallister (Coleoptera: Chrysomelidae), a classical biocontrol agent, in different ratios (1-pair, 2-pairs and 3-pairs) at three different growth stages (flowering, pre-flowering and young) of Parthenium weed to estimate the effectiveness of this agent in reducing its growth and reproduction potential. The damage (defoliation %) recorded was between 44-57% at flowering, 40-54% at pre-flowering and 100% at young stage of the weed. The defoliation significantly reduced the weed biomass, height and seed production. The damage inflicted by Z. bicolorata was more pronounced when it was applied in higher density and at early growth stages of the weed.