The Mexican beetle, Zygogramma bicolorata Pallister (Coleoptera: Chrysomelidae) an effective biocontrol agent of P hysterophorus, was introduced in 1983 from Mexico to Bangalore. Both adults and larvae of beetle can feed on leaves. This investigation has been formulated to know the infestation, feeding and damage behaviour of Z. bicolorata on the host plant (P hysterophorus) and effectiveness to the control of Parthenium. Studied on bionomics of Zygogramma showed that the Mexican beetle lay small, elongated, light yellowish eggs on leaf surface of host plant (Parthenium) in the laboratory conditions. The average incubation period hatching percent, longevity and fecundity were observed. The adult Zygogramma beetles feed and oviposit on Parthenium leaves while the newly hatched larvae feed voraciously on the terminal and auxiliary buds, leaves, stem parts etc.