Efficacy of aqueous leaf extracts of medicinal plants viz., Eucalyptus globulus, Ocimum sanctum and Piper betle were evaluated against the hatching of eggs, mortality of nymphs and mortality and fecundity of adult mealy bugs (Maconellicoccus hirsutus Green) under in vitro conditions. Totally seven concentrations 1, 5, 10, 20, 50, 70 and 100 percent were tested against mealy bugs. The results revealed that with the increase in the concentration of extracts, there was a corresponding decrease in hatching, nymphal & adult mortality and fecundity in all the extracts tested. The hatching of eggs was minimum at 100 percent concentration in E. globulus (20.00%) followed by O. sanctum (30.42%) and P. betle (36.06%) over control (96.73%). The growth and development of nymphs and adults were adversely affected after the treatment. The nymphs did not attain the adult stage and there was total mortality of nymphs in different treatments. The duration of nymphal stage was reduced by 2-10 days in E. globulus, 1-7 days in O. sanctum and 1-2days P. betle at higher concentrations. However, at lower concentrations it was prolonged by 2-4 days in all the three extracts. At lower concentrations the mortality of adults was on par with the control. Similarly the adult duration was also reduced by 3-4 days at 100 percent concentration and prolonged by 5-6 days at lower concentrations in all the extracts. With the increase in concentrations of the extracts there was decrease in the fecundity of eggs. Thus, the leaf extracts of E. globulus was found to be most effective followed by O. sanctum and P. betle against mealy bugs.