Phytolarvicides represent a prosperous store for the discovery of novel pesticides that are effective, cheap and environmentally safe. The main targeted insect pests are mosquito vectors of most serious human and animal diseases in the world, such as the mosquito Culex quinquefasciatus. The frequent cases of filariasis incidences are leading cause of illness and death in urban areas and Cx. quinquefasciatus is a primary vector for its transmission in India. Bioassays showed that larvae are highly susceptible to the selected plant extract, Ocimum basilicum and synthetic insecticide, imidacloprid as well as their combinations. Histopathological examination showed that a range of pre-emergent effects can occur; such as prolongation of instar and pupae durations, inhibition of larval and pupal molting, morphological abnormalities and mortality especially during molting and melanization processes. Thus, synthetic insecticides and the active ingredients in plant extracts have important effects on physiological processes of the mosquito, therefore, could implicate its life cycle and vectorial capacity for disease transmission and thus, present new source for the development of insecticides against mosquitoes and other vectors.