The present invention relates to a new herbal synergistic formulation, comprising essential oil of medicinal plant Foeniculum vulgare and other plants and said formulation useful as insecticide against mosquito larvae; in addition, the formulation of the present invention has toxic action against larval stages of malarial vector, Anopheles stephensi due to the property attributed by the synergistic combination of essential oil of plant genus Foeniculum vulgare and other essential oils derived from medicinal plants.