A topical insecticide is provided which can be safe to use and avoids many common deleterious side effects of conventional topical insecticides. The insecticide contains an insecticide and an insect growth regulator effective for killing fleas, flea larvae and flea eggs. The insecticide is formulated by dissolving an insecticidal (tetrahydro-3-furanyl) methylamine derivative or a chloronicotinyl insecticide and an insect growth regulator (IGR) in a solvent containing a quaternary ammonium salt to increase the solvency of the IGR component, thereby providing an insecticide having high insecticidal activity.