A topical formulation that inhibits the growth of microorganisms comprises an effective amount of an insecticide dissolved in an oil phase comprising a water-miscible or water-soluble surface active agent, a suspending agent, and a non-ionic surfactant, and an aqueous phase comprising one or more preservatives, where the aqueous phase is buffered to a specific pH. The overall antimicrobial efficacy in an emulsion formulation can be enhanced by buffering the aqueous phase of the emulsion to a pH at which the preservative system inhibits growth for a wider variety of microorganism types in the formulation relative to the equivalent formulation in which the aqueous phase is not buffered.