Conventional aerosol as a delivery system for hydrophobic insecticides, formulated with hydrocarbon or freon type propellants [(A46) or Freon 11/12] are derived from matrices based on non-aqueous organic solvents, i.e., either hydrocarbons or halogenated hydrocarbons. Such systems pose potential environmental hazards like high flammability (hydrocarbon emission) and depletion of the ozone layer from fluorinated hydrocarbons, and emission of chlorinated hydrocarbons as cancer suspect agents. Totally aqueous systems are not easy to formulate in a single phase system as are aerosols. While O/W microemulsions are described in the literature, their use as trigger spray or aerosol systems produced low knockdown rates (speed of ‘kill’). A W/O microemulsion which will accommodate high levels (⩾ 35%) of conventional hydrocarbon propellant (A46) would be safer and will improve the knockdown rate.

This paper describes our effort in successfully formulating such W/O microemulsion systems. A systematic approach to stabilize W/O microemulsions that can accommodate high level of water (25–40%) as well as high level of hydrocarbon oil and hydrocarbon propellant (40–50%) based on partial phase diagrams produced several prototype formulations. These formulations matrices essentially consist of: nonylphenol ethoxylates as primary emulsifiers and long chain (C₈) alkyl pyrrolidone/pentanol/ glycerol as cosurfactant/cosolvents, C₁₂ hydrocarbon and water. Mixed pyrethroids and propellants can be loaded at appropriate levels.

Examples of prototype formulations, stability data, and biological efficacy are provided. A working model that would explain the high biological performance is also provided.