Aerosols for Insects

by RANDALL LATTA and L. D. GOODHUE

INSECTICIDES in aerosol form have become, since 1940, an important means of controlling insects. Aerosols had just emerged from the laboratory and were undergoing practical testing when the United States entered the Second World War. They were immediately adopted by the military forces to combat disease-carrying insects, such as the malaria and yellow-fever mosquitoes. Because of accelerated research and widespread use during the war period, the aerosol today is a household word and item.

The modern insecticidal aerosol is simply a very fine spray, so fine that the individual particles will stay suspended in air for some time. By staying suspended, the minute particles of insecticide have greater opportunity to touch an insect than larger particles, such as those in an ordinary fly spray, which rapidly fall to the floor or ground. Likewise, the ability of an aerosol to remain airborne makes it possible to drift an aerosol cloud for long distances under the proper meteorological conditions. When the particles of an aerosol are liquid, a fog is formed, whereas solid particles form a smoke. At present the most efficient aerosols are composed of liquid particles and are true fogs, not smokes.

Aerosols are efficient when properly used because of the minimum of wastage. For example, enough aerosol to contain only 5 milligrams ( one small drop) of pyrethrins will kill all yellow-fever mosquitoes in a thousand cubic feet of space.

The particles in an aerosol range anywhere from 0.5 to 35 or 40 microns in diameter. (One micron is 1/25,000 of an inch.) Above this size, particles tend to be airborne for shorter and shorter periods as their weight increases.

Particles of an aerosol come in contact with an insect by settling on it through gravitational fall or by striking it through motion of the particles or the insect. When aerosols are used in confined spaces, such as a room, the contact is partly by the first method, and the time necessary for enough particles to accumulate on a resting insect to cause its death is related to the rate at which the particles settle.

It would be necessary, for instance, to expose an insect 85 times as long to an aerosol composed of particles 1 micron in diameter as to an aerosol composed of particles 10 microns in diameter. On the other hand, too large particles fall too rapidly, are not diffused as well by convection currents, and reduce the number of chances of coming in contact with an insect. For example, one 40-micron particle will make 64 10-micron particles. By biological testing, an average size of 10 to 20 microns was found most effective for aerosols under confined conditions.

Outdoors, aerosols drifting parallel to the ground strike objects in their paths according to the weight and velocity of the particles and the shape of the object. Biological tests have shown that below a certain range of size the ability to strike decreases rapidly, so there would have to be a corresponding increase in the amount of insecticide. Above that range practically all particles are deposited on insects in their paths; therefore greater size would be of no benefit. The range was determined to be in the neighborhood of a diameter of 10 microns.

Because of the small dosages necessary in confined spaces, and the airborne characteristics in outdoor applications, aerosols generally leave minute deposits and are considered an unsatisfactory means of applying insecticides where a residual effect is desired, although overdosing or repeated applications may leave an effective insecticidal deposit.

Aerosols are made by several methods. It is believed that the method by which they are generated has little effect on their action upon insects, provided that they are composed of the same elements and are of the same particle size. They can be generated (I) by incomplete combustion of materials containing insecticides; (2) by spraying solutions of insecticides in oil on a hot surface, thus vaporizing the liquid which immediately condenses into a fog when the vapor is cooled by contact with air; (3) by dissolving insecticides in a liquefied gas, such as dichlorodifluoromethane, and forcing the solution through a small orifice by the pressure of the gas, where it is broken into a fine spray, which is further reduced in particle size by the immediate evaporation of the liquefied gas; (4) by heating a mixture of water and an oil solution of the insecticide until the water is converted into superheated steam, and passing this mixture of oil and steam through nozzles where it is broken up into aerosol-sized droplets; and (5) by mechanical means, such as atomizing nozzles.

The original experiments with foglike aerosols were made by spraying solutions of insecticides in oil onto a hot plate. This new method was found to be highly effective in comparison with other methods of dispersing insecticides known at that time. It was soon supplanted by the liquefied-gas method. Later the method was reconsidered in attempts to create large aerosol clouds for outdoor applications. It was determined that the particle size produced was too small for high efficiency against insects under such conditions.