Aerosols and Insects

W. N. Sullivan, R. A. Fulton, Alfred H. Yeomans.

An aerosol, like fog or mist, is an assemblage of particles suspended in air. An insecticidal aerosol has particles whose diameters range from 1 to 50 microns from 1/25,400 to 50/25,400 inch.

Insecticidal aerosols are dispersed in air by burning organic material, atomizing mechanically, vaporizing with heat, or liberating through a small opening an insecticide that has been dissolved in a liquefied gas. In the last the liquefied gas evaporates and leaves small particles suspended in air.

Many householders have become acquainted with aerosols in small containers so-called bombs, although of course they are not explosive. A more general application has been in use a long time. The Mono Indians of California knew the value of smoke in stupefying insects so that they could be easily collected for food. They prepared a smooth floor under trees containing the full-grown larvae of the pandora moth and built a smudge fire. The smoke caused the caterpillars to drop to the ground in countless numbers. They were then raked into the fire, partly cooked, dried, and later eaten as a stew.

Another example of an aerosol was seen in the Northeastern States one day in September 1950, when the sun turned an eerie purple and darkness came at 2 p. m. A mass of cold air had drifted down from northwestern Canada and brought along the smoke from forest fires in the Alberta and Mackenzie district an illustration of how aerosols can be dispersed in air currents from one point through large areas.

Aerosol bombs were developed in1941, when L. D. Goodhue and W. N. Sullivan of the Bureau of Entomology and Plant Quarantine discovered that aerosols produced by spraying a solution of liquefied gas and insecticide through a small hole into the air were highly toxic to mosquitoes and flies. The aerosol solution was made by dissolving pyrethrum and sesame oil insecticides in a liquefied gas commonly used in household refrigerators and called dichlorodifluoromethane. The liquid has a vapor pressure of approximately 75 pounds per square inch at room temperature.

The aerosol solution is held in a strong steel container with an outlet tube to the bottom. In operation, the vapor pressure of the liquefied gas is sufficient to force the solution out of the tube and into the air through an orifice that may vary from 0.013 to 0.024 inch in diameter. The gas immediately evaporates and the tiny particles of insecticide are dispersed as a fine mist.

The scientists knew the gas was nontoxic and non-inflammable, and they found it to be nontoxic to man and animals when they mixed it with insecticide. The ease of application, the high concentration of insecticide, and the ability of the small aerosol particles to disperse and to stay suspended in the air for a long time fulfilled requirements for a good household insecticide. A public service patent was issued on the invention and assigned to the Secretary of Agriculture for the free use of the people of the United States. Licenses are issued, royalty free, for the manufacture, use, and sale of products produced under the patent.

So urgent was the need for a better way to kill mosquitoes and flies in war zones and so good was cooperation of the Department of Agriculture, the military, and industry that our troops used aerosol bombs within a year after they were discovered. Throughout the war the bombs were highly efficient against disease-carrying insects in barracks, mess halls, tents, and foxholes. They became standard equipment in long-distance airplanes, in which they were used to prevent the spread of hitchhiking insects. Occasionally one was used to cool the beer of a jungle fighter. In all, more than 40 million aerosol bombs were made for the Armed Forces.

At the end of the war aerosol bombs were made for civilian use. Strong, inexpensive containers and suitable pushbutton valves were developed. New low-pressure propellants and solvents were perfected. The earlier formulations were modified to include combinations of pyrethrum, a pyrethrum syneigist, and DDT or methoxychlor.

Making low-pressure aerosol containers is a growing business. It has expanded to include deodorants, disinfectants, and other products, besides insecticides. It amounted to 33 million dollars in 1949, with prospects of going above 100 million dollars in later years.

THE AEROSOL BOMB is a good servant in kitchen, pantry, living room, bedroom, and cellar. Before it is used, the windows and doors of a room should be closed. Pets, birds, fish bowls, and food should be removed or covered. The container should be held upright with the opening away from the face, so the aerosol will go toward the ceiling. The operator walks around the room to give a good initial distribution. The bomb should not be held closer than 3 feet to any object, or the aerosol may stain furniture, wallpaper, curtains, or draperies. It can be held 6 to 12 inches from baseboards and cracks where insects like roaches and ants crawl or hide.

In b treating the average room (1,000 cubic feet) to control such fliers as the mosquitoes, house flies, sand flies, black flies, gnats, and moths, the bomb valve should be opened to release the aerosol for about 6 seconds. This dosage will also kill some types of ants, but is not effective against the larvae Of clothes moths. A 15-second release will kill fleas, wasps, and hornets. Roaches can be decimated but it takes at least 2 minutes of spraying per room and a whole bomb (12 ounces) for good results in the cellar. Spiders are hard to kill with aerosols.

The room should be kept closed for 10 or 15 minutes after treatment for flying insects and ants, 30 minutes for fleas, and 1 hour or more for roaches. Then the room may be aired out, but that is not necessary.

Gas-propelled aerosols are widely used to control insects in greenhouses. They cut the usual time for treating greenhouses from 48 man-hours to io minutes, eliminate black spot on roses, and can increase production 25 to 50 percent, depending on the degree of infestation.

The formula originally developed for greenhouses contained 1 percent of hexaethyl tetraphosphate and go percent of methyl chloride. Parathion partly has replaced hexaethyl tetra-phosphate because it has lasting effect. Because parathion gives unsatisfactory control of resistant spider mites and aphids, new materials and formulas had to be found. Tetraethyl dithiopyrophosphate came into use in localities where resistant insects have been found. A newer material, octamethyl pyrophosphoramide, applied as an aerosol, has appeared promising against resistant greenhouse insects.

Liquefied gas aerosols have been used also against such insects as pea aphids on peas in the field. The liquefied gas is released close to the peas through nozzles on a boom that has a shield above it. The aerosol thus is distributed so that much of it is held near the plants for a long enough time.

The work that led to the development of the aerosol bomb started with a study of insecticidal smokes. Insects were subjected to a burning mixture of derris or pyrethrum, cornstalks, and sodium nitrate. The mixture burned like Fourth-of-July fireworks, and the smoke did kill insects, but such dispersal of nonvolatile or slightly volatile insecticides was wasteful.

The next step was to spray oil solutions of rotenone and pyrethrum on a hot plate. On contact with the heated surface (about 375 C.), the droplets were partly vaporized and formed particles of aerosol size. Aerosols so produced are called heat-generated. It is an efficient way to produce insecticidal aerosols.

Aerosols were then produced in the same way by spraying them onto the inner walls of a tube heated with electricity. After that, the hot exhaust of a small gasoline engine was used as the source of heat energy. From that came the suggestion that an Army smoke-screen generator be used to produce insecticidal aerosols to treat large areas for mosquitoes and flies.

Smoke was formed in the generators by running a mixture containing a little water in oil through coils passing through a combustion chamber, which was heated by an oil or gasoline burner. The oil-water mixture was completely volatilized by the heat and condensed into a smoke on contact with the outer air. The aerosol particle thus created was ideal for an Army screening smoke because it gave a good scattering of transmitted light and remained suspended in the air for a long time. It turned out, however, that the particle size was too small for efficient insect kill. A larger particle size and better insect kill was obtained by using a 50-50 mixture of water and oil and operating the machine at a lower temperature.

Another Army smoke generator used incomplete combustion to produce smoke. It was later modified to an insecticide aerosol generator by using a gasoline motor to drive a rotary air pump. The pumped air passes through and is heated in a gasoline-burning combustion chamber regulated at 482 C. The hot air then passes through special nozzles into which the insecticide is injected. The particle size is regulated by the flow of insecticidal solution through the nozzle.

Several methods have been used since the war to generate aerosols on a large scale. One machine uses a number of spinning disks to break up the solution. One uses the exhaust gases from a small pulse-jet engine. Another employs steam to atomize the solution as it issues from a nozzle.

Many small indoor types of aerosol generators have been placed on the market. They use electrically driven spinning disks, rotors, and pressure pumps, electrically generated vaporizers, and steam atomizers. One of the machines uses extremely high pressure generated by hand-pumping the liquid solution against a fixed charge of nitrogen. There are also convenient packages of mixtures for partly burning and releasing the insecticide as a fine aerosol smoke.

FOR BEST RESULTS, the farmer or health official must study his problem in detail before applying an insecticide with an aerosol generator. The machines can be set to produce different particle sizes. The choice of machine depends on the use to which it is to be put, whether for insects in confined spaces, flying insects, or those that attack his field crops. These principles may guide the prospective purchaser:

Aerosols are used indoors effectively as a way to control flying insects and to apply a light deposit on the top of exposed horizontal surfaces.

The particle size has a bearing on the effectiveness of the aerosol. The particle size is critical for the amount that collects on an insect as it flies through the aerosol. Particles that are too small are deflected from the flying insect as smoke is from a moving automobile. Particles that are too large settle rapidly, and their dispersion is poor; therefore their chance of touching the insect is also poor. When an insect does collide with an oversized droplet, the excess insecticide is wasted. Our research has shown that the best particle size to use for flying insects is between r and 20 microns mass median diameter.