Aerosols for Insects

L .NSECTIGIDES 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 Uquid, a fog is formed, w^hereas solid particles formi 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 examiplc, 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 anyw^here 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 w^eight increases. Particles of an aerosol come in contact with an insect by settling on