Thermal-generated aerosols are strictly for large-scale use. They are applied to outdoor areas by drifting the aerosol cloud across a desired area. Usually the generator is mounted in a truck and moved back and forth across a front at right angles to the direction of air movement. The cloud must be applied under meteorological conditions which will hold it close to the ground. Inversion conditions (a layer of colder air next to the ground) are the most favorable and usually occur during the periods near dawn and dusk. A wind movement of from 1 to 3 or 4 miles an hour is desirable.

The concentration of aerosols, and therefore their effectiveness, is greatest nearest the generator and diminishes with the distance. In order to obtain a high degree of control at a given distance, it is necessary to overdose the intervening area. For this reason it is more economical to treat in narrow strips than in deep areas. Mosquito adults and larvae were controlled downwind from the point of generation for distances ranging from 1,200 feet in heavily wooded areas to nearly a mile in open terrain. Gypsy moth larvae in naturally infested forests were apparently completely wiped out for distances up to 900 feet. Furthermore, enough residue remained to be repellent to migrating larvae, which prevented reinfestation in all except the marginal areas.

In recent tests by H. A. Jaynes and his associates, adult onion thrips on young cabbage plants were effectively reduced for a distance of 200 feet but the nymphs were not controlled; cabbageworms on cauliflower were reduced 85 percent at this distance; species of Empoasca were controlled for 150 feet in parsnips, in a weed field for 600 feet, and on alfalfa for 250 feet; beet armyworms were reduced over 90 percent in a weed field for 500 feet, completely killed for 100 feet in broccoli, and over 70 percent for 200 feet in tomatoes; Lygus bugs were reduced 90 percent for 250 feet in alfalfa. Unsatisfactory results were obtained against insects on very low growing plants or with low dense foliage, such as red spiders on celery or aphids on potatoes.

Thermal-generated aerosols were found to be very toxic to adult horn flies on cattle. Herds can be quickly treated and enough residue remains to prevent reinfestation for several days.

Aerosols have been created by various types of mechanical dispersion, particularly by various methods of atomization. In one method, an oil solution of insecticide is sprayed into a stream of high-velocity exhaust gases, where it is broken up into aerosol-sized particles. A simple venturi arrangement is attached to the exhaust pipe of an airplane or vehicular motor for this purpose. One type of a proprietary generator utilizes this principle also by burning gasoline or fuel oil in a forced draft of air, thus producing a high velocity-gas stream which is used in the same manner to break up an oil solution into an aerosol. Other kinds of atomizing nozzles and also whirling disks are being investigated.

THE AUTHORS

Randall Latta is an entomologist in the Bureau of Entomology and Plant Quarantine. Since 1942, in addition to other duties, he has been in charge of the development of new methods and equipment for applying insecticides. He made the initial tests with army screening smoke equipment in an attempt to convert it to make insecticidal aerosols, and later supervised a study of the use of heat-generated aerosols for outdoor control of insects. As a part of these studies, the Department carried on cooperative work with the National Defense Research Committee on the fundamental characteristics of aerosols in relation to particle size and speed of motion.

L. D. Goodhue was a chemist in the same Bureau until he resigned December 28, 1945. He also is a native of Iowa. Dr. Goodhue was a cooperator in the original studies on insecticidal aerosols; he was a co-inventor of the liquefied-gas aerosol and has been a leader in the development of these aerosols since their origination. He Played an important part in the military adaptation of aerosols, through his chemical studies on formulations.