An oil particle 1 micron in diameter will settle 10 feet in 26.5 hours. A particle of 15 microns will settle 10 feet in 15 minutes. In unheated buildings air currents are at a minimum, but heating sets up air convection currents that are a great aid to dispersion. Sometimes large-volume air blowers are used to aid dispersion. In an unheated room with a ceiling height of 8 feet, aerosols with a mass median diameter of 5 microns disperse fairly uniformly over an area 30 feet from the source, those of 15 microns over an area of 15 feet, and those of 25 microns less than 10 feet.

The deposit as a result of an aerosol settling is about 95 percent on the top of horizontal surfaces and the rest on walls and ceiling. The amount of deposit on a horizontal surface depends on the concentration of the aerosol above the surface so that if the aerosol is evenly dispersed throughout a room the resulting deposit will be proportional to the height above the surface.

In large closed warehouses that contain packaged food, the problem of flying and exposed crawling insects can be controlled by aerosol treatments. Aerosols of various particle sizes were tested; a size of about 5 microns mass median diameter was selected as most effective and easiest to apply. These small particles are produced by thermal aerosol generators that can be operated outside the warehouse; the fine particles are introduced through an open door. The aerosol is carried first to the ceiling. By the time the treatment is complete, the aerosol is well distributed throughout the interior by convection currents. The door is then closed. Overnight the particles penetrate into most of the cracks and crevices and settle on the top of exposed horizontal surfaces.

It is sometimes necessary to limit the time of application in some closed interiors. The particle size then must be large enough to settle out in the time available. A 10- to 15-minute exposure time is the minimum for satisfactory results. An aerosol having a mass median diameter of 15 to 20 microns is sufficient for the short-exposure application.

Equipment sometimes limits the particle size. When heat from thermal generators causes excess breakdown of the insecticide, equipment that produces larger particle sizes must be used. They then must be released from more than one point to cover adequately areas whose dimensions are larger than the distances of uniform deposit. Heated rooms will about double the dispersion area; rooms with high ceilings will add slightly to the dispersion.

When treating greenhouses, it should be remembered that foliage injury can be caused by a particle size larger than the foliage can tolerate with the type of formulations used.

Some formulas that we have used indoors are: (1) 1 pound of technical DDT dissolved in 7.5 pints of Sovacide 544C (Socony Vacuum) to make 1 gallon. (2) 1 pound of technical DDT dissolved in 2 quarts carbon tetrachloride; to it are added 3.5 pints of No. 3 fuel oil to maker gallon. This formula is relatively safe from explosion. (3) 1 quart of 10 percent pyrethrum in deobase; 1 pint of piperonyl butoxide and 1 pint No. 3 fuel oil are added.

Because of the explosion hazard when oil solutions are used indoors, not more than 1 gallon of the solutions should be used per 100,000 cubic feet. They should not be released near an open flame. Workers should wear proper respirators. The third formula, which contains pyrethrum, is recommended for use around foodstuffs.

The formulations should contain a proportion of relatively nonvolatile oil to maintain the desired particle size while it is suspended in the air. In closed warehouses, 1 pound of DDT in 1 gallon of solution per 100,000 cubic feet of space, applied about every 2 weeks in summer, will provide protection against insect infestation.

The main problem in applying the aerosols outside, other than for temporary control of flying insects, is to put down a uniform deposit. To do that the aerosols are applied as wind-borne clouds. For best results the wind should be light, steady in direction, and moving at 1/2 to 8 miles an hour. The air temperature at ground level should be a little cooler than at 6 feet or more. This surface inversion keeps the aerosol cloud close to the ground; it is most important when low-growing crops are treated and least important for trees having a canopy of foliage. Good inversion usually occurs from 1 hour after sunset until sunrise but may exist all day if rain has cooled the ground.

The dosage depends on how much has to be deposited on an acre to kill the insect. The deposit is heaviest nearest the point of release and decreases as the distance from the release point increases, because the larger particles settle first. Under the best conditions, only 25 to 50 percent of an aerosol containing particles less than 50 microns in average diameter is deposited over an open area in swaths up to 2,000 feet; most of it drifts beyond the area under treatment. In wooded places the deposit would be greater. When more than one swath is used, however, the dosage can be cut about io percent for each successive swath because of overlapping up to a total of 50 percent.

The swath width should be selected according to the location of accessible roads, to places where the wheels of the machine will do the least damage to the crop, and places where oil deposits will not injure the foliage.

Some recommended particle sizes in microns mass median diameter for various swath widths and wind velocities are as follows:

At least one-fourth of the aerosol solution should be nonvolatile. Results are best with a concentrated solution. A popular formula is 5 to 7.5 pounds of DDT dissolved in 2 gallons of benzene or xylene plus 3 gallons of SAE 10 W motor oil or agricultural oil. Oil-soluble technical BHC may be used in the formula in place of DDT. The operators should wear protective masks and clothing.

AEROSOL GENERATORS are useful in military and civilian situations in which mosquitoes and flies create problems of public health. They make it easy to clean up infestations in towns or camps and are particularly effective along the seashore. They are less well suited for the control of agricultural pests outdoors. Some problems require the use of fungicides and insecticides together, and the fungicides may be too bulky for efficient handling in aerosol generators. Field-model aerosol machines using concentrated DDT solutions have been employed successfully against gypsy moths, lygus bugs, tarnished plant bugs, pentatomids, potato flea beetles, and leafhoppers.

The aerosol machines are not suitable for treating individual trees or areas of less than an acre because the aerosol fog is placed entirely by wind drift and the initial thrust of about io feet given by the generator is sufficient only to place the aerosol in the wind.

W. N. SULLIVAN is an entomologist in the Bureau of Entomology and Plant Quarantine. He is a native of Massachusetts and a graduate of the University of Massachusetts.

R. A. FULTON, a chemist in the same Bureau, is a native of Oregon. He holds degrees from Oregon State College, the University of Wisconsin, and Stanford University.

ALFRED H. YEOMANS, a technologist in the Bureau, has been engaged in investigating and developing equipment used in insect control. He is a native of California and a graduate of Ohio State University.