50 CIRCULAR 97 7, U. S. DEPARTMENT OF AGRICULTURE 



Suspensions. — Suspensions of wettable powders may be dispersed 

 from airplanes, but then use is limited to larvicidal work. Adequate 

 agitation in the tank must be provided to prevent settling. Either 

 a centrifugal pump or a straight discharge pipe should be employed, 

 as gear or rotary pumps are not suitable because of the abrasive action 

 of the solid particles. 



Pellets and granules. — Where the area to be treated is protected by 

 trees and shrubs, some of the spray or dust is lost by sticking on the 

 vegetation. Dusts penetrate vegetation better than sprays, but in 

 dense growths it mav even be impossible to get an effective quantity to the 

 ground. In an effort to overcome this difficulty in reaching breeding 

 places of mosquitoes and salt-marsh sand flies, pellets or granules im- 

 pregnated with insecticides, which are heavy enough to penetrate 

 thick growths, have been developed. This type of preparation is also 

 being used in the control of soil-inhabiting insects. A granular form 

 of bentonite clay of about 16 3o mesh, called the quick-dispersant pellet 

 form, was found promising for mosquito control when tested in rice- 

 fields by the Arkansas Agricultural Experiment Station and when 

 tested in wooded or grassy salt-marsh areas in Florida by the Orlando 

 laboratory. In Arkansas the pellets were impregnated with xylene 

 solutions, whereas in Florida benzene solutions were used. Acetone 

 was not suitable, as it sometimes caused the granules to swell and dis- 

 integrate. The solvent should evaporate to dryness so that the pellets 

 will flow freely from the dust hopper. 



Another advantage of a granulated insecticide is that it can be 

 applied throughout the day at wind speeds much higher than is 

 possible with ordinary sprays. Also, the residues will probably be 

 greatly reduced on forage in livestock areas. On the other hand, the 

 effective swath width is much narrower than for sprays, more flying- 

 time is required, and suitable equipment must be installed in the 

 plane. 



Dusts. — The paris green dust formerly used in anopheline control was 

 applied at the rate of about 10 pounds of a 10-percent dust per acre. 

 Thus a payload of 500 pounds was sufficient to treat only 50 acres. 

 A similar load of 5-percent DDT solution applied at the rate of 2 

 quarts per acre would cover about 120 acres, or more than twice as 

 much area. At 1 pint per acre of the 20-percent solution the same load 

 would cover nearly 10 times that area. DDT dusts are effective at 

 considerably lower dosages than paris green, but are limited to 

 larvicidal treatment, whereas solutions are useful against adult mos- 

 quitoes as well. Moreover, a DDT dust is generally less effective 

 against culicine larvae than a DDT spray. For these reasons dusts 

 now are seldom employed except where planes equipped for crop 

 dusting are the only- ones available. 



Factors Affecting Aerial Spraying 



Altitude, wind velocity, and swath width. — The altitude at which spray 

 missions are flown ranges from about 20 to 150 feet, depending on the 

 nature of the terrain, the height of the vegetation, the size of the 

 plane, the wind velocity, the droplet size, and the desired swath width. 

 Wind velocities less than 5 or 6 m. p. h. are the most favorable, but 



