INSECTICIDES AND REPELLENTS 51 



operations are still feasible at 10 to 12 m. p. hi. if the spray is fairly 

 coarse, a low altitude can be maintained, and the area to be treated is 

 sufficiently wide so that a reasonable proportion of the drifting spray 

 falls within it. Great care must be taken in judging the drift of the 

 spray. At velocities above 12 m. p. h. operations are best postponed. 



In large-scale spraying in Alaska with a C-47 equipped with under- 

 wing spray booms, the plane was flown at an altitude of 100 to 1 50 feet, 

 the swath interval was 800 feet, and the output of 20-percent DDT 

 spray was at the rate of either l / 2 or 1 pint per acre to give 0.1 or 0.2 

 pound of DDT. The speed of the plane was about 140 m. p. h. 

 In other operations with the C-47 and other planes of equivalent size 

 the swath width has ranged from 300 to 900 feet. 



With smaller planes the swath width is usually set at 100 to 125 

 feet and the planes are flown at an altitude of 50 feet or less over open 

 areas. Over wooded areas the planes must, of course, be kept high 

 enough to clear the tallest trees. Planes applying thermal aerosols 

 are flown at low elevation wherever possible (20 to 30 feet) to reduce 

 loss of spray due to drifting. 



Spray pattern and droplet size. — The delivery rate of the spray and 

 the effective swath width at different elevations must be determined 

 before control operations are undertaken. Information on the spray 

 pattern and range of droplet sizes is also desirable. For this purpose 

 samples of the spray are obtained on waved or stationary slides 

 placed 10 to 20 feet apart in one or more lines at right angles to the 

 line of flight. In one method glass microscope slides, 1 by 3 inches, 

 are given a heavy deposit of zinc oxide smoke in which the falling 

 droplets leave circular craters. The droplets per square inch or 

 centimeter are then counted and the diameters of the craters measured 

 under the microscope with an ocular micrometer. Another method 

 commonly used is to treat the slides with a thin oleophobic coating, 

 such as Dryfilm, on which the impinged droplets appear partially 

 flattened. Measurements and counts are made under a microscope, 

 or a portion of the slide is photographed and the measurements are 

 then made from an enlarged projected image of the negative. To 

 convert the measured diameters into actual droplet diameters the 

 spread factor must be known. Since it varies with different solvents, 

 the factor should be determined for each type of spray solution. 



Slides waved at head height directly under the plane provide 

 information on the entire droplet spectrum; the stationary slides 

 show the range of droplet sizes and the numbers reaching the ground 

 at increasing distances from the center of the line of flight. Both 

 the proportion of droplets of different sizes or size groups and the 

 mass median diameter are usually given. The latter is the size in 

 microns of droplets at the midpoint of the cumulative volume index, 

 obtained as follows: Multiply the cube of each diameter by the 

 number of droplets of that size and divide the total of these products 

 by 2 for the midpoint. The median diameter is also of some value 

 but is considered less useful for comparison of the performance of 

 different sprays and equipment. 



Another centering constant, known as D , has been used by some 

 investigators and gives more uniform mean measurements of spray 



