52 CIRCULAR 9 7 7. U. S. DEPARTMENT OF AGRICULTURE 



samples than the mass median diameter. It is based on the ratios 

 of surface area to volume of the different-sized droplets. A rough 

 approximation may be obtained with the equation 



D =S<Pn/Sd 2 n 



in which S indicates the summation of the diameters, d, cubed or 

 squared, times the number of droplets, n, of that size. However, 

 this simple equation is unduly affected by the presence of a few 

 large droplets, and for the scattered measurements usually obtained 

 in small sample counts the differential equation developed by Japan- 

 ese workers and a graphic method of solution as suggested by Amer- 

 ican workers should be used (Kruse et al. 11). This method is simple 

 to perform. 



Effective swath width and dosage. — The effective swath width can 

 be estimated roughly from the number of droplets per unit area, 

 if the minimum number required to give an effective kill with the 

 spray equipment and formulation employed is known. This infor- 

 mation may be obtained by placing dishes of larvae at each slide station 

 and observing the mortality at the end of 24 hours. The average 

 droplet count at the farthest station showing mortalities of 90 to 

 95 percent is then taken as the minimum effective number. 



The approximate volume of spray per unit area can also be esti- 

 mated by determining from the diameter measurements the volume 

 of the droplets recovered . The size of the samples that are measured, 

 however, is limited, and the effect of evaporation of the solvent on 

 the size of the droplets is difficult to estimate. Much more accurate 

 estimation of volume or dosage is obtainable with an indicator dye 

 or by chemical determination of the actual amount of DDT in samples 

 collected on glass or metal plates. In the color method 0.5 to 1 

 percent of an oil-soluble dye, such as Du Pont oil red, is added to 

 the spray solution, and samples are usually obtained on two plates 

 from 6 to 12 inches square at each station. After the spray has 

 settled, the plates are collected and the pairs placed together face to 

 face for transport to the laboratory, where they are washed with a 

 measured amount of acetone and a color reading is taken with a 

 spectrophotometer. The readings are then compared with a standard 

 curve for conversion to a quantitative basis. Because this dye 

 fades rapidly in bright light, the plates must be collected and placed 

 in a darkened container as soon as possible after the spray has fallen. 

 Figure 22 is a typical graph of the spray deposit recovered at different 

 distances from the line of flight. 



Considerable attention has been given to the development of a 

 more rapid method of estimating spray deposits in control operations, 

 especially one that does not require the addition of a dye or other 

 material to the spray solution itself. The use of white paper cards 

 treated with the dye solution as tested by the Division of Forest 

 Insect Investigations of the former U. S. Bureau of Entomology and 

 Plant Quarantine and the Division of Forest Biology of the Canada 

 Department of Agriculture appears to be a promising method (Davis 

 and Elliott o, Brown 2). Oil droplets falling on the cards make 

 spots that can be counted and measured. If the spread factor for 

 different-sized droplets on such cards has been determined, a rough 



