INSECTICIDES AND REPELLENTS 27 



0.1 pound of DDT per acre is recommended for initial trials. This 

 amount is contained in approximately 0.4 pint (6.4 fluid ounces) of a 

 25-percent concentrate. By comparison, 25 times this amount of a 

 1-percent oil solution (5 quarts), or 300 to 600 times as much oil alone 

 would be required for equivalent results. The great savings in trans- 

 portation is apparent. 



When the emulsion is dispersed throughout the water, 0.1 pound 

 per acre is equal to 0.1 p. p. m. in water 4.43 inches deep. Heavier 

 dosages, up to 1 p. p. m. in still pools, will prevent breeding for several 

 weeks. Where fish are present, heavy dosages are not recommended, 

 but there are many situations, such as temporary pools, shell holes, 

 log ponds, and some borrow pits, where mortality of fish is not a 

 problem. 



A quick-breaking oil emulsion rather than a stable one is sometimes 

 employed when a surface concentration of the larvicide is desirable. 

 It can be prepared readily by reducing the amount of emulsifier to 

 about 3 percent or by adding up to 4 parts of fuel oil or kerosene to 

 the standard 25-percent DDT emulsifiable concentrate. 



Wettable powders containing either 50 or 75 percent of technical 

 DDT are available. When mixed with water they form suspensions, 

 which may be used for spraying mosquito-breeding places. When 

 properly dispersed, suspensions are very effective larvicides, but 

 heavier dosages are needed than for emulsions or oil solutions. The 

 principal advantages of wettable powders over emulsifiable concen- 

 trates or oil solutions are that they are in a more concentrated form 

 and are packaged dry for transportation. There is no fire hazard or 

 solvent problem, and the water is obtained near the site of operations. 

 They may also be diluted with a dry carrier and applied in dusts when 

 this method of dispersal is advantageous. The chief disadvantages 

 are that they require frequent agitation to remain in uniform suspen- 

 sion and they are very abrasive to gear pumps when used in power 

 sprayers. 



DDT Dusts 



DDT dusts are highly effective against anopheline larvae, and are 

 particularly useful where long-distance drifting is needed to treat 

 wide breeding areas. Dusts also penetrate thick vegetation better 

 than liquids, provided the vegetation is dry. In localities where 

 adults habitually rest on vegetation around the breeding places, the 

 dust adhering to the plant cover provides additional adult control. 

 Rather heavy dust applications to abandoned septic tanks, cisterns 

 and cellars, or to rubble-filled areas, vehicle dumps, and old tires have 

 also been effective in military control of various species of disease- 

 bearing Aedes and Culex. 



DDT dust is much more toxic than paris green, which was previ- 

 ously used extensively for malaria-vector control. A dosage of 0.1 

 pound or less of DDT per acre is sufficient under usual conditions. 

 This requires 1 pound or less of a 10-percent dust, or 10 pounds of a 

 1 -percent dust. Because of the physical properties of technical 

 DDT, the preparation of a good dust requires special equipment for 

 grinding with an inert diluent, such as talc. For this reason it is not 

 feasible to prepare the original mixture in the field. A 10-percent dust 

 (No. 5, table 3) is chiefly used in the control of lice. Since this con- 



