72 CIRCULAR 9 7 7, U. S. DEPARTMENT OF AGRICULTURE 



If such measurements are difficult to obtain, the larvicide may be 

 applied at 0.025 to 0.05 pound of DDT per acre of water surface. 

 These amounts are contained in Y 2 to 1 pint of a 5-percent DDT solution 

 or emulsion, and are applied to 32 feet of stream width if the treat- 

 ment interval is % mile or 8% feet for intervals of 1 mile. The appli- 

 cation should be spread over a period of 5 to 15 minutes. With an 

 average stream depth of 6.4 inches, a velocity of 2 m. p. h., and an 

 application time of 10 minutes, these rates would apply 0.05 to 0.1 

 p. p. m. to the treated portion of the stream. 



Although it is convenient to compute the dosage on the area of 

 the treatment interval, actually the treatment is limited to a portion 

 of the stream the area of which depends on the velocity of the water 

 and the application time. In the example given above the amounts 

 applied to the treated segment would be 0.073 to 0.146 pound per acre 

 instead of 0.025 to 0.05 pound as computed. The larvicide becomes 

 dispersed in the water by the turbulent action of the stream, and the 

 dosage must be high enough to be effective during the time that 

 the treated segment passes over the attached larvae. 



For aerial applications where different-sized streams are to be 

 treated in one operation, the dosages are estimated on a swath-acre 

 basis. In the Alaska tests a single spray swath across the streams 

 with a Piper Cub plane delivering 0.2 pound of DDT per acre, 

 based on a swath width of 100 feet, completely eliminated the larvae 

 for a half mile downstream. Similarly, with a C-47 plane covering 

 an estimated swath of 800 feet, 0.1 pound of DDT per acre (% pint 

 of a 20-percent solution) gave complete control for \){ miles in one 

 test and for 2 to 2}<> miles in five other tests. These dosages were 

 equivalent to about 0.0005 pound of DDT per foot of stream width 

 for the small plane and 0.002 pound for the large plane, much lower 

 than the dosages recommended for hand application. These dosages 

 should be increased for dense forest cover or deep, swift streams. 

 For initial trials in aerial operations it is suggested that the control 

 area be covered with parallel flights at intervals of % to }{ mile with 

 the smaller plane and ){ to 1 mile with the larger one. 



In a large-scale treatment of the Saskatchewan River in western 

 Canada for the control of Simulium arcticum Malloch, a DDT solu- 

 tion applied by airplane at the rate of 0.13 p. p. m. of DDT for 36 

 minutes destroyed practically all larvae for about 100 miles down- 

 stream. A total of 615 pounds of DDT, or about l / 2 pound per foot, 

 was applied during repeated runs across the river where the width 

 was about 1,200 feet. The flow of water at this point was estimated 

 at 35,220 cubic feet per second. Chemical analysis of the water 

 5 miles downstream showed that the section of treated water had 

 lengthened considerably, with a consequent reduction in concentra- 

 tion of DDT. It is suggested that all streams containing black fly 

 larvae be treated within at least 5 miles from the area to be protected. 



At the dosages recommended no harmful effect on fish would be 

 expected, as they w^ere found to withstand much larger amounts. 

 Some reduction of aquatic insects other then simuliids may occur 

 but should not be serious except possibly for a short distance below 

 the points of treatment. 



