INSECTICIDES AND REPELLENTS 



71 



and Africa a filarial disease, onchocerciasis, is known to be transmitted 

 by these insects. In the United States the pest species include the 

 southern buffalo gnat, plagues of which sometimes follow overflow of 

 the lower Mississippi River. Other species are annoying in the 

 Adirondack^ and various other parts of the United States, western 

 and northern Canada, and Alaska. 



The adults can be killed with DDT sprays, but in Alaska they 

 appeared less susceptible than mosquitoes to this means of control, 

 and the reduction in numbers was of short duration owing to rapid 

 re-infiltration into the sprayed area. 



The larvae of black flies attached to rocks and vegetation in flowing 

 streams have proved susceptible to control with small amounts of 

 DDT applied to the streams in oil solutions, emulsions, or suspensions. 

 TDE and some of the other new insecticides were also effective but 

 usually less so than DDT. 



In Alaska and northern Canada the effective dosages of DDT in 

 experimental treatments from the ground ranged from 0.1 to 0.7 

 p. p. m. applied over a period of 15 minutes, or a dosage product 

 (parts per million times minutes) of 1.5 to 10.5. 



When control operations are planned, an initial dosage of 0.1 

 p. p. m. for 15 minutes at intervals of 1 mile or for about 4 minutes 

 at intervals of ){ mile along the infested streams is recommended. 

 The dosage or interval can then be increased or decreased as the results 

 indicate. The approximate volume of stream flow per second is 

 estimated from measurements of the average velocity of the water 

 and the cross-sectional area of the stream at the point of application. 

 The larvicide may be applied with a hand sprayer or allowed to flow 

 into the stream from a container adjusted to empty in the desired 

 time (fig. 29). 



Figure 29. — Treating a stream with a DDT emulsion for the control of black 



flies. 



