DDT Formulations 83 



having it run. If your spray solution contains less than 5 % 

 DDT, the deposit left after evaporation of the solvent will be 

 correspondingly thinner. Of course, you can re-spray a gW^n. 

 surface a number of times, permitting the solvent to evap- 

 orate between sprayings. In this way you can build up a 

 good deposit of DDT on the surface. But, five sprayings 

 with a 1 % DDT solution have proved less effective than a 

 single application of a 5 % spray. And anyway, why make a 

 lot of extra work for yourself by using a 1 or 2 % DDT spray 

 when a 5 % spray will do the job with one application. 



But just because you buy a 5 % DDT spray don't get 

 the idea that your problems are over. It is up to you to see 

 that it is properly applied. Too many people put the stuff 

 in their spray guns, take three squirts at the wall, and expect 

 their house to be free from insects all summer. And when 

 they find the insea population thriving, they blame the DDT 

 and not themselves. 



You see, a residual spray works like this: You spray 

 the walls, lamp cords, screen door, and other places where 

 inseas light, and when the solvent evaporates the DDT 

 crystallizes out and remains on the surface. When a fly or 

 a mosquito rests on or walks across such a treated surface, he 

 picks up some DDT and dies, usually in one to three hours, 

 with tremors and spasms. 



Residual sprays do not need knockdown agents, al- 

 though it would be nice if something could be added along 

 with the DDT so that the insects dropped dead as soon as they 

 hit the wall. Pyrethrum and the thiocyanates are not of 

 much help because they do not remain effective for very long. 

 Nevertheless, many residual sprays now contain pyrethrum 

 or thiocyanates — about 1 % of pyrethrum extraa or 2 to 3 % 

 of the thiocyanates. This is a good idea, for then the spray, 



