176 The Control of Mosquitoes 



than fifteen per cent, of phenols, or of a greater 

 specific gravity than 0.97, will not make a satis- 

 factory larvacide. 



From time to time, preferably from each lot, a 

 specimen of the larvacide should be tested for its 

 larvacidal powers. If a i to 5000 emtdsion does 

 not kill full-grown Anopheles larvae in ten minutes, 

 the product is unsatisfactory. 



This larvacide, however, loses some of its effi- 

 ciency when exposed, in emulsion, to the action of 

 the air, and still more when exposed to contact 

 with algas and other organic matter. Under 

 the latter conditions its efficiency is lowered con- 

 siderably after twenty-foiur hours' contact, and 

 after a few days' exposure its larvacidal power is 

 practically extinct. 



The advantages of this phenol-resin soap larva- 

 cide are : 



1. High toxicity to mosquito larvas. A i to 

 5000 emulsion kills full-grown Anopheles larvae in 

 from three to ten minutes. 



2. Concentration. Being effective for practi- 

 cal use in a I to 5000 emulsion, only a relatively 

 small quantity of the larvacide need be transported 

 to a given body of water. 



3. Uniformity of toxic power. This proauct, 

 when carefully made, is uniform in toxicity. 



