MOSQUITO CONTROL 279 



squads to thoroughly flush the ditch below the gate and remove all 

 mosquito larvse and algae, 



Larvicides 



The ditching, draining, and clearing of waterways insure a regular 

 flow, carry off" all surplus water, and reduce but do not prevent mosquito 

 breeding. It is- necessary to use some additional means of control and 

 for this purpose various larvicides have been applied, but principally 

 kerosene, crude oils of parafl^n and asphaltum base, as well as 

 creosote oils. 



The question of the eff"ect of oils on mosquito larvae is most thor- 

 oughly discussed by Freeborn and Atsatt, who find that the toxicity of 

 tlie petroleum oils as mosquito larvicides increases with an increase in 

 volatility, the more volatile oils producing the more marked lethal 

 eff'ects. The volatile constituents of the oils contain the principles 

 tliat produce tlie primary lethal cff^ccts. The lethal eff'ects are produced 

 bv the penetration of the tracheal tissue by the volatile gases of the oils. 

 In the heaviest and least volatile oils having a boiling point greater than 

 250° F., this action may be supplementary or apparently secondary to 

 tlic eff'ect of actual contact of the oil with the body tissue, or perhaps 

 to mechanical means such as suff'ocation or plugging of the trachea. 

 Tlicy found that oils which killed very quickly did so by means of the 

 volatile gases, whereas in the case of oils with slow eff'ectiveness the 

 mechanical suffocation may be the cause of death. 



This paper is so recent that it has not been possible to obtain a 

 mass of evidence on the practical eff'ectiveness of dift'erent grades of oils 

 used as larvicides. Kerosene and crude oil are the oils most commonly 

 used in general practice. Le Prince and Orenstein prefer crude oil to 

 kerosene because of the film made by kerosene, its greater expense, inflam- 

 mability, and liability to be wasted because of its transparency. 



These authors have set down a number of requirements for a good 

 larvicide: 



1. It shall have a high toxic power, so that a small quantity may 

 suffice for a large volume of water. 



2. It shall kill rapidly in order that subsequent dilution and weaken- 

 ing by rain have as little effect as possible. 



3. It must be uniform in its toxic power and capable of standard- 

 ization, 



4. It should mix freely with brackish and alkaline waters. 



5. It must be harmless to man and domestic animals, when in the 

 dilution necessary for iarvicidal action, 



6. It shall not be susceptible to rapid deterioration. 



7. It must be inexpensive. 



