HOW TO KILL MOSQUITO LARV.E 105 



receptacles, then, should be siphoned off occasionally, 

 and search made in the lower parts of them for con- 

 cealed larvae. 



Petroleum is useful where breeding-places cannot 

 be removed or destroyed, and partly refined oil is 

 better than either the crude petrol or the refined 

 product. The first does not spread evenly, the 

 purified oil is less effective. The oil blocks the 

 breathing pores of any larva with which it comes 

 in contact, and the insect dies of suffocation. 



Corrosive sublimate, permanganate of potash and 

 crude carbolic acid, all have been used as larvicides 

 at times, but in this connexion it is well to remember 

 the deadly characters of two of these substances, 

 and that cattle may use the water for drinking, with 

 fatal results. Fish used for food also may be 

 poisoned, as well as the mosquitoes. Vegetation, 

 too, may suffer, and such vegetation as is destined 

 for food is frequently only produced by extensive 

 irrigation systems. Petrol, then, seems to be the 

 best and safest larvicide in use, but it can only be 

 regarded as giving temporary relief, and as being 

 subsidiary to drainage operations. 



Recently a well-known entomologist has stated 

 very decidedly that, wherever an insect pest exists, 

 there is also some enemy of the pest that will keep 

 it within reasonable bounds. This statement is 

 too sweeping, and it fails to recognize that, though 

 a slayer of the pest may exist, yet its numbers may 

 be such that its efficiency is reduced to a minimum 

 in practice. For instance, the ladybird (Coccinella) 

 is a well-known slayer of greenfly (Aphides), but its 



