Report No. 6 {1 9 1 7), 



A NOTE ON TREATMENT OF SWAMPS, STREAM 



BEDS, PONDS, WELLS, POOLS, AND OTHER 



MOSQUITO-INFESTED AREAS FOR THE 



DESTRUCTION OF THEIR LARVAE 



BY 

 HENRY C. WILSON, 



PiSClCULTURAL ExPERT TO THE GOVERNMENT OF MADRAS. 



THE UTILITY OF FISH AS LARVICIDES. 



When we come to consider that the great majority of fresh 

 water fry rely for their sustenance chiefly on aquatic larvas it 

 seems surprising at first that any larvee remain to come to maturity 

 in waters stocked with fish. 



Immediately after the absorption of the yolk sac the fry com- 

 mence feeding on minute organisms, such as microscopic Crusta- 

 cea, and, later, on the eggs and larvas of aquatic diptera. 



That they play an important part in keeping down fly pests, 

 such as mosquitos, etc. is an undoubted fact and, if it were not for 

 their presence, we would have millions of mosquitos where we have 

 hundreds now. There is of course a limit to their usefulness in this 

 respect, for, so many mosquito-breeding places exist that are 

 unsuitable for fish, and even in suitable waters surface weeds, 

 etc. protect the larvae from their enemies. 



Chiefly owing to this latter reason the practical utility of fish as 

 larvicides has been decried. The absurdity of this opinion is 

 evident as the fish are not likely to abstain voluntarily from eating 

 the most important item of their natural diet and consequently must 

 take a continuous and heavy toll of larvae. If the waters are con- 

 served, then a greater percentage of larvae would be destroyed- 

 Even in the foulest of ill-kept waters they do a certain amount of 

 good, and a careful observer will often see them searching the 

 weed and debris for larvae and seldom unsuccessfully. 



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