IMPOUNDING WATER TO CONTROL BREEDING OF MOSQUITOES, 21 



of these streams to the surrounding topography does not permit 

 drainage. In the absence of a drainage outlet, the Bureau of Ento- 

 mology conceived the idea of clearing a section of one of the bayous 

 and impounding the water to note the effect of a change from the 

 swampMke conditions of the natural bayou to the lakelike conditions 

 of the impounded area, on the capacity of the bayou for Anopheles 

 production. 



It is important to control breeding of Anopheles in bayous for the 

 reason that these streams offer a near-by source of mosquitoes, since 

 the houses on a plantation in the Delta are located on the roadways 

 along the bayou banks, where one of these streams sections or 

 bounds a property. 



A section of Bayou Walnut at Mound, La., was cleared of all 

 vegetation and the water in this area impounded by means of a 

 cross-levee, or fill, and spillway. This served to keep the water over 

 the bed of the stream above the dam at a suffici^ height to suppress 

 the further growth of vegetation. 



It was found by comparative studies that this clearing and im- 

 pounding was effective in preventing the breeding of Anopheles in 

 the bayou where formerly such breeding was common. 



Cooperative work on the part of the United States Bureau of Fish- 

 eries demonstrated that the mosquito-eating top minnow {Gamhusia 

 afflnis) is generally distributed in the region, but that under natural 

 delta conditions this minnow is found coincident with prevalent 

 breeding of Anopheles. It was also demonstrated that this minnow 

 has established itself in important numbers along the margins of the 

 impounded area. One of the important factors in the natural con- 

 trol found to exist in the impounded water is believed to be the 

 greater freedom for action which the condition of an open surface of 

 water gives to these fish and to aquatic predacious insects. The fish 

 are noneffective in control under natural conditions by reason of the 

 protection afforded the mosquito larvse by the aquatic and marginal 

 vegetation and the vegetable debris upon the surface of the water. 

 Other factors in the natural control in the impounded zone are con- 

 sidered to be wave action, influence of a greater depth on breeding, 

 absence of shelter for adults along the course and consequent reduc- 

 tion of oviposition, and the depletion of the food of Anopheles larvse. 



The important points to be considered in impounding water in a 

 bayou for mosquito control are the preliminary clearing of all vege- 

 tation, the provision for a permanent level of water sufficiently high 

 to suppress the further growth of aquatic and semiaquatic vegetation, 

 and the maintenance of a clean margin. 



A further point in the construction of the dam is provision to pre- 

 vent the work of crawfish, which, otherwise, work through the fill 

 and cause serious leakage. The water level in the project under dis- 



