PRINCIPLES OF MOSQUITO CONTROL 283 



with pools after the utmost low water reached by gravity has been at- 

 tained. Pumps of various types have been used, and thus far the low- 

 head centrifugal kind has been most satisfactory. One twelve-inch pump 

 of this type, with contributory ditching so arranged as to permit it to 

 work at its limit of efficiency, has been sufficient to remove the surplus 

 water from 1,000 acres. Such a pump cannot do this, however, unless 

 the water is removed and a reservoir of surface soil from eight to 

 twelve inches thick has been dried out before the mosquito season 

 starts. With water at the marsh surface an extra heavy rainfall would 

 so fill up the marsh that mosquitoes would probably escape before the 

 water could be drawn off. This would be especially likely to occur when 

 the rainy period is followed by cloudy weather, reducing evaporation 

 to the minimum. 



In dealing with marshes of this type, either provision must be made 

 to bring in brackish water with its supply of killifish to be circulated 

 through the ditches, or the ditches must be pumped dry and kept so. 

 Flooding is probably the better plan because pumping is likely to be 

 followed by the growth in the bottom of the drains creating blockages 

 and eventually breeding places. 



ENCLOSED SHRUNKEN AND SEWAGE -POLLUTED 

 SALT MARSH 



The third type of marsh, enclosed, shrunken, and polluted, must be 

 enclosed to keep the sea out, must be pumped dry, and the sewage chan- 

 nels from the highland diked so as to prevent the escape of sewage wa- 

 ters from these channels into the marsh. Drainage, pumping, and the 

 circulation of sea water must be provided as in the case of the enclosed 

 shrunken and sewage free marshes. Of course, at the beginning, on any 

 portion of such marsh, breeding may be more or less troublesome in the 

 ditches but after the circulatory system has been at work for a period 

 of time breeding will be as well taken care of as in the enclosed shrunken 

 and sewage free marsh. 



UPLAND MOSQUITO PROBLEMS 



Although the upland is never entirely covered with water, there are 

 besides occasional permanent pools, swamps, and streams the homes 

 and business houses of hundreds of thousands of people, involving water 

 containers of all sorts, artificial ponds and pools, house roofs, rain gut- 

 ters, rain pipe leaders, cisterns, cesspools, storm sewers, sanitary sew- 

 ers, and sewage disposal plants. Summer season rainfall greatly modi- 

 fies this fundamental picture. When the rain comes in sufficient amounts 



