PRINCIPLES OF MOSQUITO CONTROL 273 



any marsh will produce mosquitoes. In every case, however, study will 

 show that to produce mosquitoes, warm water free from killifish must 

 lie on the marsh surface long enough for maturity to be reached or 

 mosquitoes will not be produced. 



Briefly stated, the control of the salt marsh mosquito is a matter of 

 so ditching the marsh that none of the waer which remains upon it will 

 stagnate, but all will rise and fall with the tide and be everywhere pene- 

 trated by large numbers of killifish. The only exception to this rule is 

 the permanent salt marsh pool which, being constantly stocked with 

 killifish, turns off no mosquitoes. While it is true that low-lying sections 

 of the marsh widely open to the tide, which are covered by practically 

 every high tide, do not breed mosquitoes, it is also true that such areas 

 under a series of low tides will sometimes turn off considerable broods. 



High-lying or shut-in meadows over which the tide rarely sweeps are 

 the breeders of salt marsh mosquitoes. Even where an extra high tide 

 has stocked the holes with killifish the water dries up on the marsh 

 surface generally and in the temporary pools, destroying the killifish. 

 Water from rain and tides just high enough to trickle over the marsh 

 through the grasses then fill the holes and the larvae hatch and mature 

 and a brood of mosquitoes gets on the wing. 



Shortly after 1900 Dr. Smith began to experiment to determine the 

 type of ditching that would cause the waters of the marsh to rise and 

 fall with the tide and to afford access of the killifish to all parts of the 

 marsh waters. After many trials he settled upon a ditch ten inches wide 

 and thirty inches deep, with perpendicular, smooth sides, as the one 

 most nearly fulfilling the requirements. This depth normally reaches 

 the bottom of the sod and is maintained, except where a natural slope 

 requires a deeper cut to insure the free flow of the water. As the upland 

 is approached the sod and underlying mud become less than thirty 

 inches deep and a sand or clay bottom appears. The ditches are allowed 

 to become shallow as the sod gets thinner. Neither sand nor clay subsoil 

 is cut into except as the drainage of a pool or area behind the barrier 

 requires such action. More recently it has been found that where the 

 water is comparatively low in saline content, as along the course of 

 rivers and the sections of bays adjacent to their mouths, the cutting of 

 a thirty-inch ditch is followed by the washing out of underlying mud, 

 causing the ditch banks to soften and the ditches to start development 

 as natural marsh guts. It has been found also that in such places such 

 undesirable results are largely eliminated by cutting the ditches so shal- 

 low that they do not go below the matting of grass roots. In recent 

 years it has been found that on most marshes ditches as shallow as 



