June. 1929] MOSQUITOES OF NEW HAMPSHIRE 15 



FILLING 



The filling up of pools, swamps and low areas is a permanent method 

 of eliminating mosquito breeding. The initial cost, though often some- 

 what high, is the only expense ; for such areas, if properly filled, need 

 no future inspection or treatment. Swamps or low areas near towns 

 may often be filled by using them as dumps, oiling the unfilled portion 

 each year until the filling is completed. Care must be taken that cans 

 and other receptacles do not increase the breeding places. 



DRAINING 



Drainage of swamps, marshes and pools is a permanent method of 

 mosquito control. Periodic inspection is necessary in order to make re- 

 pairs or to clean out obstructed ditches or drain pipes. In years of 

 greatly excessive rainfall the drainage may be inadequate to carry off the 

 water rapidly enough and some mosquito breeding may occur in drained 

 areas. In such years some oiling may be necessary. The methods used 

 for fresh water areas are similar to those employed in drainage for agri- 

 cultural or industrial purposes and are well understood. 



The drainage of salt marshes is partly an engineering problem, and 

 surveys should be made and the ditches laid out before work is begun. 

 In the open salt marshes which occur in New Hampshire, the object is 

 not to drain them completely, but to open them up so that the normal 

 tides run in and out freely, and no isolated pools hold stagnant water 

 longer than a week. The high-lying or enclosed salt marshes, over which 

 the tide sweeps only occasionally, are the greatest breeders of salt-marsh 

 mosquitoes. Wherever the tide reaches regularly the killifish come with 

 it and devour all larvae and pupae. 



Mosquitoes do not breed in all parts of every salt marsh, and ditching 

 is on'y necessary where observation shows that breeding occurs. Salt- 

 mansh draining is accomplished by cutting parallel ditches through the 

 marsh, an average of about 300 linear feet of ditch being required per . 

 acre. The ditches are ten inches wide and thirty inches deep, with 

 smooth, perpendicular sides. Ditches twenty-four inches deep may be used 

 in .small marshes. No ditch depending on a single outlet should be over 

 one-fourth mile long, and each ditch should have a strong tidal outlet, for 

 the greater the tide sweep the less likely it is to become obstructed. 



The ditches are opened, ar far as possible, into the natural creeks; 

 but where this is impossible, large main ditches, thirty inches deep and 

 as wide as necessary, are cut as outlets. Short spur ditches may be run 

 to large isolated pools which do not drain readily. Small pools may be 

 fil'ed with sods taken from the ditches. Special spades are used in cut- 

 ting the ditches, and for extensive operations power ditchers are often 

 used. The state of New Jersey has done the greatest amount of work in 

 salt marsh drainage, and most of the above information on this subject 

 is taken from the results of their experiments. 



