PRINCIPLES OF MOSQUITO CONTROL 275 



trenching the salt marsh for mosquito control. It has been clearly shown 

 during that period that no salt marsh is so well ditched that it will at 

 all times be free from mosquito breeding, except the large permanent 

 pools which are always well stocked with killifish. There are times when, 

 because of high tides, continued rain and cloudy weather, the water de- 

 rived from high tide or heavy rainfall or from both fails to be drawn off 

 in time to prevent the maturing of the last remnant of the brood. More- 

 over, it may be said that the ditch mouths become plugged with sand 

 or seaweed through the action of the waves and that by one means or 

 another ditches farther up in their courses become plugged with pieces 

 of sod, accumulations of hay, and other rubbish. All of which proves the 

 need to patrol the ditched salt marshes throughout the mosquito breed- 

 ing season. 



It can be said, however, that during this period no case has come to 

 the author's knowledge in which the ten-inch wide trenching of the or- 

 dinary high-lying salt marsh has failed to eliminate all but a small 

 percentage of the brood which started. 



The great differences of opinion as to the amount of trenching neces- 

 sary to free an acre of breeding salt marsh from danger are due to the 

 different kinds of salt marshes. 



Marshes filled with holes and depressions require more extensive 

 ditching than others. Some marshes are protected from the tide by 

 dikes, and the natural drainage water is removed by tide sluices or even 

 by pumps. These require a larger amount of drainage, the extra amount 

 being necessary in cutting outlets for the primary system. It seems 

 probable that between 200 and 300 feet of ten by thirty-inch ditching 

 is the average. To this must be added an amount of hole filling and shal- 

 low spurring which will add about ten per cent to the acre cost. For- 

 tunately, large portions of the salt marsh, particularly in the southern 

 part of the state, are so low-lying and open to the tide as to be swept 

 by every tide which is a little higher than the ordinary, and are on that 

 account so free from breeding as to require no ditching. In a given area 

 which includes a considerable amount of this kind of land, the required 

 number of feet of ditching per acre will be materially reduced. 



The plan of trenching has not undergone marked changes. By 1912 

 two general plans were in use, the first of which might be called the par- 

 allel system, and the second, the pool-connecting system. In the par- 

 allel ditching scheme more generally used the territory to be ditched 

 was divided into districts on the basis of the possible outlets and each 

 block of territory crossed by parallel ditches, lying sufficiently close to 

 remove the surface water. Holes and depressions were spurred into 



