26 MISC. PUBLICATION 336, U. S. DEPT. OF AGRICULTURE 



attempted prior to 1933, when advantage was taken of the oppor- 

 tunity offered by the programs of the Federal Emergency Relief and 

 the Civil Works Administrations {28, 37, 53, 73). Although this 

 work was not long continued, many valuable experimental and sur- 

 vey data were obtained, which have been useful in a number of coun- 

 ties that have since become encouraged to provide funds for continu- 

 ing the operations. 



Mosquito -breeding conditions in salt marshes and the methods em- 

 ployed to overcome them are extremely varied. In general, the 

 mosquitoes breed on the parts of the marsh that are not covered 

 by daily tides, usually in pot holes and depressions of various sizes, 

 but sometimes over extensive level areas. By the present control 

 practices a system of ditches (pi. 4) is installed (1) to provide for a 

 fairly rapid run-off of surface water following the occurrence of 

 heavy rainfall or of high storm tides, (2) to permit free circulation 

 of tidal water into low areas that are otherwise landlocked, and (3) 

 to give larvae-eating minnows access to the pools and ponds or other 

 places where the larvae accumulate as the surface water is drained off. 



The ditches are usually laid out parallel, 100 to 300 feet apart, with 

 cross-connecting ditches as needed. Various types of heavy ma- 

 chinery for digging and cleaning the ditches have been developed 

 by the northern workers. Hand labor is also used extensively and 

 special ditching spades are frequently employed. In New Jersey, 

 where the marshes are generally well sodded, the standard ditch is 10 

 inches wide by about 20 inches deep, with the sides perpendicular. 

 In other areas, especially in the South, the width, depth, and slope of 

 the ditches have to be varied to meet other soil conditions and other 

 types of marsh vegetation. Many, or perhaps most, of the southern 

 marshes are not adapted to the use of machine ditchers, at least of 

 the types so far developed. 



Another important method of treating certain classes of marsh is 

 the installation of dikes and tide gates. These prevent the entrance 

 of high tides, and the tide gates, opening at low tide, provide for the 

 run-off of rain water. Under special conditions the tide gates may be 

 reversed to permit the entrance of high tides and to impound the 

 water on the marsh. This is effective in reducing Aedes breeding, 

 since it is the alternate drying and flooding of the marshes that 

 brings about hatching of their eggs. 



Recent experimental work by members of the Bureau of Ento- 

 mology and Plant Quarantine in the vicinity of Savannah, Ga., 

 has shown that the shutting off of the tides from marshes by means 

 of dikes and tide gates so that the margins of the marshes become 

 dry eliminates much breeding of the sand fly (Culicoides) as well as 

 of the mosquito. It is probable that this practice can be combined 

 with ditching in other sections where the sand fly is a serious problem. 



In the southern half of Florida, where Aedes taeniorhynchus is the 

 predominant salt-marsh mosquito, the marsh vegetation is dominated 

 by growths of several species of mangrove. This presents special 

 problems for the mosquito-control organization (pi. 4, B\ fig. 8), 

 as the mangrove forms dense forests through which it is necessary 

 to cut rights-of-way 15 to 25 feet wide before ditches can be dug. 

 Dynamite ditching has been employed in such marshes and compares 

 favorably in cost with hand labor. 



