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



the larvae per dip in a series of dips. This method is utilized prin- 

 cipally in connection with anopheline surveys. The relative impor- 

 tance of the area in mosquito production can be expressed numerically 

 by multiplying the average niunber per dip by a factor representing 

 the extent of the breeding area (size times percentage of breeding 

 surface). The productivity of a breeding place per unit of surface 

 can also be determined by the use of cloth nets or screen cages placed 

 over the water (25), 



Breeding places may be divided into two general classes, permanent 

 and temporary. The two classes frequently intergrade, however, and 

 the status of a given area may change over a period of time. Ano- 

 pheles and Culex occur typically in the permanent breeding places, 

 whereas most Aedes and Psorophora are found in the temporary 

 collections of water produced by rainfall, floodwaters, or high tides. 

 The status of the breeding places, particularly the permanent ones, 

 as to productivity may change greatly during the course of a season 

 or from year to year, owing to changes in the amount of aquatic 

 growth or flotage, the abundance of natural enemies, and other causes. 



The importance of temporary breeding places of Aedes and 

 Psorophora is frequently very difficult to determine, because con- 

 siderable time may elapse between broods. One may visit suspected 

 areas repeatedly without finding larvae, and such areas must be 

 classified as potential breeding places until more definite evidence is 

 obtained. The type of vegetation, especially in salt marshes, is often 

 an indicator of the suitability of breeding conditions. Breeding- 

 occurs on the parts of the marsh that are above the normal daily tidal 

 range, and the elevations are indicated by the type of plant growth, 

 since many of the plant species are restricted rather sharply by the 

 height of the water table and the frequency of tidal coverage. More 

 definite information on suspected breeding areas can sometimes be 

 obtained from samples of sod taken from dry depressions by scooping 

 off a thin layer of topsoil with a small shovel. Samples from differ- 

 ent parts of the area are placed in containers and covered with water 

 io cause hatching of the eggs, which may begin within 15 minutes and 

 continue for several hours. Glass containers are preferable, as the 

 small larvae are more easily seen when these containers are held 

 against the light. If the sods are very moist at the time of collec- 

 tion, drying in the air for a week or so may be necessary to induce 

 hatching of the eggs. 



ENGINEERING SURVEYS 



During a mosquito survey much information will be accumulated 

 which will be valuable in determining the feasibility of an anti- 

 rnosquito project and the best methods to be employed in handling 

 specific problems. Where the malaria or the salt-marsh mosquito is 

 to be controlled, an engineering survey is then undertaken to lay out 

 the detailed plan of ditching and other operations, and to determine 

 the approximate costs. Whether the local situation can be handled 

 successfully within the means at hand, or at a cost commensurate 

 with the probable benefits, is one of the first questions to be decided. 

 Matters of the legality of the proposed work (including jurisdiction 

 over the area involved), the probability of obtaining necessary ease- 

 ments on private property, etc., must also be considered. 



