CONTROL MEASURES FOR NEARCTIC MOSQUITOES 
355 
anopheles larvae. It is practiced on im¬ 
pounded waters by pre-impoundage clear¬ 
ing of the basin, water level fluctuation, 
direct removal and poisoning. In smaller 
lakes mechanical removal is often advan¬ 
tageous. Vegetation growth (other than 
grass) along drainage ditches is often pro¬ 
ductive of conditions favoring anopheles 
breeding and it should be systematically 
removed and the channel regraded, often 
as frequently as twice during the anoph¬ 
eles producing season. 
Drainage 
Anopheles control drainage has during 
the past 25 years been highly developed in 
its special field and today it remains of 
prime importance as an anopheles control 
measure. 
The purpose of anti-malarial drainage is 
to remove the proven breeding places of 
malaria vectors and is therefore directed at 
residual water rather than general runoff. 
Consequently the ditches are usually small 
or medium sized and should be cut to an 
even, regular grade. Although open earth 
ditches are not durable, they have a wide 
field of usefulness where swamps, sheet 
water, or other large areas are to be elimi¬ 
nated (particularly in rural areas). Un¬ 
der such conditions after the first rush of 
water is over the flow is intermittent. 
Probably the greatest recent advance in 
anti-anopheles drainage in the United 
States has been the development of durable 
ditch linings and stabilized banks. While 
the first cost of a lined ditch is of course 
greater, the total cost over a period of years 
is usually much less than that of earth 
ditches. A durable invert of concrete, 
brick and mortar, or stone and mortar, is 
installed in the ditch bottom and the sides 
stabilized by sodding. This practically 
permanent type of construction is being 
used wherever the density of population 
can meet the first cost. It is particularly 
applicable to drainage systems in urban 
and semi-rural areas. In the rural areas 
of at least one state it has been found eco¬ 
nomically feasible to construct sod lined 
ditches and to install durable linings of 
concrete only at such places in. the ditch as 
may be subject to scour. This construction 
adds greatly to the life of a ditch at a mini¬ 
mum of extra cost. 
The development of durable linings with 
stabilized banks has necessitated some 
changes in the established practices which 
are applied to earth ditches, such as per¬ 
mitting smaller cross sections, necessitating 
flatter bank slope, and providing lined 
inlets. Many miles of ditches with durable 
linings have been installed in malarious 
sections and their use is rapidly increasing 
as more economical methods of installation 
are being perfected. 
Tile and similar underground drains 
constructed with poles, large rocks, or even 
old tin cans, are an important method for 
the elimination of anopheles producing 
areas. They are of use in intercepting 
seepage outcrops, drying marshes, or other 
wet areas, or lowering the ground water 
level. They are relatively simple and in¬ 
expensive to construct; they keep the water 
under ground; they operate satisfactorily 
over long periods, and of course do not dis¬ 
figure the surface or take up valuable space. 
Their proper installation requires consider¬ 
able engineering skill and a study of the 
underground flow of water. Randle (1940) 
has described the construction of this type 
of anopheles control drainage as developed 
in Mississippi. 
In a flat country with pervious soil a line 
of open joint tile laid directly beneath the 
ditch bottom has often been found very 
effective in drying ditches having almost 
no fall which would normally produce 
anopheles. When a tile line is utilized in 
this manner, weeds and grass in the ditch 
are not cut and no maintenance work on 
the ditch itself is needed. 
Under some conditions, particularly in 
limestone districts, vertical drainage may 
be utilized to remove surface water and 
isolated ponds. The method depends on 
locating a limestone fissure to which the 
water can be led, and consequently is not 
always successful. 
