i8 
Mosquitoes and 
keeping Anopheles and other mosquitoes under control, and have 
sometimes been artificially introduced with successful results; 
but they are not of much use where there is any considerable 
quantity of algae or other vegetation present, and reliance should 
never be placed on them alone. Besides fish, ducks have been 
found very efficacious. 
Oiling the surface of the water with kerosene, petroleum or 
some other oil is most important, and is the method which 
has been used perhaps more than any other. Exact prescription 
cannot be given as to the amount to be applied, owing to the 
variation in density and spreading qualities, but enough should 
be used to form a complete film on the surface. With the lighter 
oils spraying is the best method of application, but if heavier oils, 
such as sunflower oil, are used they are apt to collect into globules 
and may have to be spread on the water with a cloth or with 
brooms, instead of or after spraying. The disadvantage of the 
oiling method is that the larvae can often remain at the bottom 
long enough for the oil-film to become discontinuous, which soon 
takes place in a high wind. Apart from this, oiling is not satis¬ 
factory for the weedy margins of ponds and streams, and it is 
often necessary, therefore, to employ a larvicide which will mix 
with the water and kill the larvae in it. 
A great variety of miscible larvicides have been recommended. 
The following preparation has been used with great success in the 
Panama Canal zone :— 
(i) “ 150 gallons of carbolic acid is heated in a tank to a 
temperature of 212° F.; then 150 lbs. of powdered or finely 
broken resin is poured in ; 30 lbs. of caustic soda is then added 
and solution kept at 212° F. until a perfectly dark emulsion, 
without sediment, is obtained. The mixture is thoroughly 
stirred from the time the resin is put in until the end.” 
One part of this emulsion in 5,000 parts of water is said to be 
sufficient to kill Anopheles larvae in from five to ten minutes, or 
even less. The emulsion is diluted one-sixth, and sprayed on to 
the water, and can be used even in streams with a moderate 
current. 
Other larvicides. which are recommended are 
(ii) Commercial cyllin, one teaspoonful to a gallon of water, 
or rather stronger, enough to make the water remain quite 
milky after being stirred with a stick. 
