352 
MALARIA 
spring from indoor aquaria the fishes mul¬ 
tiply rapidly. 
In the tropics it is usually best to build 
a concrete tank for use as a Gambusia nur¬ 
sery (Mulligan and Majid 1936). This 
may be from 4 to 10 feet or more square. 
It is a good plan to plant rushes in the cor¬ 
ners and to add algae. The nursery may 
have to be screened against birds or ani¬ 
mals preying on the fishes. In Madras we 
have had excellent results with three such 
nurseries (30x30 ft, 14x7 ft and 10 x 
16 ft), feeding finely chopped raw meat to 
the Gambusia daily. Fishes have flour¬ 
ished in these concrete tanks with maxi¬ 
mum temperatures of air and water up to 
105° F. 
We have transported Gambusia in the 
tropics successfully in various types of 
native clay pots, adding some algae to each 
pot to prevent splashing. It is necessary 
to allow free access of air and to avoid 
overcrowding and splashing during trans¬ 
portation. 
The numbers of Gambusia to be used in 
various types of breeding places to control 
mosquito larvae will depend on local condi¬ 
tions. More are required where the surface 
area is large or vegetation abundant. In a 
well, ten Gambusia may be enough to keep 
down all breeding. Ten to 50 may be 
needed in a cistern. Low-lying culverts 
holding water for a considerable time often 
may be kept free from larvae by stocking 
liberally with Gambusia, at least 5000 per 
acre. 
Before stocking a well with Gambusia it 
is wise to treat the water with chloride of 
lime or “bleaching powder,” using 12 to 
16 ounces to 800 gallons of water. This 
kills smaller fishes and brings the larger 
ones up for removal by netting. Sometimes 
a second treatment is required. The water 
becomes potable in a few days but it is best 
to wait 10 days before introducing Gam¬ 
busia (Russell and Jacob 1939b). 
Too few fishes or too much natural cover 
for larvae are primary causes of failure in 
mosquito control with Gambusia. The 
former defect is in turn usually due to (a) 
natural enemies, such as larger fishes or 
live-bait fishermen; and (b) neglect. The 
latter defect may consist of floatage or 
vegetation, especially when matted. In 
many situations, the cost of removing 
larger fishes or of keeping down larvae- 
cover would be excessive and consequently 
fish should not be used. In other places, a 
little help by removing matted algae is all 
that the fishes require. Sometimes it is 
advisable to combine fluctuation in water 
level with the use of Gambusia to provide 
cleaner edges and so less shelter for larvae. 
Some years ago Gambusia fishes were 
overrated in mosquito control. Now the 
tendency is to underrate them. Extensive 
trials throughout the world during the past 
two decades have proved that Gambusia 
can give effective results in certain types 
of breeding places and considerable help 
in others. 
