350 
MALARIA 
that shade itself played no part in the 
control of breeding places of A. minimus 
by dense shading. The densest possible 
shade that could be produced in a field ex¬ 
periment was still attractive to gravid fe¬ 
males. In fact, gravid A. minimus was 
strongly attracted by shade and would not 
oviposit in an unshaded position. Breed¬ 
ing continued until death of vegetation 
left the edge of the flowing drain com¬ 
pletely denuded. In other words, the 
controlling effect against A. minimus was 
due entirely to secondary effects of shade, 
e.g., increased water movement. Effective 
control could be obtained by complete re¬ 
moval of all vegetation at the edges with 
full exposure of bare edges to light. 
Thompson (1940) notes that indiscriminate 
use of shading plants might do more harm 
than good. 
In Assam, such plants as duranta, lan- 
tana, hibiscus, and tarapat have been 
planted along streams to control breeding 
of A. minimus. In Java, Tithonia diversi- 
folia, a Mexican marigold, was used against 
A. maculatus. In Cuba, Carr (1938) used 
Ficus benjamina with excellent effect 
against A. albimanus. Good results have 
followed shading Malayan wells by erect¬ 
ing small thatched huts over them. In 
Assam woven fiber mats were used effec¬ 
tively to shade the edges of pools. But in 
South India we were not able to control 
A. culicifacies by shading casuarina pools 
(Russell and Jacob 1939a) nor have pre¬ 
liminary experiments with well-shading 
been successful. Breeding of this species 
in field channels can be controlled by plant¬ 
ing rice. When these plants are a foot or 
more high culicifacies disappears. We are 
not able to state yet whether this is an ef¬ 
fect of shade or of a barrier although the 
latter appears more probable. 
Carr (1938) noted a double effect of 
shade. It not only of itself tended to drive 
out A. albimanus but it also prevented 
algae and other vegetation from flourish¬ 
ing, thus depriving larvae of protection 
against fishes. 
This naturalistic method of shade culti¬ 
vation is not so simple as may appear. The 
degree of shade required will vary and 
should be determined by experiment for a 
given species of mosquito. The shade plant 
must be carefully chosen. It should be 
easy to secure, should grow rapidly and be 
hardy enough to endure local climate and 
storms. Preferably, it should be ever¬ 
green and its foliage, fruit, and wood not 
attractive to man or animal. After plant¬ 
ing, it may require cultivation and fenc¬ 
ing and it must be inspected regularly and 
maintained. 
Certain Assam ravines have been con¬ 
trolled successfully by planting and culti¬ 
vating shade while allowing natural jungle 
growth to return, so that the end result has 
been dense, permanent shade which has 
given perfect control. It is, of course, 
much easier to preserve natural shade than 
to reproduce it. Undoubtedly, the method 
has possibilities in American tropics 
against A. albimanus, but it should always 
have preliminary experiments, such as 
Carr (1938) carried out in Cuba. 
Use of natural enemies. Excepting 
fishes, not much can be said for use of nat¬ 
ural enemies of mosquito larvae in malaria 
control. Such plants as Utricularia prob¬ 
ably destroy some larvae but have had little 
experimental study in this regard. Un¬ 
doubtedly, parasitic bacteria, fungi, vi¬ 
ruses, and protozoa kill mosquitoes. Some 
species have been actively used with suc¬ 
cess against certain agricultural insect 
pests, but none has had a suitable experi¬ 
mental test against mosquitoes. So, too, 
certain nematodes, annelida, crustacea, and 
numerous hexapoda are natural enemies 
of mosquito larvae but have not yet been 
proved to be useful in malarial control. 
Even Notonectidae and Dytiscidae, per¬ 
haps the most active invertebrate predators, 
have not yet had suitable experimental 
triads. Various amphibians, reptiles, and 
birds have been observed to include mos¬ 
quito larvae in their usual diets, but no 
convincing attempts have ever been made 
to stimulate them to give more than their 
natural help in controlling larvae. Only 
in the case of certain fishes has that poten¬ 
tial, destructive force against mosquitoes 
