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suffocation. Among the wells, tanks and pools of Bombay over 50 have been
found in which large amounts of Wolffia were present, and in which mosquito
larvæ did not exist. In a great many other collections of water a scanty growth
of the weed did not appear to interfere with the breeding? of mosquitoes and
in pools where reeds and largo weeds broke the surface film of Wolffia, larvæ
were to be found with ease. Experiments with the weed in vitro showed that
its action is purely mechanical. Anopheles larvæ placed in water covered with
a layer of Wolffia immediately assume an almost vertical position, hanging in
the water head downwards in a manner resembling that of Stegomyia larvæ.
After a longer or shorter interval dependent upon the thickness of the film of
weed, the larvæ died and sank to the bottom of the water.
195. Experiments tried with ordinary duckweed (Lemna) and Azolla were
unsatisfactory, neither of these plants appearing to have any effect upon
mosquito larvæ. Both Lemna and Azolla were present in tanks and pools in
Bombay and in almost every case mosquito larvæ were found living amongst
the weed in considerable numbers.
Section 4.
196. ARTIFICIAL MEANS FOR DESTROYING MOSQUITO
LARVÆ.—During the investigation a number of experiments have been carried
out with a view of gaining some idea of the most effective larvacides from the
point of view both of cost and efficiency. The cheapest larvacide is sea-water.
Unfortunately mosquitoes quickly adapt themselves to breeding in salt water ;
but larvæ that have been hatched out in fresh water die after a few hours, when
sea-water is added to the water in which they are living so as to form a
mixture containing 75 per cent. of sea-water. Common salt, lime, sulphate of
copper and sulphate of iron, destroy mosquito larvæ when added to the
water in which they are breeding. Sulphate of copper is the most active of
these substances but the cost is prohibitive. Sulphate of iron is active only
when the solution approaches saturation.
197. The larvacides most commonly used are crude petroleum (otherwise
known as liquid fuel or pesterine} and kerosene. Of these kerosene is more
effective than pesterine, but a mixture of about equal parts of kerosene and
pesterine is more efficient than either oil by itself. Pesterine has the advantage
of cheapness, and its colour shows when it has been applied; but it does not
spread so easily as kerosene, is more apt to collect in large blobs and is much
slower in its action. Unless it is used in sufficient amount and carefully spread,
it is of very little use, and as it tends to collect in large blobs with free spaces
in between, mosquito larvæ easily escape suffocation. In wells in which
many gallons of pesterine have been placed, mosquito larvæ have been after-
wards discovered, even when there was sufficient oil present to have covered
the surface of the water many times over. Kerosene costs more than pesterine
and is more liable to be stolen but it spreads better on water and is decidedly
quicker in its action. Unfortunately like pesterine it is liable to be blown to one
side of a pool if there is much wind and it may be found necessary to repeat
the oiling of a pool three days in succession before all the larvæ and pupæ
are destroyed»
198. In a series of experiments conducted in the New Dock Works early
in 1909 it was found that of 66 pools of water containing numerous larvæ
which were carefully oiled with a mixture of pesterine and kerosene, applied
to them by means of a spray, 47 contained many live larvæ the following day.
They were all treated once more but on being examined again the next day
scanty larvæ were found in 22, and after being treated a third time a few
larvæ were still present in 7. In another series of 44 pools most carefully
oiled by being sprayed repeatedly with a mixture of kerosene and pesterine, 12
showed a number of living larvæ the next day. These observations show that
where it is a matter of importance, as in the New Dock enclosure, that larvæ
should be destroyed thoroughly, it is not sufficient to make a routine ap-
plication of pesterine or kerosene once a week, as many mosquitoes continue
to reach maturity in spite of it,
199. During the experimental destruction work in the New Dock it was
found that the only way to limit the breeding of mosquitoes there with much
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