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Finally, D.D.T. can be made up in 'bomb' form with a gas such
as freon, being then liberated in a mist which remains suspended in
the air for some minutes, eventually blowing away without settling.
This method is therefore employed only against flying types of insects.
The lethal power of D.D.T. is exceptionally high, but the time
taken varies with different insects. Thus mosquitoes and flies quickly
succumb, but bugs and cockroaches require several hours before dying.
In using D.D.T. the first principle to accept is that it is in no
sense a fumigant, since it gives off no vapour. It can therefore form
no substitute for the older insect-repellants. The action of D.D.T. is
purely that of a contact-poison, reaching the insect's brain by absorption
through its body or its feet. For this reason it is useless to apply
D.D.T. powder to clothing in the hope of preventing moth. Mothlarvae
are able to browse contentedly upon one face of a lapel while
D.D.T. powder lies idly upon the other. The only way to protect
clothing with D.D.T. would be to spray or impregnate the whole of
the fabric with a liquid solution in such a way as to leave a residual
coat of D.D.T. crystals upon every surface. Similarly, in dealing
with bugs, the method is to use a spray of D.D.T. in kerosene, paying
attention not only to the bedstead but also to the walls surrounding the
bed, together with the adjacent edges of the floor and ceiling. Afterwards,
any bug desiring to return to its feed can only do so by crossing
a sprayed surface.
This introduces the second principle, that D.D.T. is a stable
substance possessing valuable clinging properties. Consequently any
surface sprayed with D.D.T. will remain lethal to bugs for weeks or
months. The chemical will thus not only achieve a large initial kill,
but will remain to mop up succeeding generations as they hatch out.
This residual activity places D.D.T. far in advance of anything we
have possessed in the past. Indeed, experiments suggest that by the
use of D.D.T. distemper we may be able literally to make bug-proof
a room or an entire house for long periods. Unfortunately, in dealing
with flies in the home, the clinging property of D.D.T., although useful,
avails us less. Flies breed out of doors, and no amount of D.D.T.
can hope to take the place of proper disposal of garbage. Unless this
rule be followed, flies may die in the sprayed room, but others will
Still come in to take their places. In any case, food which has been