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A feature is introduced in ship work which does not generally apply on shore—the avoidance
of risk of fire which may arise from a low flash point product particularly when applied in galleys or
similar places where fires or lights may cause an explosion.
This risk has been overcome by the development of the so-called wettable powders which do
not rely on the use of inflammable solvents as the medium for the distribution of the insecticide on
surfaces or in corners and similar places difficult of access.
A number of useful and indeed effective insecticides have thus been developed but they have
all suffered in some degree from the fact that they are soon covered by dust or grease and subsequently
are removed in the ordinary process of cleaning the surface to which they have been applied.
While their killing properties may be high they act only for a limited period which in the case of
cockroaches for instance, may not even be long enough to destroy the nymphs hatching out after
the original application has killed the adults. Consequently unless there is a repeated re-application
the infestation reappears.
The incorporation of insecticides in paint applied to bulkheads and to any surface carrying an
infestation is no answer to the problem since the bulk of insecticides incorporated in ordinary paint
remains hidden beneath the surface and only that part of it lying on the surface can do its job.
The development of resins which have been found to possess the property of extruding the
insecticide incorporated in the resin over a long period until the whole insecticidal content of the
resin is exhausted seems to have provided a complete answer to the problem.
A considerable amount of research into certain types of synthetic resins notably the Urea
Formaldehyde resins, carried out under the auspices of the Agricultural Research Council and tried
out in ships in collaboration with the Port of London Health Authority, has produced highly successful
results. The resin in the form of a colourless lacquer incorporating at various stages in the trials
insecticides such as DDT, BHC, Aldrin or Dieldrin or a mixture of two of them, has been applied to
surfaces by two methods, either by painting or preferably by spraying using a special type of sprayer
that will effectively dissipate the comparatively thick lacquer.
One solvent used to dissolve the resin and incidentally the insecticide has been xylene. The
flash point of xylene is in the neighbourhood of 78° F. and it is obvious, therefore, that precautions
must be taken to keep any heat in the form of a naked light, live coal, cigarette ends, etc., away from
the area of treatment.
It must be confessed that shipping companies in the Port of London have raised no difficulty
whatsoever in this regard having satisfied themselves of the absence of any fire risk in the area under
treatment, but the comparatively low flash point is a difficulty which will undoubtedly soon be overcome
by further research into the composition of the lacquer and the solvents that can be employed
with it. Once the lacquer is dry (a matter of an hour or two) it is entirely non-inflammable.
So long as a substance such as xylene is being used as a solvent the operator must wear protective
overalls and a gas mask which will prevent him from inhaling the vapour. This is found to
present no difficulty and is a well known precaution adopted when using solvents of this kind in the
painting industry. It is obvious that the operator must be experienced in the habits of insects and
particularly of cockroaches if he is to apply the lacquer in the appropriate places most effectively.
There are two practical methods of treatment. The spot treatment which consists of spraying
areas around all cracks, crevices, electrical conduits, etc., leading to insect harbourages so that the
insects must come in contact with the treated surface when they emerge in search of food.
The second and most efficient method is to apply the lacquer during the ship's refit over the
whole surface of the compartments that experience has shown to form the main breeding grounds
(galleys, pantries, stores and native crew quarters). All fixtures and fittings behind which insects
harbour should first be removed so that when the fittings are replaced the lacquer behind them will
effectively prevent infestation. The lacquer application can be applied in place of the last coat of
paint. In addition it has been shown that pigments can be added to the lacquer as desired without
impairing the insecticidal properties and this would ensure that existing colour schemes could be
retained.
The property of the lacquer is to extrude on to its surface the crystals of the insecticide incorporated
in the body of the lacquer in the form of a bloom. The extrusion takes the form of microcrystals
less than ten microns in length in the case, for instance, of Dieldrin and these minute crystals
are much more toxic to insects than the larger crystals deposited by ordinary insecticide sprays.
Any irritation of the surface even by the insect walking across it will stimulate a further extrusion
of crystals as also will the cleansing of the surface in the ordinary course of removing dust or grease
or other dirt. The lacquer is tough and hard and resistant to washing soda so that it can be washed
down repeatedly and this will serve to promote a further bloom of the insecticide.
Practical experience has shown the continued extrusion of the insecticide over a period of two
years, the surfaces showing no diminution in their insecticidal action but remaining at maximum
efficiency during this period and probably even longer. When the insecticidal reservoir is exhausted
the action ceases abruptly, dropping immediately from the peak plateau of high toxicity to zero.
There is thus no gradual loss in toxicity as with ordinary insecticidal deposits and at no time is the
insecticide present in sub-lethal concentration. With this new type of insecticidal treatment, therefore,
there should be no development of resistant races of insects.
Of the various insecticides now tried the present experience seems to favour Dieldrin which is
approximately eight times as lethal as DDT to most insects. Dieldrin is, of course, more toxic to
warm-blooded animals but, with resin formulations the great bulk of the Dieldrin is safely sealed
within the resin and only a minute proportion is exposed at any time on the surface.
The phenomenon of blooming and of repeated regeneration must be controlled by careful formulation
in which insecticide, resin, plasticisor and solvents are balanced within the concentration limits
for a metastable solution. If insufficient insecticide is used the surface bloom may not materialise.
If an excess is employed the quantity of solute which the resin can maintain in metastable solution
may be exceeded and crystallisation w ill occur throughout the coating on the evaporation of the
solvent.
Experimental work in the Port of London has been directed primarily to the destruction of the
German cockroach (Blatella germanica) being one of the most persistent and difficult of insects to
eradicate. The lacquer is proportionately more effective against the American cockroach, flies,
mosquitoes, fleas and bugs and from all appearances it is also the answer to infestations of that most
difficult of infestations to cure, the Pharaoh Ant.
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