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The volatility of the Aldrin and BHC causes the evolution of a vapour readily toxic to insects,
while the action of DDT and Dieldrin is relatively postponed due to the lower vapour pressure.
The presence of active DDT and Dieldrin is exhibited by a crystalline "bloom" which is
exuded from the reservoir in the film of clear transparent lacquer. This phenomenon is not
precisely understood, but the innumerable and microscopic crystals are of a size and shape easily
picked up by the insects and their toxicity is not reduced by any substance contained in the
lacquer film. Moreover, the "bloom" may be generated spontaneously in a matter of hours with
a sufficient concentration of DDT or Dieldrin and crystals can also be induced by the mechanical
stimulation produced as an insect runs over the lacquer surface or by the friction created during
human cleaning operations.
Should the crystals be carried away by some external mechanical operation such as washing
or rubbing, they will regenerate from within the film until the reservoir of insecticide is
exhausted. Accordingly, three formulations have been dispensed for trials, each having a
common lacquer formula of resin, xylol and butanol to which has been added the following
proportions of insecticides by weight:-
(a) 20% DDT and 2% BHC.
(b) 10% DDT and 5% BHC.
(c) 10% Dieldrin and 5% Aldrin.
Application.
In the early stages of investigation, lacquer with these formulations was brushed on various
surfaces such as unpainted wood, painted surfaces, glass, brick and tiles and did not indicate any
significant variation of insecticidal performance despite reasonable exposure to the elements.
Insects were allowed to contact the crystals and Houseflies, Yellow Fever Mosquitoes and Flour
Beetles all succumbed after various durations of contact ranging from seconds to one hour.
Three years ago, by an arrangement between the Colonial Office Research Unit, the
Agricultural Research Council and the Port of London Health Authority, it was decided to do
several experiments aboard ships in collaboration with the shipowners. Ships which were
persistently infested with insects, particularly cockroaches, and those engaged in diverse trade
routes were selected. Most of the selected ships were freighters which regularly traded to the
Port of London and for that reason were available for intermittent inspection, and the records
compiled by the ships' officers were collected and examined. Two ships, however, which are
engaged consistently on the Far Eastern trade were included in major tests and records of the
results obtained over long periods mailed to London.
In the earliest tests, the treatment was confined to the pantries and galleys which were
heavily infested and the entire surfaces of deckheads, partitions and lockers were lacquered with
a clean brush, but it was soon realised that a more effective treatment could be given by spraying
the lacquer with a pneumatic paint sprayer. The spread produced a clean, uniform film and the
spray penetrated into insect harbourages inaccessible to the brush.
For large scale applications in confined spaces it was found necessary to wear protective
clothing and a respirator owing to the vapour evolved, particularly when the mechanical sprayers
were used. Wherever possible, the maximum amount of ventilation was introduced to minimise
any physical irritation from the vapour and, in the absence of naked lights, to reduce any fire
hazard to a minimum. Having achieved excellent initial results by a complete coverage,
attempts were made to secure the same results by reducing the area of treated surfaces to include
the runs and harbourages only, thus making a considerable saving in material and labour costs.
Such operations were limited to all obscure places, warm pipes and electrical circuits, being the
surfaces where insects live for the greater part of their existence and over which they crawl in
search of subsistence.
In order to avoid dirty smears or sealing of dust, all the surfaces to be treated were appropriately
cleaned and no freshly painted surface was covered until it had set sufficiently to avoid
excessive absorption of the lacquer.
Results.
The lacquer dried in less than six hours, leaving the surfaces with the appearance of having
been varnished. The odour of the solvents became negligible as the ventilation and drying
continued.
An immediate "knock-down" of insects was obtained and insects continued to perish as
they emerged from harbourages and contacted the treated surfaces.
The average area covered by the brush method was approximately 70 square yards and by
sprayers nearly 80 square yards per gallon.
Every care being taken to exclude fire hazards brought its reward, and no evidence of danger
occurred at any time. It was established by holding a naked flame to a dry treated surface, that
the lacquer changed to a hard, charred crust, but did not support combustion.
When the crystals did eventually appear as a "bloom", they were not readily visible to
the naked eye except when viewed in a strong light at a suitable angle. Even so, this "blooming"
did not affect the decorative effects of painted surfaces, but could be better recognised on
unpainted surfaces in lockers, etc. The needle type crystals varied in length from 5 to 120
microns and the density of the DDT "bloom" appeared greater than that of Dieldrin, but that
is due to the opaque nature of DDT crystals.
All the formulations proved very effective, but the ultimate order of merit as far as ship
trials are concerned are as follows:'—(c), (a), (b).
Subsequently, the reports from the ships on various trade routes and inspection visits proved
that a single treatment did in fact eradicate cockroaches for at least two years, and there is good
reason to believe that this period will be extended provided the "bloom" is not obliterated and
destroyed by a coat of paint or varnish.
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