INSECTICIDES
Item
- Title
- INSECTICIDES
- extracted text
-
j_.hr. s Medical Coil-:;;
o,M-!G;\LORE-56C03a
THE ROSS
I HI STI TOTE
! K FORWiftTBfflN AND
ADVISORY SERVICE
BULLETIN No. 1
JULY, 197 6
SiMSEffiUSOES
Published by THE ROSS INSTITUTE
THE LONDON SCHOOL OF HYOIENE AND TROPICAL MEDICINE
Keppel Street (Gower Street), London, WC1E 7HT
Introduction
The first edition of this bulletin closely followed the introduction
of the first residual insecticide, DDT, which was hailed as a wonder
chemical as it marked an immense step forward in the control of insect
vectors of disease. Progress was rapid, other residual insecticides were
produced, and eradication of malaria became a practical goal for some
countries.
Now the easier victories against malaria have been won and many
difficult areas remain. Insects have developed resistance to many
insecticides, and it has been realised again that insecticides are only one
of the weapons to be used in the fight against vector-borne diseases.
And, as with all weapons, they need to be correctly used. This bulletin,
again revised by Dr. G. Davidson from his immense experience of
their use, aims to give a detailed practical guide to the choice and
application of insecticides for the user. It considers methods for each
insect group of medical importance and pays due attention to inseticidc
resistance problems. We trust it will continue to serve the needs of
those who seek to control vectors and nuisance insects in the field.
Pififfs, 130
DAVID J. BRADLEY, MA, DM, MRCPath, MFCM
Professor of Tropical Hygiene, Director of the Ross Institute.
[2]
OCCUPATIONAL HL’ALI ?!
St. John’s Medical Collcj3ANGALORE-560034
CONTENTS
PAGE
...............................................................
2
INSECTICIDES...........................................................................
Insecticides in current use ...
...
...
...
...
Insecticide Formulations for the Residual Spraying of
Buildings ...
...
...
...
...
...
...
Residual Spraying
...
...
...
...
...
...
Organisation of Residual Spraying ...
...
...
...
Other Insecticide Formulations
...
...
...
...
Precautions in the use of Insecticides
...
...
...
4
5
8
10
15
16
19
THE CONTROL OF SPECIFIC INSECT PESTS
General Considerations
...
...
...
...
Anopheline Mosquitoes
...
...
...
...
Culicine Mosquitoes ...
...
...
...
...
Houseflies
...
...
...
...
...
...
Sand-flies
...
...
...
...
...
...
Fleas ...
...
...
...
...
...
...
Bedbugs
...
...
...
...
...
...
Reduviid Bugs
...
...
...
...
...
Ticks ...
...
...
...
...
...
...
Lice ...
...
...
...
...
...
...
Cockroaches ...
...
...
...
...
...
Mites ...
...
...
...
...
...
...
Biting Midges
...
...
...
...
...
Black-flies
...
...............
...
...
...
Tsetse flies
...
...
...
...
...
...
Scorpions
...
...
...
...............
...
Ants ...
...
...
...
...
...
...
Termites
...
...
...
...
...
...
Repellents
...
...
...
...
...
...
...
...
...
...
...
...
...
...
...
...
...
...
...
...
...
...
...
...
23
27
29
30
32
32
33
33
34
35
36
37
37
38
38
39
39
40
40
BIBLIOGRAPHY...........................................................................
41
GLOSSARY
42
INTRODUCTION
...........................................................................
[3]
THE ROSS INSTITUTE INFORMATION
AND ADVISORY SERVICE
Bulletin No. 1
Re-written July, 1976.
(Originally issued May, 1949, revised March, 1952; re-written June, 1955;
re-written February, 1960; revised March, 1964; reprinted October, 1967,
June, 1969, February, 1971 and October, 1973.)
Insecticides
by G. DAVIDSON, D.Sc.
The Ross Institute of Tropical Hygiene
Insecticides are now an accepted means of control of insect-borne
disease throughout the world. Indeed the complete eradication of some
diseases, in particular malaria, has been achieved in some parts by their
use. However as a result of this widespread use, insects have now
appeared which are resistant to them. Considerable research is con
tinually going on on this problem of resistance with some solutions
already being applied. These include the discovery of new insecticides
to which these resistant insects are susceptible.
This booklet enumerates the common insecticides available at the
present time and describes their use against the main insect disease
vectors of the world, as well as other nuisance insects. The main
principles which have emerged from a considerable volume of work on
the resistance problem are outlined and remedies suggested where such
a problem arises.
As is often the case where new and revolutionary methods are intro
duced there is a tendency' for the indiscriminate use of these methods
and the complete abandonment of previous ones. Such has been the
case with insecticides and one seldom hears at the present time of any
major attempt at permanent removal of insect breeding places as one
did before insecticides were available. It must be remembered that
effective control of insects by insecticides can only be achieved by their
continuous use and this may in many circumstances be much more
costly in the long run than the actual removal of insect breeding places.
This has usually been found to be the case in urban areas where, though
high initial costs may be involved in the removal of these breeding places,
e.g. drainage of mosquito breeding sites and efficient sewage and rubbish
disposal, such measures may' lead to complete eradication of insect pests
and thus eliminate the need of recurrent expenditure on insecticides.
[4]
In many rural areas, on the other hand, breeding places may be so
numerous, scattered and inaccessible that their control by such
permanent means is out of the question. In such circumstances insecti
cides arc the answer and an undoubted boon.
Though insecticides in general, and those stable and persistent ones
in particular, have come under attack in recent years from people
concerned with their effect on non-target organisms including man
himself, they must remain the principal weapon of attack against the
insect pests of medical, agricultural and veterinary' importance until
such time as suitable alternative methods are discovered. Undoubtedly
more attention will have to be paid to their methods of distribution,
particularly in outdoor situations, so that they arc more specifically
directed against the pests, and to the search for less persistent, bio
degradable compounds with a narrower range of activity with regard to
insect species.
INSECTICIDES IN CURRENT USE
Insecticides used in the field of public health arc usually contact
insecticides and fall into two groups: the non-rcsidual and the residual.
Of the non-rcsiduals the best known is pyrethrum which is the basis of
ordinary ‘ flit ’. Pyrethrum is a natural insecticidal mixture derived
from pyrethrum flowers. The dried, powdered flowers can be used
as such as a dusting powder but more usually a solvent extract is used
diluted with kerosene as an atomised space spray. The insecticidal
constituents of pyrethrum are unstable in light and air and so have
virtually no residual effect.
Pyrethrum, being a very quick-acting insecticide, is of considerable
value in the immediate alleviation of biting nuisance. It is usually used
in the form of a space spray as a 01 per cent, solution of pyrethrins in
kerosene and applied from a flit pump, a form of atomiser. At the rate of
1 fluid ounce per 3,000 cubic feet (29-6 ml. per 85 m3) in a closed room
kept closed for some ten minutes after application, such a spray will kill
most insect pests which are in that room at the time of spraying. When
the room is opened again, however, further insects entering will be
unaffected and a further spraying will be necessary to kill them.
It must be emphasised that for the most efficient use of space sprays
rooms must be closed when sprayed. It is realised that such conditions
cannot usually be met in the tropics. Here efficient screening and the
daily use of pyrethrum sprays in rooms closed as well as possible to kill
the odd insects entering through opened doors or windows arc
recommended.
There is a tendency for some of the larger insects, e.g. blowflies,
cockroaches, etc. to recover from an almost immediate knockdown by
pyrethrum and nowadays a little residual insecticide, e.g. BHC, is added
[5]
to proprietary formulations to eliminate this tendency. Modern formu
lations also contain synergists, e.g. piperonyl butoxidc, which increase
the efficiency of the pyrethrum. An example of a domestic spray
formulation is:—
Pyrcthrins
Piperonyl butoxide
’Lindane
Odourless petroleum distillate
Percentage Ff’/L
0-05
0-4
0-1
99-45
Dispersion methods have also been improved by the development of
aerosol containers, stout containers in which the insecticide is in solution
in liquefied gases under pressure (propellants). Release of pressure
causes the insecticide solution to be atomised in the most effective
manner to ensure that the small particles arc picked up by flying insects.
These aerosol containers are usually operated for 3 seconds per 1,000
cubic feet (28 m3) and deliver 1 gm. of formulation per second.
New synthetic pyrethroids, e.g. bioallethrin, bioresmethrin are now
on the market which are even more efficient than the natural compounds.
Examples of available formulations containing these new pyrethroids
are:—
Ordinary flyspray:
Pyrethrins
Bioresmethrin ...
Piperonyl butoxidc
Odourless kerosene and propellants to
Percentage IV/V
0035
001
0-175
100
Aerosol:
Bioallethrin
Bioresmethrin ...
Piperonyl butoxide
Odourless kerosene and propellants to
Percentage IV/IV
0-2
0-02
0-4
100
Non-residual aerosol sprays have been universally adopted by
quarantine authorities throughout the world for the disinfestation of
transport, particularly aircraft, to prevent the spread of disease and
vectors from one country to another. The World Health Organisation
•Where organochlorine resistance is evident this may be replaced by 0-5
fenitrothion or 0*5 diazinon.
[6]
recommends the adoption of a standardised aerosol with the following
composition for such disinfestation work:—
•
•
T
.
Percentage by
weight
Pyrethrum extract (25 % pyrethrins)
...
1-6
DDT technical
...
...
...
...
30
Xylene ...
...
...
...
...
...
7-5
Odourless petroleum distillate
...
...
2-9
Dichlorodifluoromethane
...
...
...
42-5
Trichlorofluoromethanc
...
...
...
42-5
Two per cent, resmethrin or 2 per cent, bioresmethrin in the
usual propellants are also recommended for this purpose.
Ground pyrethrum flowers or bioallethrin have been incorporated
in slow-burning, joss-stick-typc coils which produce an insecticidal
smoke both toxic and repellent to insects. Such coils, burning for some
nine hours, arc widely used in houses in many parts of the tropics,
particularly in the East.
An exciting new development is the discovery of stable pyrethroids
with residual effects comparable with conventional residual insecticides.
One whose code name is NRDC 143 has been given the appropriate
common name of permethrin.
Residual contact insecticides are stable, organic chemicals which,
when applied to a surface, remain toxic for some time, usually several
months, to insects alighting on or walking over that surface. Particles
of these insecticides, either in crystalline form or in oil solution, are
picked up by the insect’s feet and dissolve in the waxy outer layer
of the cuticle; penetration into the insect follows, and so the nervous
system is affected, leading to paralysis and eventual death. Their action
is relatively slow; often several minutes contact is required and death
may not occur for several hours. Thus a room treated with residual
insecticide may not show dead insects inside it because the affected
insects may leave the room and die elsewhere.
The residual contact insecticides most used in the control of insects of
medical importance are the chlorinated hydrocarbons (organochlorines).
However, cases of resistance to these insecticides are now quite common
and other groups of chemicals especially organophosphates and carbamates are being investigated with regard to their toxicity both to man
and to insects. Many are no more poisonous to man than DDT, c.g.
malathion, temephos (= difenphos = abate), fenchlorphos (= ronnel),
fenitrothion (= sumithion) and trichlorfon (= dipterex), all organo
phosphates and carbaryl (= sevin) and propoxur (= arprocarb),
carbamates. Unfortunately none of these compounds has yet been
shown to reach the standard of efficiency and persistence of the common
organochlorines currently in use, particularly on mud surfaces. Also
[7]
cases of resistance to them are now appearing particularly in houseflies
and in mosquitoes.
Of the many chlorinated hydrocarbon insecticides, only three have
been used to any great extent in the control of insects of medical
importance. These are, in order of their discovery:—
DDT (dichloro-diphenyl-trichlorocthane) of which the para-para
isomer is the most insecticidal.
Gamina-BHC (the gamma isomer of benzene hexachloride—trade
names: ‘ Gammexane ’ and ‘ Lindane ’).
Dieldrin (hexachloro-epoxy-octahydro-dimethano-naphthalenc) of
which the endo-exo isomer is the most insecticidal.
DDT and dieldrin are both very stable and persistent, the former
less toxic to most insects than the latter. Both are slower in action and
less toxic than gamma-BHC which is, however, slightly volatile and so
not as persistent as the other two. All three are to some extent irritating
to some insects, e.g. mosquitoes and houseflies, causing them to leave
a treated surface sooner than they would leave an untreated one. This
irritant property is most marked in the case of DDT, which often
causes the insects to leave a treated surface before they have picked up
a lethal dose of the insecticide.
In many countries DDT remains the sole organochlorine allowed
for pest control. BHC (or HCH as the World Health Organisation
prefers to call it) and particularly dieldrin are now considered unsafe
by many on toxicity grounds. In any case resistance to both compounds
is commonplace. Some countries have even banned the use of DDT
mainly because of its potential long-term pollutant effects on the
environment.
Insecticide Formulations for the Residual Spraying of Buildings
All the residual insecticides are toxic to most insect pests in verysmall dosages and thus for their efficient application some dispersion
medium is necessary. Most are insoluble in water and the use of solvents
for their dispersion introduces additional costs. For this reason
emulsions and water dispersible powders have been formulated,
requiring only water for their dilution. Conflicting results from the
use of these formulations were obtained in different parts of the world
for some years; studies of the fates of these formulations on different
tvpes of surface have explained to some extent these conflicting results.
The facts emerging from these studies are:—
(1) The most efficient formulation is that which, when applied to a
particular type of surface, leaves the insecticide on the surface in
a form readily detachable or absorbed by the insect.
(2) When dissolved in oil, insecticides are most toxic to insects, since
penetration through the insect cuticle is more rapid.
[8]
The evaporation of the solvent from an oil film of insecticide
usually' causes crystallization in a form not easily detached by the
insect.
(4) The speed of evaporation and consequent loss of toxicity depends
on the solvent and the type of surface on which it is applied.
Impurities in the insecticide or the addition of heavy oils may
slow down evaporation and so prolong the toxicity. Evaporation
from fibrous surfaces is in general slower than from smooth ones.
(5) The application of oil solutions to absorbent surfaces such as mud
leads to a rapid loss of effect as the insecticide is lost from the
surface. On fibrous surfaces such as paper and some woods this
loss by absorption is not always permanent and the insecticide may
return or 1 bloom ’ on to the surface in an easily detachable form.
(6) Emulsions (droplets of solution of insecticide held in suspension
in water with the aid of an emulsifier) behave in general like
solutions. Absorption on porous surfaces is, however, less rapid
and, if heavy solvents are used in their preparation, the process of
evaporation and crystallization is slower.
*(7) Waler dispersible or wettable powders are the best formulations
for use on absorbent surfaces; they are composed of particles of
the insecticide ground with an inert filler, e.g. diatomite, talc, etc.,
with the addition of a wetting agent to ensure an even suspension
when mixed in water. After application the water carrier is
absorbed leaving the insecticide on the surface in a readily
available form.
(8) The toxicity of solid insecticide particles depends on their size;
the smaller the particle the more readily it is picked up by the
insect. Particles of 10 to 20 microns have been found to be the
most toxic.
*(9) The proportion and particle size of the inert filler in a wettable
powder are important because of the possibility of masking of the
insecticide particles. Some filler is essential in the grinding
process involved in the manufacture of wettable powders as the
pure insecticides arc not easily ground alone (the heat generated
in grinding may cause melting and clumping of the insecticide).
Most wcttable powders nowadays have at least 50 per cent, of
filler but some with only 25 per cent, exist; obviously the less
filler, the less masking.
(10) After the evaporation of the water on hard, non-absorbent
surfaces, the wetting agent of these water dispersible powders,
being of a sticky nature, tends to cause adherence of the insecticide
particles to the surface so that they are not easily picked off by
alighting insects.
(3)
‘These statements do not necessarily apply in their entirety to liquid insecticides.
Here wettable powders consist of filler particles impregnated with insecticides.
[9]
On absorbent surfaces there is considerably less loss of insecticide
from wettablc powders than occurs with solutions and emulsions.
(12) On the most absorbent surfaces such as mud this loss from
wcttable powders may, however, be sufficient to cause a serious
loss in efficiency of the non-volatile DDT and dieldrin. This has
been clearly shown in laboratory tests but was not so marked in
field trials. With the volatile gamma-BHC, on the other hand,
no such loss in efficiency occurs for, through absorption may occur,
this insecticide continues to kill by a fumigant action in its vapour
phase. In fact, some degree of absorption is advantageous where
this insecticide is used, because loss by volatilization is slower than
on non-absorbent surfaces and so a more persistent but still lethal
effect is obtained.
(13) Humidity has now been shown to affect the efficiency of insecticide
deposits. Thus a sprayed surface giving only low kills in
conditions of low humidity (‘ in the dry season ’) may start to give
and maintain high kills when the humidity increases (‘ in the wet
season ’)•
All these facts may, at first sight, appear confusing, but the main
practical deductions are that solutions and emulsions are only efficient
when applied to relatively non-absorbent surfaces such as wood, metal,
painted surfaces, etc. On these surfaces emulsions will usually produce
a more persistent effect than solutions but the latter may be preferred
where marking is undesirable. For absorbent surfaces such as the
usual mud of dwellings in the tropics wcttable powders are the only
formulations which given an adequate available surface deposit. Very
often both types of surface are present in one and the same building.
The usual situation is one in which the roof is of non-absorbent
materia! and the walls of absorbent material. I Icre, wettable powders
are usually used throughout.
(11)
Residual Spraying
For the control of such insect pests as adult mosquitoes, houseflies,
sandflies, etc., which rest at random on the walls and roofs inside
buildings for at least part of the day or night, a uniform adequate dosage
of residual contact insecticide on all these surfaces is required. To
obtain such a uniform dosage a direct spray of liquid formulation is
applied, sufficient to wet thoroughly the surface without running off
and coarse enough to adhere to the surface. A very fine spray such as
that produced by an atomizer, e.g. an ordinary flit-gun, is unsuitable as
the tiny droplets produced tend to bounce off the surface being sprayed.
The ideal nozzle size for residual spraying has been found to be — in.
to £ in. in diameter (0-8-1-2 mm.), the smaller for solutions and
emulsions, the larger for wettable powders.
Undoubtedly the most efficient spray nozzle is that producing a
fan spray in one plane and this is the one usually used for surface
spraying. Distance between nozzle and surface has proved to be im[10]
portant in that it affects the proportion of spray actually adhering to the
surface. The normal distance for maximum efficiency is 18 inches
(45 cm.). This means that for the efficient spraying of high roofs and
ceilings extension lances arc necessary. Spraying is best carried out in
vertical bands which are the width of the fan-spray.
The simplest of the spraying machines is the stirrup-pump type.
Little maintenance of these machines is necessary and the operator can
see if such formulations as emulsions and wettable powders arc staying
suspended in the water as spraying proceeds. Efficient manipulation
requires the services of two operators, one for spraying and the other
for pumping and, if necessary, stirring. In this last respect a useful
addition to these machines is an attachment to the pump handle which
moves up and down in the liquid while pumping is being carried out.
To eliminate the necessity for continuous pumping and so the
necessity for a second operator, various types of pneumatic pumps have
been designed, normally for carriage on the operator’s back. These are
filled with the liquid formulation and air is pumped in to a certain
pressure. The World Health Organisation recommends an operating
pressure of 40 pounds per square inch (2-8 kg/cm2.) in conjunction
with a fan-type spray nozzle with a spray angle of about 80° and an
output of 0 2 U.S. or 0-17 Imperial gallons (757 ml.) per minute. Held
at a distance of 18 inches (45 cm.) from the surface to be sprayed, such a
nozzle and pressure should deposit a swathe of spray 30 inches (75 cm.)
wide, of which the middle 14-16 inches (35-40 cm.) is effective. In
practice, therefore, the marginal 7 or 8 inches (17-5-20 cm.) of one
swathe should be overlapped by the margin of the next swathe to
produce uniformity in deposit.
As the machine empties its two or three gallons (9 or 13-5 litres) of
spray liquid the pressure almost invariable falls, and a consequent
reduction in discharge rate ensues. A pressure control valve inserted
between tank and spray lance can overcome such variation so long as the
pressure in the tank exceeds that at which the control valve operates. The
type of machine in common use to-day is shown in the accompanying
illustration.
[11]
The usual type of machine employed for house spraying and the one
recommended by WHO.
[12]
Pneumatic machines require a certain amount of simple main
tenance, mostly involving the replacement of washers to ensure that
there is no loss of pressure through leaks.
For large-scale work, various types of motor-driven compressors
exist, but these need skilled maintenance. They are provided with
several leads and spraying lances so that several sprayers can operate
from the one machine, spraying several rooms or houses at one and
the same time. They are usually mobile, either mounted on wheels
or on a motor vehicle. Ease of access to buildings to be sprayed is
essential where these machines are used.
With all spraying machines a careful check should be kept on the
size of the nozzle apertures. Continuous use eventually causes an
increase in aperture size especially when wettable powders are employed.
This leads to over-application and wastage of insecticide. The problem
of erosion has been overcome to some extent in recent years by the use
of specially hardened steel, ceramics and plastics in the manufacture of
nozzles.
As already stated, residual spraying entails the application of
specific dosages of insecticide in a uniform manner. To calculate these
quantities and dilutions of the formulations to give these specific
dosages, three things must be known:—
(1)
The Pure Insecticide Content of the Formulation.—By pure is meant
the para-para isomer content of DDT formulations, the gamma
isomer content of BHC formulations, the endo-exo isomer content
of dieldrin formulations and the pure organic phosphate or
carbamate content of formulations of these insecticides. Specifica
tions arc usually given by manufacturers but if the formulations
arc being used in very large quantities it is advisable to have the
insecticidal content checked by chemical analysis from time to
time, as it may vary from consignment to consignment. Com
mercial DDT on which most of the formulations are based
contains about 80 per cent, of the para-para isomer, crude BHC
usually 13 per cent, of the gamma isomer (where manufactured as
such, gamma-BHC is of 99 per cent, purity and called Lindane),
and commercial dieldrin not less than 85 per cent, of the endo-exo
isomer. Most solution and emulsion concentrates of all these
insecticides contain about 20 per cent, w/v of the pure insecticide.
Most DDT and dieldrin wettable powders contain 50 per cent, of
the commercial insecticide, but 75 per cent, wettable powders are
available. Modern BHC formulations are now based on Lindane
and wettable powders containing 50 per cent. gamma-BHC are
readily available.
Organic phosphates and carbamates are also formulated as
emulsion concentrates and wettable powders and percentages of
active ingredients vary from compound to compound. Many of the
[13]
phosphates are liquids and wcttable powders are in fact impreg
nations of filler particles with insecticide.
The settling out in storage and transit of emulsions and wcttable
powders is not uncommon and leads to variations in insecticide
content at differing depths in the containers. This can be checked
by chemical analysis of samples from different levels and corrected
by thorough mixing before use.
(2)
The Suspension Properties of Emulsions and Het table Powders.—No
emulsion or wettable powder will stay in suspension in water
indefinitely but obviously those settling out rapidly will make for
unevenness in application. Crude tests by the addition of a small
quantity of the formulation to the right amount of water in a
glass tube arc usually sufficient to indicate satisfactory dispersion
or otherwise. Visible settling out should not occur for at least ten
to fifteen minutes. The World Health Organisation has now laid
down a detailed stringent test for the suspension properties of
wcttable powders and only those which meet the requirements of
this test should be used. Because of the importance of efficient
dispersion of emulsions and wcttable powders, manufacturers’
instructions regarding their dilution (c.g. creaming with a small
quantity of water prior to final dilution) should be rigorously
observed; dilution of these formulations should only be carried
out immediately prior to spraying.
(3)
The Application Rate of the Machine.—The application rates of
spraying machines can be fairly accurately determined by spraying
sheets of cloth of known area and weight with water, at a speed
consistent with the thorough wetting to a point of run-off aimed
at in ordinary surface-spraying, and recording the increase in
weight after spraying. The usual rate lies between one-half and
one Imperial gallon per 1,000 square feet (approximately 2-25-4-5
litres per 100 m2). The World Health Organisation, in recommen
ding a standard type of machine operating at a specified pressure
combined with a specific nozzle size, advises the training of opera
tors to cover 1,000 square feet (100 m2) in the time taken for the
machine to deliver one gallon (4-5 litres) of spray.
Usual application rates for the residual spraying of domestic
premises in terms of grammes of active ingredient per square metre
(multiply by 100 to obtain milligrammes per square foot) arc:—
DDT—2
gamma-BHC—0-4
dieldrin—0-6
organophosphates—2
carbamates—2
The following table gives some examples of quantities and dilutions
of some formulations needed to give such application rates, assuming
[14]
that the insecticide content of the formulation refers to the pure
insecticidal isomer where such exists and that application is at the rate
of one gallon per 1,000 square feet (4-5 litres per 100 m2):—
Insecticide
content
Dilution required
Insecticide
Formulation
. 50% 14 oz. per gallon of water
DDT ... Wcttable powder
(=90 gm. per litre)
. 75% 91 oz. per gallon of water
DDT ... Wcttable powder
(=60 gm. per litre)
DDT .. Solution concentrate .. . 20% 1 part to 31 parts of kerosene
DDT ... Emulsion concentrate 20% 1 part to 31 parts of water
. 25% 28 oz. per gallon of water
Malathion Wcttable powder
(=180 gm. per litre)
These dosages are those commonly recommended for the residual
spraying of houses for the efficient control of most house-haunting
insect pests, e.g. mosquitoes, houseflies, sandflies, bed-bugs, etc.
Organisation for Residual Spraying
A common mistake in the carrying-out of residual spraying
campaigns is to entrust the actual spraying to the lowest-paid type of
worker. It cannot be over-emphasised that the success of such cam
paigns depends on the application of an adequate, uniform dosage of
insecticide on all possible resting places of the insect pests to be
controlled. To achieve this requires considerable skill and the operators
involved should be recognised and treated as skilled labour.
Of equal importance is the necessity' for adequate and reliable
supervision and in this connection a small manageable team is preferable
to a larger and consequently more scattered one.
A simple unit, which can be multiplied according to the size of the
control scheme is:—
1 supervisor whose function should also be to record the houses
sprayed and those missed (for future spraying).
4 spray-operators, using pneumatic machines (if stirrup-pumps
are employed the number of operators would need to be eight).
I mixer, who is responsible for diluting and mixing the formula
tion to refill the machines.
1 driver, with a small vehicle for transporting equipment, water
and personnel.
Before spraying is started it is advisable to explain in simple terms
to the occupants of the houses the aim of the scheme, to warn them to
leave their houses open on specified days and to remove their cooking
and eating utensils, water, foodstuffs, small furniture, clothing, etc.
Larger furniture such as beds and tables should be left inside the
[15]
houses and sprayed with the walls and roof; this furniture often serves
as an additional resting-place for most house-haunting pests.
Under the most favourable conditions of house concentration and
ease of access to them one machine operator should be able to spray 40
houses of usual tropical village size in one day. This may fall to as low as
10 per day in areas where houses are scattered however.
The time required to complete the spraying of an area must
obviously be taken into account in organising a control scheme. If
continuous control is required all the year round, the time taken should
fall within the persistence time of the insecticide employed. If control
is seasonal, spraying should be completed before the pest season starts
and the toxic effects of the insecticide should persist until the end of
the pest season.
Other Insecticide Formulations
In addition to the formulations used for the residual spraying of
buildings, numerous other methods of dispersion of residual insecticides
exist and some of these arc now described.
(1) Dusts.—These are finely’ ground mixtures of insecticides and
light, inert diluents such as china clay, talc, etc. DDT dusts usually
contain 5 to 10 per cent, of the insecticide, I3HC dusts 0-5 per cent, of
the gamma isomer and malathion dust 1 per cent, of the insecticide.
Dusts are commonly used in the control of agricultural pests and against
cockroaches, fleas, fly maggots and lice, the precise location of which is
usually known. Dusts are also useful as mosquito larvicides on breeding
waters covered with considerable vegetation, where control by' oil-films
is impossible.
Various methods of distribution of dusts are employed ranging
from the simplest by hand to the use of dustguns and aircraft.
(2) Aerosols, Fogs, Vapours, Smokes and Fine Sprays.—Aerosols are
suspensions in the air of solid or liquid particles of insecticide, so fine
(usually’ less than 50 microns) that they' remain suspended for some
considerable time. They are produced in a variety of ways from several
kinds of machines:—
(i) Atomizer, in which a stream of air is made to impinge on a
stream of insecticide solution, as in the ordinary flit-gun.
(ii) Aerosol container, in which the insecticide is dissolved in or
mixed with liquefied gases under pressure. When the pressure
is released, the carrier liquid boils off, dispersing a cloud of
insecticide particles.
(iii) Fogging-machines.—Several types of larger machine exist for
the production of aerosols. In the Todd Insecticidal Fog
Applicator (T.I.F.A.) for example, an atomized spray is
introduced into a hot or cold blast of air and further
[16]
fractionized. The hot blast of air is more commonly used
and the aerosol termed thermal. In the Microsol machine a
solution of insecticide is introduced between discs rotating
at a very high speed (about 15,000 r.p.m.). The powerful
centrifugal forces produced by these spinning discs is respon
sible for the distribution of the solution in very fine droplet
form. The Swingfog machine operates on a pulse-jet system
and the Micron Sprayer on a rapidly rotating atomiser and
air-blast.
(iv) Smoke generator, in which the insecticide is mixed with a
slow-burning chemical; when ignited it produces a smoke
bearing fine particles of insecticide.
(v) Aircraft.—Two different principles are employed in the
production of sprays from aircraft. The insecticide solution
may be gravity- or pump-fed into a boom (beneath the aircraft
wings) bearing nozzles at intervals along its length. The spray
produced may not be very fine on emergence but is further
broken up in the slipstream of the aircraft. Insecticide
solution may also be distributed by introduction into the
exhaust system of an aircraft, in a similar way to the production
of a thermal aerosol from a fogging machine.
Aerosols are primarily used where rapid measures and good
penetration are required and where residual effect is not of first
importance. In fact surface deposits from aerosols, especially on
vertical surfaces, are too small to produce much residual action.
However, this concentration of fine particles suspended over long
periods is very effective against flying insects; to stimulate flight
pyrethrins are often included with residual insecticides dispersed in
this form.
The larger aerosol machines have been very effectively used for the
control of insect pests of stored products such as grain, tobacco, hides,
etc. in warehouses, ships, trains, etc., and in the control of insects
resting in dense vegetation, e.g. some mosquitoes, black-flies and tsetse
flies. Also because of their rapid operation they have been found to be
particularly effective where rapid control measures are considered more
essential than the slower methods of applying residual deposits. Thus
in epidemics of insect-borne diseases, such as fly-borne diseases and
plague, the vectors can be rapidly reduced in numbers not only in
dwellings but in outside breeding grounds.
Smokes have similar uses to liquid aerosols without the necessity of
elaborate machinery for their production, but arc not usually quite as
effective. One reason is that a proportion of the insecticide is de
composed by the heat required for the production of the smoke.
The dispersion of insecticides by aircraft is expensive but often the
only effective method of control of many widely distributed insect pests.
The method has been extensively used in North America for the control
[17]
of outdoor-resting mosquitoes (mainly culicines) and their larvae and the
larvae and adults of black-flies, which occur in enormous numbers over
very large areas in that part of the world. Smaller-scale trials have also
been made against tsetse-flies in Africa. Efficient aerial spraying can
only be carried out under certain meteorological conditions and much
depends on the particle size of the spray emitted and the density of
vegetation to be penetrated.
Gaining considerable popularity nowadays is the dispersal of neat
liquid insecticides by ultra-low-volume (ULV) dispersal methods
which result in the dissemination of very’ small quantities over large
areas. Malathion is often used in this way in the United States. It is
cither dispersed from the air using slow-flying aircraft or from ground
equipment in the form of cold aerosol machinery mounted on slowmoving vehicles.
(3) The Incorporation of Residual Insecticides in Whitewashes,
Distempers, Paints and Resins.—The mixing of insecticides with white
washes and distempers has little or no advantage over the spraying of
the insecticides on to such surfaces as the insecticide is diluted and
masked by the whitewash or distemper. In addition, lime may possible
cause a slow decomposition of DDT and BHC. With oil-bound paints a
great deal of the insecticide is lost in solution in the paint and is not
available to contaminate the insect. With a high concentration of
insecticide, i.e., more than sufficient to saturate the paint oils,
‘ blooming ’ of crystals of insecticide may occur on the surface; they
are highly toxic to alighting insects.
This principle of ‘ blooming ’ from saturated solutions has been
used with marked success in surface-coatings of urea-formaldchyde
resins containing some 20 per cent. DDT. ‘ Blooming ’ from these
surfaces continues over long periods (as long as one year after applica
tion) and such surfaces withstand the usual methods of cleaning—in
fact, rubbing the surface encourages ‘ blooming ’. However, the expense
of this formulation prohibits its widespread use.
(4) Insecticidal Pellets or Granules.—Pellets or granules of certain
types of clay, impregnated with insecticide, which disintegrate slowly in
water and release the insecticide, have been developed mainly as
mosquito larvicides for use in shallow-water breeding places with much
vegetation, where successful control by oil-films is impossible, e.g. rice
fields. Some residual effect is claimed because of the slow release of
insecticide. They can be distributed by hand or from the air and their
heavier weight compared with dusts ensures penetration through dense
vegetation to the water.
(5) High-spreading Oil Larvicides.—The control of mosquito larvae
by the use of oil has long been practised; to produce a continuous film of
oil toxic to larvae required some 20 to 25 gallons of oil per acre (9001,125 litres per 4,047 m2) in the past. The addition of spreading agents,
e.g. certain resins, to oil, increases its spreading pressure considerably
[18]
and enables a marked reduction to be made in the quantities necessary
to cover a given area completely. The addition of small quantities of
the residual insecticides greatly increases the toxicity of such thin films.
Thus 5 per cent. DDT in a high-spreading oil applied at only one
quart per acre (1-14 litres per 4,047 m2) will produce efficient larval
control.
The dispersion of such small quantities over such large areas
presents a problem far from solved as yet. Spraying from the air can
only be the answer where large areas of water are involved and where
economically feasible. The problem is partially solved by the use of a
small oiler which squirts, at each operation of the plunger mechanism,
1 ml. of oil 15 to 20 feet (4-5 to 6 m), a sufficient quantity to cover 2
square yards (1 -7 m2) of surface.
Other larvicides dependent on continuous film production and
being presently developed are lauryl alcohol, various mononuclear
lipids—fatty substances which penetrate the tracheal system of both
larvae and pupae—and Flit® MLO, a petroleum hydrocarbon. The
latter is used at the rate of 1-5 gallons (4-5-22-5 litres) per acre
(4,047 m2), low volumes being sufficient for the control of anophelines
but higher volumes being necessary for the more polluted culicine
breeding places.
Precautions in the use of Insecticides
All the chlorinated hydrocarbons, organic phosphate and carbamate
insecticides are poisonous to some extent to man. The following
precautions arc essential for persons habitually using dieldrin, organic
phosphates, or carbamates. (Where DDT or BHC are used the special
veil described in item (3) and the gloves described in item (7) are not
considered necessary):—
(1)
Operators should be carefully instructed in handling, and should
be informed that there is a risk in swallowing insecticide, or in
excessive inhalation, or in contact of the material with the skin.
(2)
Work should be carried out under adequate supervision.
(3)
Operators should wear an effective protection to the head and face
and it should be regularly cleaned. Such a head protection is
illustrated and consists of a veil of plastic netting suspended from
the brim of a light broad-brimmed hat or tropical helmet.
(4)
Separate working clothes, which cover the entire body, should be
used, removed at the end of each working day, and washed as
frequently as possible. Ordinary cotton clothing is suitable.
(5)
Operators should wash, using soap or detergent, at the end of each
working spell, and whenever insecticide is spilled in quantity on
the skin or clothes.
[19]
Reproduction from an article by Wolfe, H. E. el al (1959) in Hull Wld. Hlth.
Org. 20, (1), 1-14, by kind permission of the editor.
Operators should not smoke or eat whilst on duty, or at any time
after it without having first washed their hands.
(7) Operators should wear impervious rubberised gauntlets.
(8) Operators should not work more than 4 to 5 hours a day on
spraying.
(9) Equipment should be maintained in good condition, particularly
so that it does not leak or spill over the operator. In addition
the lowest spray pressures compatible with good spray pattern
should be used thereby lessening the bounce of spray from the
wall and operators should avoid as far as possible any spray drift.
(10) Insecticide should be handled with implements, scoops, spoons
and mixing rods and not with the hands, and containers such as
buckets should have handles, eliminating the tendency to soil the
hands when handling them.
A very great risk occurs when handling strong oil solutions, as in
mother-concentrates of emulsions, which should be handled through
threaded taps, or by pumps arranged to prevent contact of the solution
with the skin.
(6)
[20]
There is probably little real risk to the occupants of treated houses
but as a safety measure all foodstuffs should be removed or carefully
covered before a house is sprayed. There is some risk to domestic
animals, especially with dieldrin, and casualties have occurred amongst
cats, which can pick up a large dose by licking contaminated fur, and
chickens which have pecked along the foot of treated walls.
The signs of acute poisoning by the chlorinated hydrocarbons
include convulsions, accompanied by destruction of the liver tissue. Acute
poisoning due to swallowing should be met by emetics, c.g. a tablespoon
of salt in a glass of warm water. More chronic poisoning due to the
continued intake of smaller quantities is heralded by nervous symptoms
which include hyperexcitability, anxiety and tremors. In addition, there
is a very marked loss of appetite which quickly leads to loss of weight.
The nervous symptoms should be countered by the use of pheno
barbitone (a maximum single dose of 3 grains (200 mg.) could be given
if medical help were not immediately available and pending its arrival).
On medical advice it may be necessary to increase this dosage and to
maintain it over some considerable time. Animal experiments have
indicated that large dosages of phenobarbitonc over 2 weeks or more
may be necessary to keep poisoned animals from showing hyper
excitability or convulsions and to enable them to eat and behave
normally. The dosage is often in excess of that which would induce sleep
or even anaesthesia in a normal animal. In human beings the dosage
should be adjusted to the clinical signs.
Any person who is thought to have suffered toxic effects due to
handling chlorinated hydrocarbon insecticides should be removed
from risk of contact with the insecticide for a long period: dieldrin
is known to persist in the body in toxic amounts for many months, and
six months’ freedom from further risk should be ensured.
Symptoms of poisoning by organo-phosphorus insecticides are
similar in many respects to those of chlorinated hydrocarbon poisoning,
but include also bronchial disturbances. In severe cases artificial
respiration by mechanical means may be necessary before the admini
stration intravenously of atropine followed by 2-pyridinium aldoxime
methiodide (2-P.A.M.-iodidc). It is strongly advised that supplies of
atropine should be available in first-aid kits when organo-phosphorus
or carbamate insecticides are being applied and that the spraying
supervisor should be trained to administer atropine in emergencies.
Medical help should be sought immediately poisoning is suspected
and any administration of 2-P.A.M.-iodide left to the doctor. Organic
phosphates inhibit one of the vital enzyme systems of the body,
cholinesterase, and it is advised that people habitually handling these
insecticides should have their blood cholinesterase activity checked
periodically. Operators should be withdrawn from exposure if this
activity decreases by 25% or more from a well-established pre
exposure value.
ML ISO
'l
[21 ]
■.T.L
■
■
■'■■■
1
The insecticidal carbamates give rise to a more rapidly reversible
cholinesterase-inhibitor complex. This makes it impossible to use
estimates of cholinesterase activity in vitro as an accurate index of the
activity of this enzyme in the tissues. In cases of poisoning by carba
mates all the methods used for treating poisoning by organic phosphorus
compounds are useful with one exception: 2-P.A.M.-iodide and other
oximes arc not recommended for routine use. Recovery from carbamate
poisoning is usually quite rapid.
[22]
THE CONTROL OF SPECIFIC INSECT PESTS
General Considerations
The successful control of insects which merely act as mechanical
carriers of disease involves their complete eradication or reduction to
very small numbers; an example is the housefly, which can pick up the
germs of such diseases as typhoid and dysentery on its body or ingest
them. Usually, eradication can only be achieved by a combined attack
on larvae and adults. With insects which act as true hosts to organisms
of disease, i.e. in which the organism undergoes a cycle of development
inside the insect, successful control entails the reduction of length of life
of the insect vector to below the time it usually takes for it to become
infective. This can be achieved by attack on the adult insect only and
may not require drastic reductions in overall numbers. In the case of
malaria, for instance, the parasite (Plasmodium vivax or P. falciparum—
the extrinsic cycle of P. malariae is considerably longer) may take about
two weeks to complete its development within the anophelinc mosquito.
The mortality required amongst an anophelinc population to prevent
any one individual living this length of time depends on:—
(«) the density of the anopheline vector population;
(6) the frequency of biting man;
(c) the natural mortality.
It has been estimated that a continuous mortality of about 50 per cent.
would be commonly sufficient to intercept malaria transmission by
species A of the Anopheles gambiac complex or A. funestus in tropical
Africa, mosquitoes which occur in moderately large numbers, have a
low natural mortality and feed usually every two days almost exclusively
on man. In contrast, in India A. culicifacies, which may occur in very
large numbers but feeds mainly on animals and has a fairly' high
natural mortality, would only require a continuous mortality of some
30 per cent, to intercept transmission. Thus a more efficient insecticide
will be required in the former case than in the latter. DDT with its
highly irritant nature may not in fact produce high enough mortalities
to intercept transmission by the more efficient vectors and particularly
those which do not spend all the time between egg-layings inside
houses.
Measurement of the real mortality amongst insects exposed to
residual insecticides involves the recovery, dead or alive, of all the insects
coming into contact with them and the recording of delayed mortalities
amongst those insects caught alive. This involves the fitting of window
traps to sprayed houses. The window-trap resembles a lobster-pot in
that entrance, from the inside of the house, is possible but escape from it
impossible. All other major sources of light are blocked off and the
window trap is then the main attraction for insects wishing to leave.
Mosquitoes entering the house through gaps in the eaves to feed on the
occupant would rest on the walls or roof after feeding, some to die later
[23]
inside the house (these can be collected from the floor) and some to
attempt escape from the house and to be caught in the window-traps.
The latter are kept for some hours afterwards and the delayed mortality
recorded. Thus the total mortality is represented by:—
Total dead on floor
number dying later in window-trap
Total dead on floor
total in window-trap
Huts treated with DDT never show a complete mortality even on
the day after spraying but on most types of surface and against most
anopheline species (in the absence of resistance) it will produce at least
50 per cent, mortality over a period of 6 months when applied at the
rate of 2 gm./nr. Thus most spray campaigns using DDT in areas of
perennial malaria transmission are based on a twice-yearly spraying
cycle. Where transmission is seasonal and the season lasts less than 6
months only one application per year will be necessary providing it is
completed before the season starts.
BHC will usually produce a complete mortality for one or two
months after its application but its effect will deteriorate rapidly in the
third and fourth months particularly on non-absorbent surfaces and in
hot climates. However longer persistences may occur on active absorbent
muds where presumably absorption slows down volatilization.
Dieldrin having the stability of DDT without its marked irritant
effect and the toxicity of BHC without its volatility can produce very
long persistences. Unfortunately its human toxicity and the frequency
and level of resistance it produces now limits its use.
Organophosphates and carbamates are by comparison with the
organochlorines relatively unstable compounds especially in the
presence of water and alkali. They do not therefore have such long
persistences especially on mud surfaces. Many field measurements of
mortalities produced by them have shown most of the kill to be produced
by the deposits persisting on the non-absorbent parts of the building
sprayed and some would advocate only spraying these surfaces in the
interests of economy. Malathion, and in particular propoxur, are many
times more expensive than DDT and if, as has been indicated by some
field trials, spraying 3 and 4 times a year may be necessary, costs may be
prohibitive. However, where resistance to the organochlorines is
prevalent there may be little alternative to their use if disease control is
required.
Some results.—In some parts of the world, mainly islands, it has
been possible to eradicate some insect vectors completely, either by
attack on only the larval stage or only the adult stage. A. ganibiae,
accidentally introduced into Brazil from Africa, was successfully
eliminated by antilarval measures as were anophelincs in the island of
Cyprus. Successful eradication of A.funestus in Mauritius was achieved
solely by anti-adult methods although coincidental elimination of the
[24]
other malaria vector, .-I. gatnbiae, failed. In the coastal region of
British Guiana, >1. darlingi was eliminated by the residual spraying of
houses with DDT.
Eradication in limited continental areas surrounded by uncontrolled
terrain and not protected therefrom by natural barriers such as desert,
mountain ranges, etc., is a virtual impossibility as reinvasion from the
uncontrolled area will occur. Nevertheless, the gradual cessation of
control measures from the centre of the controlled area towards the
periphery, leaving an outer barrier zone continuously treated, should be
possible and lead to considerable saving in recurrent expenditure. Thus,
in British Guiana, control was stopped in the coastal area and only
possible courses of invasion from the hinterland controlled.
World eradication of a disease such as malaria as distinct from
eradication of the insect vectors was until quite recently considered a
distinct possibility. Now the idea has been abandoned for most of the
larger areas of the tropics. Here the future will be dependence on a
combination of control measures directed at specific localities, the
reduction of disease in which will benefit the overall development of the
country the most. These measures will undoubtedly include insecticides.
Resistance.—Resistance to all the chlorinated hydrocarbon insecti
cides is becoming more and more common among insects of medical
importance and among houseflies and mosquitoes resistance to the
organophosphorus compounds, carbamates and even to pyrethroids
has been recorded in addition. This resistance is not acquired by insects
after contact with insecticide but is inherent in some individuals of a
population. It is these individuals and their progeny which survive the
effect of contact with insecticide and replace the original mixed popula
tion. The speed with which resistance appears thus depends on the
proportion of resistant individuals originally present and the pressure of
selection. Dieldrin-resistant .4. ganibiae have been shown to be present
in unsprayed areas of West Africa to the extent of more than 10 per cent.
of the mosquitoes. Using an efficient insecticide like dieldrin which
produces nearly 100 per cent, kills of susceptible mosquitoes, selection
of a pure resistant population from such an area would occur within only
a few generations of the mosquito population and within only a month or
two after spraying.
Generally speaking, two types of resistance to the chlorinated
hydrocarbons exist:—
(1) to DDT and allied products such as DDD and methoxychlor and
to dieldrin and allied compounds, e.g. aldrin, chlordane, etc. and
to gamma-BIIC.
In some instances when one of these types of resistance has appeared
it has been possible to continue controlling the insect species by a
change of chlorinated hydrocarbon. For example, .4. sundaicus in
certain areas along the north coast of Java showed resistance to DDT,
(2)
[25]
some years after the wide-scale use of this insecticide in houses. A
change of insecticide to dieldrin was successful and no resistance to
dieldrin appeared in these areas, though it did so in areas on the south
coast, and DDT was used in these areas.
In general, dieldrin-resistance is much more common and arises
much more rapidly than does DDT-resistance. This is because
dieldrin-resistance is of a much higher degree and is dominant or
partially-dominant in its genetic expression, that is to say, that even the
individual which is impure genetically is partially or sometimes fullv
resistant and can survive field-dosages of the insecticide. DDTresistance, on the other hand, is usually of a low order and recessive in
its genetic expression. Here, only the individual genetically pure for
the resistance factor survives the field-dosage. Thus if the type of
resistance in the insect population to be controlled is unknown at the
outset it would seem preferable to use DDT rather than dieldrin or
BHC. The development of resistance may then only be slow and
sufficient mortality may be inflicted among the insect population to
achieve the object of disease eradication. These arguments apply in
particular to resistance in anopheline mosquitoes. In fact even if DDTresistance is detected in anophelines it is not advisable to change the
insecticide until it has definitely been proved by field observations that
malaria transmission is on the increase.
Where resistance to both groups of chlorinated hydrocarbons
occurs a change to one of the organophosphates or carbamates may be
necessary. Resistance to one of the compounds in these groups does
not necessarily mean cross-resistance to the members of the same group
fortunately, though some evidence of cross-resistance between some
carbamates and some organophosphates is evident. In other words, a
wide choice of alternative insecticides still exists for most insect pests
(houseflies may be an exception) though whether these can be afforded
is another question.
To prove resistance entails a comparison of the susceptibility of
the insect in question to a particular dosage of the insecticide before
and after control measures have been adopted. The World Health
Organisation has now produced several test kits for this determination
of susceptibility, particularly for mosquitoes, bed-bugs, fleas, sandflies
and lice. Alternative measures of control should not be resorted to until
real proof is established and the possibility of failure of control due to
other causes has been investigated. Other causes may be:—
(1) inadequate dosage of insecticide;
(2) poor formulation, e.g. large particle size;
(3) change in habits of insects, from indoor-resting to out-door
resting, for example.
Ideally, the detection of resistant individuals before spraying
programmes are commenced would determine the insecticide to be used.
[26]
Work on the mode of inheritance of resistance, much of which has been
done in the Ross Institute laboratories, has shown the possibility of
recognition of resistance in anopheline mosquitoes. In most species
studied a single genetical factor is responsible for resistance and by the
use of discriminating dosages of insecticides susceptible and resistant
and sometimes hybrid mosquitoes can be recognised.
Anopheline Mosquitoes
The choice of anti-larval or anti-adult measures for the control of
anopheline mosquitoes depends on local conditions. Where breeding
places are not numerous and can be controlled by permanent measures
such as drainage, canalization, filling-in, etc., these may prove more
economical in the long run than the continuous use of insecticides.
Where economics prohibit such measures, efficient larviciding may be
cheaper than adult control but it must be remembered that those larvae
which escape such control measures will produce adults which can live
their normal length of life and so become infective. Where breeding
places arc so extensive and numerous that anti-larval measures are out
of the question, as happens in many rural areas, anti-adult measures by
means of residual insectidcs may be the only methods of control.
Fortunately most malaria vectors spend some part of their life
inside houses and stay there sufficiently long to pick up a lethal dose of
insecticide, if the houses arc effectively treated. However, they do vary
in the length of time they spend in houses and obviously the longer they
do so the higher the mortality from insecticidal treatment is likely to be.
Thus A.funestus in Africa tends to spend nearly all its time in houses,
only leaving them to lay eggs. Species B of the A. gambiae complex, on
the other hand, may only spend half its time in houses, the other half
being spent resting elsewhere. The Kerteszia mosquitoes, A. bellator
in Trinidad and A. cruzi in Brazil, are examples of mosquitoes which
seldom rest in houses and so cannot be controlled by house-spraying.
These mosquitoes breed in water, held in the axils of the leaves of
bromcliad plants; control is exercised by the removal or treatment of
such plants.
Larvicides.—Except at high and often uneconomical dosages, the
residual contact insecticides have little residual effect when used as
larvicides on or in water. This may be due to their dilution or to being
washed away or to absorption by mud and vegetation. Thus mosquito
larvicides have, in general, to be reapplied at intervals within the aquatic
cycle of the species. This usually amounts to application once a week
but the interval may be as little as five days in the hottest parts of the
world.
The choice of formulation for larvicidal work depends on the type
of breeding place. Oils .which depend for their success on being able to
spread into a continuous film, can only be used on relatively open water,
[27]
though of any depth. Where vegetation impedes the spread of oil,
dusts, emulsions and pellets can be used. The two latter formulations
are more efficient in shallow than in deep water as dilution is then not
so great.
DDT has frequently proved its efficiency as a mosquito larvicide.
The required dose is minute, about one-quarter of a pound of the active
product to one acre (28 gm. to 1,000 m2) of water surface, and the
difficulties of treatment turn on spreading such a small quantity over
such a large area. Two methods are used: the distribution of DDT in
oils which have very high spreading properties, and the use of very
dilute emulsions of an oil solution of DDT in conveniently large
quantities. Preparations are therefore marketed in two types, a 5 per
cent, solution of DDT in a “ high spread oil ” and an emulsion con
centrate of DDT. The first is intended to be sprayed at the rate of about
2 to 4 pints per acre (0'28 to 0-56 litres to 1,000 m2) of water surface;
the second is intended to be diluted with water till the final strength of
DDT is between 01 and 015 per cent, and then sprayed from a
knapsack sprayer at the rate of 10 to 20 gallons to the acre (11 to 22
litres per 1,000 m2).
Among the organophosphates malathion at 3 to 6 ozs. per acre (85
to 170 gm. per 4,047 m2) has proved efficient as an anopheline
lavicide, but becoming more popular because of its very low toxicity' to
man is abate. This can be applied as a dilute emulsion at the rate of 0-5
to 1-5 fluid ounces per acre (14-43 ml. to 4,047 m2) or as sand-core
granules containing 1 per cent, of the insecticide at 5-10 lb. per acre
(2-27-4-45 kg. to 4,047 m2) in shallow, clean water or 10-20 lb. per
acre (4-54-8-90 kg. to 4,047 m2) in tidal water of high inorganic
matter content. Some considerable residual effect has been claimed for
these treatments with abate.
hnagicides.—For most house-haunting anopheline mosquitoes
residual spraying of the interior of houses at 200 mg. per sq. ft. (2 gm./
m2) of DDT will give efficient control for at least 6 months and at 40 mg.
per sq. ft. (0-4 gm./m2) of gamma-BHC for 3 to 6 months depending
on the absorptive properties of the surfaces. The optimum dosage of
the organophosphates malathion and fenitrothion and the carbamate
propoxur appears to be 200 mg. per sq. ft. (2 gm./m2) and persistences
have varied from a few weeks to several months again depending on
the prevalent surface sprayed.
As already pointed out, the residual insecticides are relatively slow
in action and therefore treatment of houses with them may give only'
little relief from the bites of incoming mosquitoes before they succumb
to these residuals. For relief from bites the frequent use of space sprays
containing pyrethrins or the newer synthetic pyrethroids and DDT is
preferred. It need hardly be said at this stage that little control of either
insect pests or the diseases they carry will be achieved by the residual
spraying of single or a few houses in amongst numerous untreated ones.
[28]
Insecticide resistance has now been established in some 44 species
of anophclines, 28 of which are malaria carriers. All 44 species have
shown dieldrin-resistance while 24 have shown resistance to DDT as
well. Among those resistant to both DDT and the BHC-dicldrin group
arc some of the most important malaria vectors in the world, e.g.
A. gambiae species A and B, A. stephensi, A. culicifacies, A. sacharoii
and A. albimanus. To make things worse the last two of these species
are now showing resistance to several organophosphates and to
propoxur, A. sacharovi in Turkey and A. albimanus in Central America,
as a result of the widespread use of these compounds for agricultural
purposes.
Culicine Mosquitoes
The control of culicine mosquitoes, some of which are important
vectors of disease, e.g. filariasis, yellow fever and various other virus
diseases, is usually more difficult than the control of anophelincs. Many
of them, e.g. Culex fatigans, are less susceptible to insecticides, while
some do not habitually rest in houses.
The most efficient method of control of Aedes aegypti, carrier of
dengue, dengue haemorrhagic fever and of yellow fever, would still
appear to be by the removal or treatment of breeding places which are
usually small and in the immediate vicinity of houses. Abate sand
granules form a most convenient method of treatment of water collec
tions and seem safe even for use in drinking water. No increase in
concentration of this insecticide could be detected in water treated at
3- or 6-weck intervals nor were any toxic symptoms exhibited in a
village population exposed to such treated water for 1} years. The
effective concentration is of the order of one part per million.
Aedes aegypti eradication programmes have in the past been based
on the treatment of the interior and exterior of all possible breeding
sites and of the adjacent wall surfaces (i.e. the perifocal method) with
conventional residual insecticides. In those territories in which Ae.
aegypti normally rests in houses, house-spraying with residual in
secticides has been successful to the point of elimination of the
mosquito; this occurred in the coastal regions of the Guiana countries
of South America where Anopheles darlingi was eradicated at the
same time.
In most other instances of culicine nuisance, anti-larval measures of
control are the most effective. Many species which habitually bite man
breed in well-defined waters quite near to habitations. Control by
removal or treatment of these breeding places is often relatively simple;
the difficulty lies in locating all of them. In general, insecticidal
emulsions, suspensions and pellets are better as larvicides than oil
solutions, especially where there is a large amount of organic matter
in the water as there often is in culicine breeding places; this prevents
the spread of oil. Higher dosages than those used for the control of
[29]
anopheline larvae are usually required and allowance must be made
for effects of dilution where water is deep. Dursban (chlorpyrifos) is a
particularly good larvicide for use on water of high organic content.
The control of widespread culicine breeding is a much more
difficult problem. Here fogs, dusts and ultra low volume (ULV) appli
cations come into their own. In the United States the control of salt
marsh mosquitoes Culex tarsalis, the chief vector of western equine
encephalitis, is usually effected by such means using malathion,
naled (dibrom), abate, dursban, propoxur and now the synthetic
pyrethroids. Dissemination may be from the air or from ground
equipment (malathion is the only insecticide allowed to be dispersed
from the ground). Application rates of malathion and abate used are
0-2-0-5 and 0-05-0-1 lb. per acre respectively (91-227 gm. and 23—45
gm. per 4,047 m2). Multi-resistances are now common in all the
species involved.
The control of Mansonia mosquitoes, the main vectors of rural
filariasis in Ceylon, has been successfully achieved by the use of the
sodium salt of methyl-chlorophenoxyacetic acid to kill the plant Pistia
stratiotes to which the larvae of these mosquitoes attach themselves.
2 oz. (57 gm.) of this herbicide per gallon (4-5 litres) of water sprayed
over the plants at the rate of 36 gallons (162 litres) per acre (4,047 m2)
kills young plants in 5-10 days and old plants within about 14 days.
Resistance to all the chlorinated hydrocarbons is widespread in
Culex fatigans and even in its susceptible state this species in the adult
form shows a high natural tolerance to them. The larvae of susceptible
C. fatigans are quite susceptible, however, particularly to DDT.
Organochlorine resistance is now widespread in Ae. aegypti and cases of
organophosphate resistance are beginning to occur.
Houseflies
Aerosols or ‘ space sprays ’ are widely used in households for
destruction of houseflies and other flying insects. While this method
cannot deal completely with a serious fly nuisance it is a reliable way of
ensuring the destruction of small numbers of intruders into larders,
kitchens, etc. Some of the more modern aerosol sprays have already
been described.
In using residual insecticides against houseflies attention is usually
directed against those areas where these insects tend to congregate,
rather than the complete spraying of the insides of houses. Such
places as doors, windows, sills, some of the outside surfaces of houses,
kitchens, porches, latrines, animal shelters and fences should receive
special attention where the control of adult flies is concerned and,
where larval control is aimed at, refuse dumps and accumulates of
animal excrement and their vicinity. The spraying of refuse dumps,
especiallv where these are large, will not always kill fly larvae but will
[30]
kill flics attempting to lay their eggs there and also new adults as they
emerge. It must be emphasised that no insecticidal treatment can
replace efficient methods of rubbish and excrement disposal.
As residual sprays the chlorinated hydrocarbon insecticides can
be used at the rates advocated for mosquito control and will suffice
to control both larval and adult houseflies providing resistance to the
insecticides is absent. Unfortunately it seems almost inevitable that
resistance will develop, and rapidly too, and the replacement of DDT
by dieldrin or BHC will soon result in resistance to all the chlorinated
hydrocarbons.
Nowadays emphasis is on the use of organophosphates with
diazinon, dimethoate, fenthion, tetrachlorvinphos (Gardona), mala
thion, naled and ronnel all being recommended but with some restric
tions on the use of some of them in dairies, milk rooms, poultry houses
and food-processing plants (see 17th Report of World Health Organi
sation Expert Committee on Insecticides. Tech. Rpt. Series No. 443,
1970).
It has been found helpful to add attractants, c.g. sugar, molasses,
to these insecticides at the rate of 2-5 parts attractant to one part of
toxicant. As an example, inalathion-sugar emulsions or suspensions at
2 gm./m2 of malathion and 5 gm./m! of sugar give good control for
1-5 weeks and diazinon and sugar at 1 gm. diazinon and 2-5 gm.
sugar/m2 give good control for 4 or 5 weeks. Spot treatment of areas
where flics are known to congregate is favoured rather than the
wholesale spraying of buildings.
A recent innovation is the use of cords or strips impregnated with
insecticide and hung in buildings. 5 mm. (3/16 in.) cords dipped in
25 per cent, diazinon-xylcnc solution and installed at the rate of 30 feet
(10 m.) of cord per 100 sq. ft. (10 m2) of floor area have been found to be
effective for 5-6 months in dairy barns in the U.S.A. Great care is
needed in the handling and preparation of these cords which should be
marked or coloured to show what they arc and to prevent them being
used for other purposes. In Scandinavia strips of gauze impregnated
with parathion and festooned from the ceilings of buildings arc favoured.
Parathion however, is the most poisonous to man of all the organo
phosphorus insecticides and so has to be very carefully handled.
Organic phosphates, c.g. malathion, trichlorfon, ronnel, diazinon,
naled, dimethoate and dichlorvos, have also been incorporated in liquid
and dry baits for spot treatments against flics. 0-1 to 0-2 per cent.
toxicant and 10 per cent, attractant applied by watering can at 0-8 to
2-5 gallons per 1,000 sq. feet (3-6 to 11-25 litrcs/100 m2) of floor area
has been found efficient. In a dry state 1 to 2 per cent, toxicant added
to granulated sugar and scattered at the rate of 2 to 4 oz. per 1,000 sq.
feet (57-114 gm./lOO m2) has been used. Two treatments per week by
these methods for 2 weeks was found to give good control for 12 weeks.
[31]
Diazinon appears to be one ot the most potent organic phosphates
for use as a larvicide. 0-5 to 1 gm./m! applied in 6 to 12 gallons (26
to 54 litres) of water to every 1,000 sq. feet (100 in2) is efficient for 10
to 14 days. Ronnel is also particularly effective against houseflies, and
has a low mammalian toxicity. In the United States it has been found
to be effective as a residual spray for 4-5 weeks at 100 mg./sq. ft.
(1 gm./m2) and at double this rate for S-9 weeks. It has also been used
successfully as a cord-impregnant.
Other suggested uses of insecticides for housefly control are to be
found in Ross Institute Bulletin No. 5.
Resistance to the organic phosphates in houseflies is already of
common occurrence. Cross-resistance patterns within this group of
insecticides are not yet clearly defined. Malathion resistance apparently
imparts resistance to only' a few other organo-phosphorus compounds
while diazinon-resistance imparts a wider cross-resistance. Diazinon
and trichlorfon seem at present to be suitable alternatives where
malathion resistance is present. Some evidence exists of cross-resistance
between organic phosphates and carbamates.
Paradichlorobenzene is said to give satisfactory control of Hybreeding for 1-2 weeks if added to dust-bins at the rate of 60 gm. or
2 oz. per bin. This is a simple means of control suitable for the
individual householder.
Sand-flies
The spraying of houses with residual insecticides at the dosages
given for anopheline control will control the adults of Phlebotomus
species, the vectors of sand-fly fever and kala-azar. The breeding places
of these insects are usually very difficult to locate and are said to occur
mainly in the rubble of old walls or similar debris. No case of resistance
in these insects is yet known.
Fleas
Concentration of insecticide treatment on the usual haunts of these
insects rather than complete spraying of houses is more efficient. In the
case of the human flea, Pulex irritant, this involves the application of
insecticide dusts, e.g. 10 per cent. DDT or 0-5 per cent. gamma-BHC,
to clothing, bedding, carpets and house-floors. Rat fleas, e.g. Xenopsylla
choepis, the principal vector of plague, can be similarly controlled by'
concentration on rat burrows and runways. Higher dosages, e.g. 50 per
cent. DDT powder or 3 per cent. gamma-BHC powder, are reputed to
kill the rats as well as the fleas. The rats are poisoned when they lick
insecticide off their feet. Fleas of domestic pets can be controlled by the
use of dilute dusts, e.g. 5 per cent. DDT, 0-5 per cent. gamma-BHC,
although care must be taken not to apply in too large quantities for fear
of poisoning animals, especially cats, which are in the habit of licking
[32]
c
themselves. Malathion dusts (4—5 per cent.) are also effective against
dog and cat fleas and can be safely applied to the host animals in
moderate quantity. Other insecticides that have been used with success
on pets for flea control either as dusts, dips, washes or sprays arc
carbaryl (a carbamate), coumaphos (an organophosphate), pyrethrum
and rotenone. All should be applied to the animals’ sleeping quarters as
well as to the animals themselves. A method suitable for some pets is
the wearing of collars made of plastic impregnated with the volatile
organophosphate dichlorvos. The vapour given off is reputed to kill
the fleas over a considerable period of time. Silica aerogels (see section
on cockroaches) should be particularly useful for the control of fleas of
domestic animals and without any toxicity risks. Several cases of
resistance to the chlorinated hydrocarbon insecticides, particularly
DDT, have been recorded in fleas and malathion resistance has been
found in X. cheopis in India and Vietnam.
Bedbugs
There are two species associated with man. Cirnex hemiplerus is
strictly tropical, while C. lectularius is world-wide in its distribution.
Though not definitely associated with the transmission of disease, these
insects are particularly irritating in their bites and obnoxious in their
smell. They are so much disliked the world over that when house
spraying was instituted as a malaria control measure it was more
welcomed in some parts as a means of bed-bug control than for what it
was primarily intended and when the bed-bugs became resistant to the
insecticides it became more and more difficult for spraymen to gain
access to the houses.
Ordinary residual house-spraying combined with special attention
to crevices in walls, floors and furniture and to beds and bedding (and
perhaps aided by the flushing effect of added pyrethrins) will effectually
control bed-bugs for long periods. Unfortunately, resistance to the
chlorinated hydrocarbons is now widespread. Malathion (0-5 to 1 per
cent, spray) is usually adopted as a substitute in such circumstances but
cases of resistance to malathion and other organophosphates are now
known. Suggested alternative sprays are ronnel (1 per cent.), trichlorfon
(0-1 per cent.), propoxur (1 per cent.) and dichlorvos (0-5 per cent.).
Care must be taken in applying some of these chemicals. They must be
applied sparingly and only to the bedding of adults and not to that of
infants. Retreatments should not be made at less than two-week
intervals. Synergized pyrethrin sprays (0-2 per cent, pyrethrins) are
also effective though two or more applications may be necessary at
intervals of 2 to 6 weeks.
Reduviid Bugs
Variously known as cone-nose bugs, assassin bugs, barbeiros,
kissing bugs or china bugs, several species are carriers of Chagas’
[33]
disease in Mexico and Central and South America. The main genera
concerned in the transmission of the disease to man are Triatoma,
Panstrongylus and Rhodnius. Numerous animal species such as
armadillos, opossums, domestic and wild rodents, birds, dogs and
squirrels act as reservoirs of the disease agents, various trypanosome
species.
Like bed bugs, these bugs spend most of their time in cracks and
crevices in human habitations though in addition they are to be found
in animal haunts and birds’ nests. As for bed-bugs, house-spraying
with residual insecticides with special attention to crevices is the usual
control method adopted. Treatment of pcridomestic infestations in
chicken houses, pigeon lofts and piggeries is often carried out at the
same time. The usual insecticides have been dieldrin at 1 gm./m2 or
gamma-BHC at 0-5 gm./m2 but resistance to both is known in Triatoma
maculata and Rhodnius prolixus in Venezuela. DDT is said to be
ineffective and various organophosphates are under consideration as
alternatives.
Ticks
DDT is not very effective against Ornithodorus moubata, the vector
of relapsing fever in Africa and India. In East Africa effective control
has been achieved by two applications of heavy dosages of crude BHC
in powder form (approximately 150 mg. of the gamma isomer per
square foot or 1-5 gm./m2) to the floors of native dwellings. The second
application, three weeks after the first, killed off those ticks which
survived the first treatment in the egg-stage. Such a dosage is ex
ceedingly high and more recent trials have indicated that lindane
wettablc powder in suspension in water, applied at 10 to 15 mg. of the
gamma isomer per square foot (01 to 015 gm./m2) will achieve control
if repeated at monthly intervals. There is no evidence of resistance
of O. moubata to BHC.
Other ticks are vectors of various rickettsial, bacterial and virus
diseases throughout the world. These include Colorado tick fever,
American spotted fever, tick-borne encephalitis, ‘ Kyasanur forest ’
disease, haemorrhagic fevers and tularaemia. Animal ticks such as
Dermacentor variabilis and Rhipicephalus sanguineus both of which
occur on dogs and are commonly found resting on vegetation, can be
controlled by area applications of the chlorinated hydrocarbons to
vegetation, c.g. DDT at 2 lb. (907 gm.) toxicant per acre (4,047 m2) or
gamma-BHC at 0-5 lb. per acre (227 gm./4,047 m2). Suspensions,
emulsions or dusts can be used. Treatment with these chemicals usually
prevents reinfestation for 30 days or more. Where resistance to organochlorines occurs gardona at 1 lb. per acre (454 gm./4,047 m2) or carbaryl
at 2 lb. per acre (907 gm./4,047 m2) may be used.
Treatment of the animals themselves can be by washes, sprays,
dips or dusts. Washes or sprays containing 1 per cent. DDT, 0-5 per
[34]
cent, malathion, 1 per cent, coumaphos or 0 05 per cent, lindane or
rotenone are suitable. Dips should be one-half of these concentrations.
Dusts should contain 5-10 per cent. DDT, 1 per cent, lindane, 4 or 5
per cent, malathion, 3 to 5 per cent, rotenone, 5 per cent, carbaryl,
0-5 per cent, coumaphos or 1 per cent, trichlorfon.
Where animal ticks invade houses spot treatments of baseboards,
floor and wall cracks, and the sleeping area of the animal with formula
tions containing 1 per cent, propoxur, 0-5 per cent, dioxathion, 2 per
cent, ronnel or 0-5 per cent, lindane have all been used successfully.
0-5 per cent, diazinon in the form of an emulsion or in solution, 1-3 per
cent, sprays of malathion or fenthion or 2 per cent, suspension of
carbaryl have similarly been used to treat indoor infestations as well as
fences, shrubs, yards and the exteriors of buildings.
Against the forest and scrub-dwelling ticks Ixodes and Hyalomma
plumbeum ground and aircraft dusting and fogging methods have been
used with the chlorinated hydrocarbon insecticides.
Lice
Three different kinds parasitise man—body and head lice and crab
lice. The last two stay on the body continuously. The first is to be found
in the clothing except when feeding.
The body louse (Pediculus humanus corporis) is the carrier of typhus
and relapsing fever both of which have been responsible for serious
epidemics in the past. 10 per cent. DDT dust (usually talc) is the
customary remedy for this pest. It causes cessation of feeding in three
hours, complete knockdown in six hours and complete kill in twenty
hours. One application to clothing will give thorough protection for
three weeks; although eggs are unaffected, all will have hatched in this
period. The dust can be effectively applied from a plunger-type
dust-gun with a long nozzle for insertion beneath clothes without the
necessity for undressing. 1| to 2 ozs. (43 to 57 gm.) is required per
person. Alternatively, sifter-top cans can be issued for individual use. A
1 per cent, lindane powder may be used where DDT resistance is
present, though here a second application within 7-10 days of the first
may be necessary. Where resistance to both DDT and lindane has
arisen 1 per cent, malathion, 2 per cent, abate, 5 per cent, carbaryl, 1
per cent, propoxur or 5 per cent, mobam (another carbamate) dusts
can be used. Resistance to malathion has been found in Burundi.
The impregnation of clothing with DDT formed an effective way
of combating lousiness during World War II. This clothing retained
its insecticidal effect after several launderings.
Head louse (Pediculus humanus capitis).—For control of head lice
liquid formulae arc preferred to dusts since they are not noticeable in
the hair. One well-tried formula (the N.B.I.N. formula) is composed of
6 per cent. DDT, 68 per cent, benzyl benzoate (which is also sarcopti-
[35]
cidal), 14 per cent. Tween SO as emulsifier and 12 per cent, benzocaine,
which eliminates irritation and acts as an ovicide. Another effective
emulsion concentrate is 1 per cent, lindane in alcohol together with an
emulsifier. These concentrates are diluted 1 part concentrate to 5 parts
water before application. Treated persons should not bathe or wash
their hair for at least 24 hours.
Crab louse (Phthirius pubis').—A single application of the N.B.I.N.
formula will effectively control crab lice; two dustings, a week apart,
with If) per cent. DDT dust or 1 per cent, lindane powder may also be
used.
Marked resistance to DDT among lice was first noticed among
troops serving in Korea. At the present time DDT-rcsistance is wide
spread and cases of BHC-resistancc are also known. Experiments are
now being made on pyrethrins and allethrin (a synthetic compound
similar to pyrethrins), with synergists, to replace the chlorinated
hydrocarbons. One formulation that has been effective in practical
use contains 0-2 per cent, of pyrethrins, 2 per cent, of stdfoxide (as a
synergist), 2 per cent, of 2, 4-dinitroanisole (as an ovicide), 0-1 per
cent, of Phenol S (as an antioxidant), and 3 per cent, of a conditioner
in a suitable dust carrier. This powder has a shorter residual action
than DDT and treatments should be repeated weekly until the
infestation is controlled.
Cockroaches
Not usually recognised as important carriers of human disease,
cockroaches, with their haphazard association between dirt, including
human and animal excrement, and human food, undoubtedly contribute
to the mechanical transmission of enteric and other illnesses. The three
main species, the German (Blattella germanica), Oriental (Blatta
orientalis) and American (Periplaneta americana) cockroaches often
present noticeable pest problems in the warmer parts of premises in
temperate climates where they are usually associated with kitchens and
central heating installations. They are more efficiently controlled by
treatment of their ‘ runs ’ than by treatment of the whole building.
Chemical control can be achieved by applying organochlorines, organo
phosphates or carbamates as residual sprays or as dusts in affected
areas. Among the organochlorines 0-5 per cent. gamma-BHC or 5 per
cent, chlordane dusts have been used as well as a fine dust made up of
20 parts of ground pyrethrum flowers, 10 parts of technical DDT and
70 parts of chalk applied at weekly intervals. Effective control has also
been achieved bv the application of urea-formaldehyde resins con
taining dissolved organochlorines, e.g. DDT. These give very prolonged
protection (one year or more) and are particularly suitable for hospitals
or ships’ galleys and food storage rooms.
Organophosphates used with success have been 2 per cent.
diazinon and 5 per cent, malathion dusts and 0-5-1 per cent, diazinon,
[36]
0-5 per cent, dursban and dichlorvos and 3 per cent, fenthion sprays.
1 per cent, propoxur spray has also proved useful. Pyrethrins may be
added to these sprays to act as a flushing agent. The fact that toxic
chemicals are being used near to foodstuffs must be remembered,
however, and every effort made to avoid contamination.
Other chemicals used for cockroach control have included sodium
fluoride, boric acid and silica aerogels. The latter are very fine abrasive
powders which remove the surface coating off the insect cuticle causing
a water-loss and subsequent death by dehydration. One such aerogel is
marketed under the name of * Dri-Die ’. Baits containing dichlorvos,
propoxur or chlordecone (an organochlorine insecticide sometimes called
kepone) are also popular methods of control.
Insecticide resistance is most widespread in the German cock
roach and involves the organochlorines, organophosphates (diazinon,
malathion and fenthion), the carbamate propoxur and even pyrethrins.
Organochlorine resistance is also known in the Oriental cockroach and
both organochlorine and organophosphate resistance in the American
cockroach.
Mites
Surprisingly, the chlorinated hydrocarbon insecticides arc relatively
inefficient against the itch-mite (Sarcoptes scabei) and benzyl benzoate
still remains one of the most successful sarcopticides.
The Trombiculid mites, Leptolrombidium akamushi and L. diliensis
arc vectors of scrub typhus in Asia and the South Pacific. Control
measures involve area treatment with insecticides in the form of sprays
or dusts, c.g. toxaphene or chlordane at 1-2 lb. per acre (454-907 gm./
4,047 m2) or lindane at 0-25-0-5 lb. per acre (113-227 gm./4,047 m2),
and personal protection with repellents. The latter are more effective
when applied to the clothing than to the skin. The best appears to be
benzyl benzoate which remains effective after the clothing has been
washed. The true repellents, e.g. dimethyl and dibutyl phthalate and
diethyltoluamide are only effective when fresh.
Biting Midges
The true biting midges (genera Culicoides, Leptoconops, Styloconops)
are tiny blood-sucking Diptera capable of easily penetrating the
common type of house-screening and mosquito and even sandfly netting.
The residual spraying of houses has little effect on reducing their
numbers; treatment of their breeding grounds, which are almost
invariably extensive swampy areas, is commonly impracticable and
always costly. Larval control has been carried out in the Americas by the
application of DDT, BHC and dieldrin as dusts or pellets, usually
dispersed from the air. Considerable relief from indoor biting can be
obtained by painting the gauze screening of houses with 5 per cent.
DDT or 8 per cent, malathion solution.
[37]
Black-flies
The larvae of the Simuliidae characteristically breed in rapidflowing water. The adults, voracious feeders, do not enter or rest in
houses to any extent and are therefore impossible to control by house
spraying. The larva is very susceptible to minute concentrations of
residual insecticides, in particular DDT. Concentrations of the order
of one part of DDT per ten million parts of water, maintained for a
period of about half an hour, has effected the control of Simulium larvae
in Canada, where the flies are a very great nuisance, and in Africa where
two species are the vectors of onchocerciasis. The DDT in solution or
emulsion is usually applied at the head of the water to be controlled, the
quantity required being related to the rate of flow of water. Control has
been achieved in this way as far as 100 miles downstream in Canada.
Successful control has also been carried out by spraying along the
affected watercourse from the air; at Kinshasa in Zaire, DDT was
applied in this way at 20 mg./m- on several successive days and more
or less eliminated the Simulium both as adults and as larvae.
DDT is said to be ineffective against pupae at larvicidal doses while
gamma-1311 C is said to be effective against larvae and pupae at 2 parts
per ten million parts of water for fifteen minutes.
One apparent drawback to these larvicidal measures, especially
where quantities of insecticide are applied at one point, is the detri
mental effect on fish. When dispersed at 1 part in ten million, DDT is
harmless but in parts of Kenya where dosages of 2 to 10 parts per
million were used fish were killed.
Abate, the organophosphate of very low mammalian toxicity and of
short persistence in the environment, is now being recommended for
widespread and long-term use as a larvicide. Aerial spraying trials have
achieved complete control over 50 km. of rivers by discharging a 20
per cent, emulsion concentrate to give a final dilution of 0-05 parts per
million of insecticide.
Tsetse-flies
Tsetse-flies are confined to tropical Africa where 5 species are
responsible for the transmission of human sleeping sickness and 10 for
the transmission of disease of animals including domestic ones. Their
presence over large areas of the continent has been largely responsible
for restricted progress in agricultural development. They are not easy
to control because of the resting habits of the adult flies in dense bush
over extensive areas. However they have remained susceptible to
insecticides and the distribution of these from the air or from ground
equipment to preferred resting sites has led to their control and even
complete eradication from sizeable tracts. Insecticides used have been
DDT, BHC, dieldrin and endosulfan and only now are organophos
phates, carbamates and pyrethroids being considered from fear of too
[38|
much pollution of the environment from the widespread use of organochlorines. Two spraying techniques have been developed. One aims
at producing sufficient deposits of insecticides on the known resting
sites of adult flics to have a residual effect. As well as ground equipment,
helicopters have been used for this pruposc. The other involves the
aerial spraying of aerosols produced by injection of solutions of insecti
cide into the exhaust system of the aircraft or delivered through atomisers
fitted to the wings. In Botswana, for example, endosulfan has been
applied in this way from Piper fixed-wing aircraft 5 times at 21-day
intervals at a total cost for fly eradication of U.S. $ 70-75 per km2.
Scorpions
The application of gamma-BHC in houses at 50 mg. per sq. ft.
(0-5 gm./m2) has proved effective against scorpions in Brazil. In addition
to the interior of houses, backyards and brick walls around the houses
were sprayed. Stacks of wood and bricks were removed and the vacated
places also treated; vacant lots were also given BHC. Five per cent.
malathion, 2 per cent, chlordane and 1-2 per cent, carbaryl sprays have
also been used successfully'.
Ants
True ants often invade buildings and can be of considerable
nuisance though never as destructive as termites. The only sure way
of controlling these pests is by tracing them back to their source and
destroying the nest (with boiling water or insecticides or general poison,
c.g. carbon disulphide). The spraying of infected buildings with residual
insecticides may considerably reduce them but is seldom completely
satisfactory.
Poison baits have been used with some success in some instances.
The ideal poison is one not too rapid in its effect so that contaminated
worker ants have time to return to the nest and feed the poison to the
queen and brood. By this means the poison is taken to the heart of the
infestation. Such a poison is sodium fluoride which however must be
put into a container to prevent accidental poisoning of children or
domestic animals. A small tin or pill-box with holes punched in the
sides to allow access of the ants is suitable. A liquid bait is apparently
more effective than a solid one and a suitable formula is:—
Water...
...
... 51 per cent, by weight
Sugar ...
...
... 42 per cent, by weight
Honey...
...
...
6-2 per cent, by weight
Sodium fluoride
...
0-8 per cent, by weight
Weigh out sugar, honey and poison, add the water, heat and stir until a
homogeneous syrup is obtained. Do not boil. The liquid should be
kneaded into a mush with one to two parts of a solid ‘ carrier ’ (cake,
minced meat or chopped liver). Prepared proprietary baits also exist.
[39]
Termites
Pentachlorophenol has been used for a considerable number of
years as the basis of wood preservatives to prevent attack by termites
and other wood-boring pests and seems to be efficient.
In the U.S.A, the insecticide chlordane is favoured for the control
of termites. To protect buildings a trench is dug next to foundation
walls 1 or 2 ft. (30 or 60 cm.) deep and 8 to 12 ins. (20 or 30 cm.) wide.
A 1 per cent, to 1 per cent, water emulsion chlordane spray is applied
to the trench at the rate of 1 gallon per linear foot (4-5 litres per 30 cm.)
and the soil for refilling the trench is also treated.
Repellents
It has been known for some considerable time that certain strong
smelling oils, e.g. citronella, lavender, cedar-wood, eucalyptus, repel
biting insects, though their period of effectiveness is not very great.
During World War II dimethyl phthalate (D.M.P.) and dibutyl phthalate
(D.B.P.) were brought into use for the protection of troops in jungles
and other places where insect-biting nuisances were a major problem.
Now a number of other chemicals are available and among these
diethyltoluamide (D.E.T.) seems to show the most promise as an
efficient and long-lasting repellent.
These repellents are usually applied to the skin of exposed parts
of the body, e.g. hands and face, though additional protection is provided
against such pests as scrub-typhus mites by impregnating the clothing
at ankles, wrists and neck. The period of protection provided by any
one of these repellents varies with the individual, the temperature, the
amount of activity that would induce sweating, the amount of rubbing
the treated surface receives and the avidity of the insects.
They should be used with caution, as they will damage such
materials as plastic watch-glasses, some types of synthetic cloth (rayon,
but not nylon), fingernail polish and articles that are painted or
varnished. They will not damage cotton or wool.
[40]
BIBLIOGRAPHY
Specifications for Pesticides used in Public Health.
Insecticides—Rodenticides—Molluscicides—Repellents—Methods.
Fourth edition. WHO, Geneva. 1973.
Equipment for Vector Control.
Second edition. 1974. WHO, Geneva.
Vector Control in International Health.
WHO, Geneva. 1972.
Manual on Larval Control Operations in Malaria Programmes.
(prepared by the WHO Division of Malaria and Other Parasitic
Diseases.)
WHO, Geneva. 1973.
Insecticide Resistance and Vector Control.
Seventeenth Report of the WHO Expert Committee on Insecticides.
WHO Tech. Rep. Ser. 1970. No. 443.
Resistance of Vectors and Reservoirs of Disease to Pesticides.
Twenty-second Report of the WHO Expert Committee on Insecti
cides.
WHO Techn. Rep. Ser. 1976. No. 585
Insects and Hygiene by J. R. Busvine.
Second edition. Methuen, London, 1966.
[41]
GLOSSARY OF INSECTICIDE NAMES
Common Names and Synonyms
(The currently used name is given first.)
Organochlorines
DDT=Gesarol
BHC=HCH
gamma-BHC=Gammexane=Lindane
DIELDRIN=Octalox
CHLORDANE= Octachlor
TOXAPHENE=Camphechlor
CHLOR DECONE = Kepone
ENDOSULFAN=Thiodan
Organophosphates
M AL AT HI ON=Cy thion
ABATE=Temephos=Difenphos=Biothion
RONNEL=Fenchlorphos=Nankor=Trolene = Korlan
FENITROTHION = Sumithion=Folithion
TRICHLORFON=Dipterex=Neguvon
NALED = Dibrom
DURSBAN=Chlorpyrifos
DIAZINON=Basudin=Exodin
DIMETHOATE=Rogor=Cygon=Perfekthion
FENTHION=Baytex= Lebaycid = Mercaptophos
GARDONA=Tetrachlorvinphos
COUMAPHOS=Asuntol=Co-Ral = Resitox
DICHLORVOS=DDVP=Nogos=Nuvan=Vapona
DIOXATHION=DeInav
Carbamates
CARBARYL= Sevin
MOBAM
PROPOXUR=Arprocarb = Baygon= Blattanex=Unden
Others
ROTENONE=Derris-Root=Tubatoxin=Cube
PYRETHRUM (Pyrethrins are the main insecticidal constitu
ents of pyrethrum.)
ALLETH RIN=Pynamin
BIOALLETHRIN
RESMETH RIN = Chryson
BIORESMETHRIN
PERMETHRIN=NRDC 143
[42]
INFORMATION AND ADVISORY SERVICE
HE primary object of the Ross Institute is the prevention of
disease in the tropics. In the course of working towards this
end it has become apparent that the co-operation of industry
is essential if rapid progress is to be made. Fortunately, this
co-operation has never been lacking, for those responsible for
directing tropical industry were quick to appreciate the immense
value to them of healthy labour and have therefore been among
the strongest supporters of the Ross Institute since its inception.
T
For this reason the Ross Institute has made it an important
matter of policy to keep tropical industry informed of the progress
of medical knowledge, and of the practical methods by which the
greatest benefit may' be obtained from its application. This series
of bulletins, which have been specially written for non-medical
people, is one of the means by which this information is made
available; other publications are issued from time to time and a
list of those now current will be found on the following page.
The Ross Institute invites all those whose work is connected
with the tropics to refer to it on any matter concerned with
health or welfare in. tropical countries-. The Director and his staff
will answer'as. 'pyomp'tly and as fuij'^as,^possible all inquiries and
requests for /t'dvicc.
5 • ‘ E-‘
\;4x............. ;*isGxlv
________ -
[43]
PUBLICATIONS OF THE ROSS INSTITUTE
The Preservation of Personal Health in Warm Climates.
(A handbook for those going to the tropics for the first time)
Ross Institute Bulletins:—
(1)
Insecticides.
(2)
Anti-Malarial Drugs. {Reprinted) April, 1975.
(3)
{Out of Print)
(4)
Tropical Ulcer. {Revised) August, 1973.
(5)
The Housefly and its Control. {Reprinted) August, 1975.
{Revised) July, 1976.
(6)
Schistosomiasis. {Reprinted) May, 1974.
(7)
Malaria and its Control.
(8)
Rural Sanitation in the Tropics. {Reprinted) May, 1974.
(9)
The Inflammatory Diseases of the Bowel.
{Reprinted) August, 1975.
/flOj
Small Water Supplies.
{Reprinted) May, 1974.
{Reprinted) April, 1975.
(11)
Anaemia in the Tropics. {Reprinted) June, 1974.
(12)
Protein Calorie Nutrition in Children.
{Reprinted) June, 1975.
These publications are revised from time to time and new and revised
editions are issued as occasion warrants. They are available at printing
cost plus postage on application to:—
The Secretary,
The Ross Institute,
London School of Hygiene & Tropical Medicine,
Keppel Street, Gower Street,
London, WC1E 7HT Tel: 01-636 8636
Printed by E. G. Bbrryuan & Sons,
London, S.E.10
- Media
1251.pdf
Position: 472 (13 views)