When DDT became available in , the East African Tsetse Research Organization soon showed in the laboratory that tarsal contact with a sprayed surface for a few seconds could be lethal to tsetse flies , but subsequent field trials with treated oxen as bait animals were not successful because they were unable to compete as tsetse hosts with the game animals and they had to be re-sprayed at too frequent intervals 8, Repeated applications to maintain a toxic deposit for longer than a pupal period were necessary for a greater and more permanent reduction , Dieldrin was first tested in 18 and soon replaced DDT in residual sprays on vegetation in Kenya Residual treatments of vegetation from ground spraying equipment were begun in West Africa in Since these early operations until the present time, DDT and dieldrin have continued to be almost exclusively the insecticides used for residual treatments of vegetation in ground spray operations.
The only real changes have been in techniques to improve effectiveness and reduce costs by reducing dosages and by more discriminative and selective applications 21, 26, 28, 48, 63, 64, 74, 75, 76, 89, 90, Applications of insecticides from aircraft for tsetse control have been made since , and the first successful scheme was carried out with DDT and BHC in South Africa where air spray operations with both fixed-wing aircraft and helicopters were combined with other control measures over a period of years to eradicate G.
A programme of research on aerial spraying for tsetse control began in page 26 at the Tropical Pesticides Research Institute TPRI 13, 17, 19, 35, 49, 50, 51, 52, 53, 54, 55, 56, 68 , and this has been the forerunner of air spraying techniques employed in Zambia page 29 and Botswana page 26 today. DDT and BHC, the first insecticides used, were replaced by dieldrin and, more recently, by endosulfan at rates as low as 6g in 0.
Since the late 's, techniques have been developed for the application of a residual insecticide to vegetation from a helicopter Insecticides used in tsetse control operations on any scale from the ground and from the air have been limited to DDT, BHC, dieldrin and endosulfan. These are all organochlorine compounds. Where insecticides belonging to this class have been used to control other insect vectors and crop pests, resistance to them has frequently developed quite quickly and become a serious problem.
Resistance to insecticides in tsetse flies has not been reported yet. Some tests in N. Nigeria in indicated that the susceptibility of a population of G. Restrictions have been put on the use of organochlorine insecticides in some countries because of persistence in the environment and undesirable side effects on non-target organisms, and these compounds may become less readily available or acceptable for tsetse control. Destruction of birds and wildlife immediately after tsetse control operations with organochlorine insecticides has been recorded 41, 65, There was a general decline in the prominence of certain insectivorous birds a year after a single application of dieldrin in Nigeria, but most of the species affected then recovered to pre-spray density 66 , and monitoring in Uganda showed that the spray programmes were not leaving dangerous doses of dieldrin in the soil or vegetation.
An ecological assessment after aerial applications of endosulfan in Zambia showed no lasting adverse effect on the insect fauna, and similar operations in Botswana have so far produced no adverse effects on people, cattle or fish.
The various studies to date indicate that residual spraying of a single heavier dose of insecticide to a selected and restricted part of the vegetation is more likely to have an impact on non-target organisms than is sequential aerial spraying. A systematic search for insecticides that could replace the organochlorine compounds in tsetse control operations when necessary was begun at the TPRI, Arusha 9, 10, 12, 14, 15 , and has continued at COPR as a part of the WHO Evaluation Programme 43, Some of the candidate insecticides have also been tested recently at the Nigerian Institute for Trypanosomiasis Research In laboratory bioassays of the toxicity of organophosphorus compounds to tsetse by topical applications in solution, none is found to be appreciably more toxic than endosulfan and dieldrin and only a few are as effective.
These include OMS, OMS, OMS, fenthion, chlorfenvinphos, dicrotophos and crotoxyphos, while phosmet, naled, dichlorvos, tetrachlorvinphos, bromophos, jodfenphos and fenitrothion are from two to four times less active. Naled and dichlorvos are too volatile. Propoxur and bendiocarb, the most active of the N-methyl carbamate tested, and two di-methyl carbamates, pyrolan and dimetilan, are similar to, or slightly less toxic than, endosulfan and dieldrin. NRDC is outstanding and is times as effective as dieldrin to G. Resmethrin resembles natural pyrethrin in that it is unstable on exposure to light, but permethrin and NRCD are light stable and involatile and at present have the greatest potential as residual insecticides for tsetse control, although some pyrethroids may be more suitable for sequential low rate applications than for single dose residual sprays because of their high toxicity to fish.
Variation in susceptibility of different species to an insecticide is another important factor in field operations.
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As early as , a 4-fold difference in susceptibility to DDT was found among 12 species and subspecies between the most susceptible G. The susceptibility within a species to an insecticide can vary according to sex and physiological condition, either because there is a real difference in the dose actually required to kill or because there is a difference in the dose reaching the flies, e. Teneral flies of either sex and old fed males are generally equally susceptible to insecticides applied in solution by topical application 11, All are more susceptible, however, than pregnant females to the organochlorine insecticides DDT, dieldrin and endosulfan 11, 43, 57 , and 4- to 9-fold increases in tolerance in old females have to be taken into account in field operations with these insecticides.
Laboratory tests to date indicate only a small increase in tolerance in old pregnant females to natural pyrethrins and a synthetic pyrethroid resmethrin, and no increase in tolerance to the organophosphorus compounds fenthion, tetrachlorvinphos and bromophos, and the carbamate propoxur 11, 43, Some of the compounds showing high toxicity to tsetse flies in laboratory tests and referred to above have already been subjected to limited field trials.
In , six aerial applications of 0. Populations of G. It was considered that unfavourable meteorological conditions during these applications and full leaf cover were the main factors responsible for the lack of effect, and further work was recommended on the basis of the good results of the first applications Subsequent aerial applications of natural pyrethrins at a higher concentration did not give high reductions of G.
In aerial spray trials in Botswana in , natural pyrethrins at 0. Earlier, applications were made from a helicopter in the Republic of Mali of cismethrin, bromophos, jodfenphos, fenitrothion and tetrachlorvinphos for aerosol contact of the flying adults in the gallery forest and of tetrachlorvinphos and methoxychlor for residual contact Operational problems limited the scope of these trials, and additional tests with these materials could be valuable. The trials emphasized the need for further studies of the penetration of spray drops into the dense vegetation of gallery forests 60 and on the resting behaviour of tsetse flies, particularly at night.
Residual spraying techniques employ DDT and dieldrin as emulsions and suspensions. The concentrates from which these are prepared are those normally used for public health work, and the only example of a formulation specially made and used for tsetse control is the dieldrin 15T emulsion concentrate. Otherwise, formulations which meet the WHO specifications Specifications for Pesticides used in Public Health, have been satisfactory.
Aerial spraying methods have used high volume rates of dieldrin emulsion 91 , but aircraft normally distribute low or very low volumes of concentrated solutions of dieldrin, DDT or endosulfan with the intention of directly hitting the flies. The desired efficiency and economy are obtained by using insecticides which are very toxic to tsetse flies, the optimum range of droplet sizes and suitable choices of aircraft operation methods and meteorological conditions.
An important factor controlling the droplet sizes in the aerosol as it reaches the standing vegetation or the ground is the volatility of the solvent used to dissolve the insecticide, and East African experience has shown that this should not be high, otherwise droplets become too small to reach the target efficiently. However, at the time of this development work the formulation had to be chosen from those made for other applications.
Also, volatility was necessary to allow suitable droplet sizes to be formed by evaporation of coarser sprays produced by the available equipment. Less volatile solvents are now being investigated see following paragraph. Insecticides, which are themselves liquid, such as fenitrothion and malathion, do not require formulation for application at ultra-low-volume rates, have not been used against tsetse. There has been little attempt during the last 20 years to improve formulations for the insecticide application in tsetse control.
A good reason for this is the desire for economy. It is cheaper to employ concentrates which are already being used for the control of other pests and to dilute these with locally available petroleum solvents than to transport specially prepared solutions over long distances. However, special formulations are being produced and the use of more involatile materials is being studied by some chemical companies. For example, trials with dieldrin and endosulfan in various solvents have been carried out in Nigeria and Botswana.
Bioassay of the residues are made with captured flies. Both investigations suggest that the less volatile solvents give the best results, although the reasons for the improvements are not clear because lower volatility influences both the persistence of insecticide in the residues and the particle size distribution of the spray. The solvents used are essentially non-volatile, e. These have been compared with diesel oil which is partly volatile and Shellsol AB which is relatively very volatile.
Extremely low volatility means high viscosity and the solutions therefore contain other more volatile solvents to reduce viscosity to a sprayable level and, in some instances, to ensure that the active ingredient remains in solution during storage. Anyone with access only to the published records of ground spraying against tsetse would have difficulty in discovering just how the insecticides are applied.
This is being remedied to some extent by the publication of more information on pesticide application equipment, a good deal of which has relevance to tsetse control. Assessment of spraying machinery is ongoing research at OSMC 79 , and many of the tests carried out are based upon recommendations by WHO 30, Spraying equipment is used to disseminate pesticides as liquids in the form of aqueous suspensions, emulsions and solutions. It is not possible to list all the equipment in current use or which is available, but the apparatus mentioned below has been used in major control operations; e.
There are three basic pattern shapes produced by hydraulic energy nozzles; the falt fan, the solid cone and the hollow cone. The limitations of the fan pattern for tsetse work are reflected in the very few occasions on which it appears to have been used.
In a campaign against G. One difficulty which arises with these nozzles is that the width of the fan has to be orientated in order to cover the surface to be sprayed. Cone-shaped sprays are produced by nozzles which consist essentially of a swirl plate with angled slits or holes which produce a vortex in the flow of insecticide, causing it to issue from the orifice in the form of a solid or hollow cone of spray depending on the presence or absence of a central hole in the core. Whether the solid cone nozzle has ever been used in tsetse control is uncertain.
The hollow cone spray nozzle is in general use and either a constant or variable spray width can be produced. For the former, the actual dimensions are determined by the design of the swirl core, the size of the nozzle disc orifice and the operating pressure. The practical limitations of these nozzles is that when spraying tree trunks or individual branches, the position of the nozzle, in relation to the surface being sprayed, must be constantly adjusted to ensure that a swath of insecticide of the required width is obtained This can be achieved by altering the length of the swirl chamber.
The angular width of the cone at a given pressure is inversely proportional to the length of the chamber between the swirl block and orifice disc. Alternatively, wastage can be avoided by adjusting the distance between the nozzle and the target surface, but this may necessitate moving the nozzle more rapidly when close to the surface. Two types of spray lances are in general use; one has a shut-off valve in the handle for controlling the flow of insecticide and produces a constant or variable cone depending upon the type of nozzle fitted.
With the other, the shape of the cone is adjusted from the handle by means of a control rod inside the lance. These consist of a trigger-operated, instantaneous shut-off valve which, with tube and nozzle, from the lance assembly. To produce a constant-shape cone, the nozzle assembly contains a swirl cone, an orifice disc and a screw-retaining cap. The shape of the cone produced for a given pressure is related to both the design of the swirl plate and the size of the orifice in the disc.
Variable cone nozzles consist of a nozzle body attached to the end of the lance by means of an adaptor. Variations in the shape of the cone are produced by altering the length of the swirl chamber between the core and the disc by screwing or unscrewing the cap. The CP 2 spray gun has been used in some countries.
This consists of a trigger-operated shut-off valve with a brass body, a plated steel trigger, a brass tube The nozzle has a plastic swirl core, of which only one model is available, a stainless steel nozzle disc and a plastic nozzle cap. Six different orifice sizes are available, with diameters ranging from 0. Also in this category is the Stoprex spray gun 3.
This has a thumb trigger in contrast to the models previously described, all of which have a four-finger trigger for operating the shut-off valve. The lance and constant-shape cone nozzle assembly are, however, similar to those of the CP, but the Rex Variable Nozzle 4 is of a different design. Variations in swirl chamber length are made by rotating a collar to which the same stainless steel orifice disc and plastic nozzle cap as those used for hollow cones are fitted. Spray guns of this type are intended for use in situations where frequent and often rapid changes in the width of the spray pattern are necessary, e.
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As with the variable cone nozzles described above, pattern shape is changed by altering the length of the swirl chamber. The disc is fixed but movement of the core is achieved by the control rod. The CP Spray Gun 5 is one example of this model which has been extensively used in Uganda but is no longer available. A some what different mechanism for adjusting cone shape is found in the GunJet No. The essential difference between these and the No.
On rotating the handle the swirl chamber is progressively opened and spray is emitted, at first in the form of a short broad cone.
As this movement is continued, the cone becomes progressively longer and narrower until a complete circle has been turned and the swirl chamber is fully open and the spray is emitted as a solid stream. The cone dimensions are related to operating pressure and orifice size and two types of fitting may be used for producing hollow cones. One is a stainless steel disc intended for the higher rates of application, while for the lower outputs the disc can be replaced by a ConeJet spray tip 7 , of which ten sizes are available. Another lance in which the spray cone is adjusted by means of a trigger-operated control rod has been used with a Platz vehicle-mounted sprayer in Zambia.
The output of each lance is controlled by the manual operation of the trigger and can be varied between a very fine cone spray and a solid jet The maximum output from one lance is about 4. The maximum throw is about 15 m in still conditions. In contrast to the extensive use made of hand compression sprayers, few organizations have adopted the lever-operated type, which is lighter but less robust.
Unless a pressure regulating device is fitted, considerable variations in output rate can occur because of changes in lever operation, which is affected by operator fatigue. Operators have also found continuous pumping more tiring than the periodic pumping of compression sprayers and a further disadvantage is that they are unable to use both hands freely to gain access to difficult places or to steady long lances when spraying the undersides of high branches, one hand being required for pumping.
Extensive use has been made of these machines because they maintain pressure and, consequently, less effort is required to operate them than nonpressure retaining sprayers. The high operating pressure also reduces the tendency to blockages. Most insecticides are sprayed or dusted onto plants and other surfaces traversed or fed upon by insects. Stomach poisons are toxic only if ingested through the mouth and are most useful against those insects that have biting or chewing mouth parts, such as caterpillars , beetles, and grasshoppers.
The chief stomach poisons are the arsenicals —e. They are applied as sprays or dusts onto the leaves and stems of plants eaten by the target insects. Stomach poisons have gradually been replaced by synthetic insecticides, which are less dangerous to humans and other mammals.
Contact poisons penetrate the skin of the pest and are used against those arthropods , such as aphids , that pierce the surface of a plant and suck out the juices. The contact insecticides can be divided into two main groups: naturally occurring compounds and synthetic organic ones. The naturally occurring contact insecticides include nicotine , developed from tobacco ; pyrethrum , obtained from flowers of Chrysanthemum cinerariaefolium and Tanacetum coccineum ; rotenone, from the roots of Derris species and related plants; and oils, from petroleum.
Though these compounds were originally derived mainly from plant extracts, the toxic agents of some of them e. Natural insecticides are usually short-lived on plants and cannot provide protection against prolonged invasions. Except for pyrethrum, they have largely been replaced by newer synthetic organic insecticides. Fumigants are toxic compounds that enter the respiratory system of the insect through its spiracles , or breathing openings.
They include such chemicals as hydrogen cyanide , naphthalene , nicotine, and methyl bromide and are used mainly for killing insect pests of stored products or for fumigating nursery stock. The synthetic contact insecticides are now the primary agents of insect control. In general they penetrate insects readily and are toxic to a wide range of species. The main synthetic groups are the chlorinated hydrocarbons, organic phosphates organophosphates , and carbamates. The chlorinated hydrocarbons were developed beginning in the s after the discovery of the insecticidal properties of DDT.
Other examples of this series are BHC , lindane, Chlorobenzilate, methoxychlor , and the cyclodienes which include aldrin , dieldrin , chlordane , heptachlor , and endrin. Some of these compounds are quite stable and have a long residual action; they are, therefore, particularly valuable where protection is required for long periods.
Their toxic action is not fully understood, but they are known to disrupt the nervous system. A number of these insecticides have been banned for their deleterious effects on the environment. The organophosphates are now the largest and most versatile class of insecticides. Two widely used compounds in this class are parathion and malathion; others are Diazinon, naled, methyl parathion, and dichlorvos.
They are especially effective against sucking insects such as aphids and mites, which feed on plant juices. The organophosphates usually have little residual action and are important, therefore, where residual tolerances limit the choice of insecticides. New product price is lower than exchange product price.
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