Chapter 7: Choice and integration of control techniques

In the field of tsetse control the term integration has usually been taken to mean the combination of two or more techniques to control or eradicate tsetse. When taken in a wider sense the concept, however, involves rather more than this and we should first clarify the definition.

The term integrated control was introduced to describe the combination of chemical and biological methods for the control of agricultural pests. It was developed as the prior sole reliance on chemicals began to decrease in efficiency due to pesticide resistance and because insecticides indiscriminately killed a wide range of insect predators, parasites and other harmless and beneficial non-target species. This resulted in some previously rare insects and mites increasing greatly in numbers thus becoming major pests.

Later, the definition was extended to include the integration of all control techniques into an effective management approach in order to minimise costs and environmental damage. Such integration is based on a sound understanding of the biology and ecology of the pests concerned. This approach, used in applicable situations, can achieve effective and sustainable control as the pest is subjected to several controlling factors, as in nature, rather than just one.

Strictly speaking, some of the combinations of techniques used for tsetse cannot be classified as integrated control but are rather a combination of methods. Integrated disease control involving the use of vector control in conjunction with chemotherapy is becoming increasingly more viable and will be discussed later in the chapter.

The techniques described in this volume will first be compared and contrasted with each other and with other techniques and examples will be given of how they can be combined to achieve integrated trypanosomiasis control or tsetse eradication.

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Control or eradication
This topic is referred to again because it is sometimes thought that these techniques will only achieve control. Although they are particularly well suited for this they also have the potential to eradicate discrete populations. However, few such populations exist which are of manageable proportions for eradication and barriers for the guaranteed long term prevention of reinvasion do not exist.

The reduction of tsetse populations of most species is in many circumstances not difficult, providing an appropriate technique is selected and a concerted effort made towards this end. What is difficult is to ensure the extermination of the last fly and consolidating the result against reinvasion. The decision of planners as to whether operations should be directed towards control or eradication is further complicated by cost considerations, all of which vary according to the local situation.

Control over a finite area could be relatively cheap when cost over the short to medium term and implemented by community participation using traps or targets. However, all the time the threat of reinvasion exists and low density populations remain, the system has to be maintained and could in the long term prove more expensive than an initial high cost eradication programme of finite duration. Conversely, eradication mostly requires constant action to prevent reinvasion and this may prove less beneficial than long term control.

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The main factor mitigating against the decision to eradicate is, therefore, the danger posed by reinvasion, we shall examine this in more detail.

Where natural fly limits do not exist, reinvasion can be slowed down, and perhaps in the short term prevented, by the placing of a delibarate barrier. Previously such barriers were implemented by periodic use of persistent insecticides, the destruction of tsetse habitat, either deliberately or through settlement, or the reduction of tsetse host animals. They were, therefore, lacking in mobility and usually maintained over a sustained period to protect vulnerable cleared and productive areas. Many were placed around infested Game and Wildlife areas where control actions were not directly justifiable.

The introduction of traps and targets now offers the opportunity to plan and implement more mobile and temporary barriers which although they may not be totally effective, especially in the long term, are of considerable use in protecting the seasonal achievements of an ongoing and progressive eradication programme. However, in terms of economics and technical achievement the question should be considered as to whether the positioning of a barrier to protect a finite cleared area can be justified if the alternative exists of gradually advancing the barrier forward and thereby progressively increasing the cleared area. This would, of course, apply more specifically to areas of eradication.

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Prior to the introduction of traps and targets the majority of vector control activities, using insecticide based techniques were aimed at eradication within the treatment area. The newer methods with lower technological demands and potential for use through community participation, have strengthened the option for cost effective control. However, the sustained implementation of a trap/target or insecticide treated animal operation could well result in eventual eradication in sections well removed from reinvasion pressures and protected by the "front line". In this situation the decision has to be faced as to whether it is necessary and economically justified to include a deliberate and more expensive barrier and cease operations in the cleared sections or to maintain the existing system.

The above examples clearly show that the decision as to whether control or eradication is the objective will depend on many factors of which economic justification and the local situation are of major influence.

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The choice between different bait methods
Ideally, for environmental and human/animal health reasons, reliance on insecticides should be eliminated wherever and whenever possible. However, as stated previously in this manual, the selection of method depends on the ultimate objective of the control activities and on the conditions prevailing in the local situation.

For surveys, to establish fly distribution and density and to monitor the progress of control, only non-treated traps are used. This is to ensure that all tsetse approaching or contacting the trap remain active and capable of being lured into the capturing device. The latter is in some cases treated with insecticide or killing agent to ensure that the flies captured do not escape.

Un-treated traps have been used effectively for control both of savanna species, with odours e.g. G. pallidipes , and for riverine species, without odours e.g. G. palpalis and G. f. fuscipes. Their use is dependent on the existence of a cheap, effective trap/odour bait combination for the species involved. Such traps have proved effective in a community based approach to control. In this respect traps may have an advantage over targets as the people involved can see the results of their endeavours through the flies caught in the cages. This helps to promote enthusiasm which may, however, decline as tsetse captures are reduced to low levels.

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The advantages of using such devices are that insecticide is eliminated, they are relatively cheap, especially if local communities are involved in their construction from local materials, and the lower technological and servicing demands.

The disadvantages may include the fact that the traps only catch that proportion of the fly population that voluntary enters the trap, whereas when insecticide is used the flies that make contact but do not enter are also affected. Also, being less efficient because of this, they may not be as effective for eradication and in emergency situations may not reduce fly numbers as quickly as other devices. Their efficiency can also be impaired by damage which allows captured flies to escape whereas insecticide treated surfaces can retain their functional ability in spite of considerable damage. Such factors must be carefully considered prior to selecting a particular device for local use.

In a purely control situation where the objective is to suppress or eliminate tsetse populations as quickly and economically as possible the more popular approach is to incorporate the use of insecticide, at least in the initial implementation.

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This can involve the use of either traps or screens but because there is no longer any need to capture and record the flies either basic design targets or simplified traps have been found adequate. They have been used effectively against a number of species either enhanced with odours, G. pallidipes, G. morsitans and G. tachinoides or without, G. fuscipes, G. palpalis. The choice of a particular trap or target will depend on the tsetse target species. For example, the S-type or all black targets are both very effective for G. pallidipes and G. morsitans, although the biconical trap has also been used for G. m. submorsitans. The biconical is more effective for G. palpalis whereas blue screens may also be used and are more economical. The pyramidal trap is probably the best design so far for G. f.fuscipes.

Research into the development of more effective designs, attractive colours and colour combinations and the identification and testing of attractive odours for all the economically important tsetse species is continuing.

The effective maintenance of traps and targets in the field depends on an efficient routine of servicing including repair and replacement, periodic treatment with insecticide, replenishing of odours, protection from bush fires and the prevention of damage and theft. In many instances where the devices, or their components, present some monetary value or have alternative usefulness to local inhabitants various degrees of theft occur. It is, therefore, essential that tsetse control be supported by an extensive public awareness and information campaign and that the cooperation of rural communities be secured.

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Where livestock are present they can be considered for use as living insecticide treated targets and may efficiently substitute artificial devices. This obviates the expense of construction, placement, protection and servicing associated with the latter and for the supply of odours. However, whereas present insecticide formulations persist on cloth for from 4 to 10 months the efficiency on animals is reduced to from 2 to 4 weeks, depending on local conditions, the insecticide used and method of application. With cattle too, the dangers of damage and theft are eliminated. The technique, therefore, has the potential to become an economical and effective control and perhaps, eradication method. It has so far been tested with good results against G. pallidipes, G. morsitans and G. austeni. There are good reasons to believe that it will be effective for all species which are attracted to livestock in reasonable numbers. A fact that can be established by an analysis of blood meals extracted from fed flies to determine preferred host species.

The use of animals has to date depended on treating as many as possible of those found in the area. However, when one considers the effectiveness of the animal in attracting tsetse compared to that of artificial devices and that only some 4 to twenty of the latter are required per km² to produce a result, then it may be possible to also reduce the number of treated animals per unit area.

Further research and development is required to perfect and economise the technique. Aspects needing investigation include the density and distribution of animals required, their effectiveness as a barrier, identification of insecticides, frequency of treatment and optimum methods of application. There is also the possibility of combining tsetse with tick control, although care must be taken not to affect enzootic stability.

In summary the success of traps, targets or treated animals all depends on the same principle of attracting and capturing or killing tsetse in sufficient numbers to ensure a progressive decline in the population. A choice of design, colour, odour, placement density and species of animal to be treated exists, the final selection will depend on the tsetse species, the local situation and the practicality and economics of implementation. In many circumstances the selection of a single "type" may be convenient and adequate, in others a combination of these options may be preferable but will need to be determined by testing in the prevailing situation. No set guidelines for the multitude of natural conditions encountered throughout Africa can be given in this manual.

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Bush clearing and game destruction
This is the oldest method of tsetse control and bush clearing, either direct or as a result of human settlement activities, is still practised. The environmental consequences of such methods deliberately practised on a large scale can, however, be far more adverse than the use of insecticides, and have led to severe soil erosion and resource degradation in some areas. Attempts to reverse the process of soil erosion by planting tree crops (e.g. cashew) has resulted in tsetse reinfestation in some areas. Game destruction destroys a valuable resource which could be utilized through game cropping or tourism, and tsetse may change to feeding mainly on livestock. If this occurs, there are often unusually high infection rates in cattle even with very low fly densities, because of the high parasitaemia in cattle and consequent high infection rates of the flies.

Although such methods are now rarely practised specifically to control tsetse, increasing population pressure, especially in West Africa, has indirectly resulted in widespread bush clearing and game destruction for agricultural purposes. This has probably had more effect on tsetse populations than man's direct attempts to control tsetse. It is unwise, however, to rely on this process to solve the problem since reafforestation, and the ability of some tsetse species to adapt to these new habitats, may reverse the process. The most notable effect on tsetse, through the destruction of host animals, occurred as a result of the Rinderpest epidemic at the end of last century which severely affected wildlife populations throughout the continent and caused notable contractions in tsetse populations and distribution.

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Residual insecticides
The principle behind the use of residual insecticides applied to resting sites is actually the same as that for the techniques described in this volume viz. to raise the death rate above the birth rate for a sustained period.

The difference is that the insecticide is applied selectively but much more widely in the environment to resting and breeding sites which a fly may visit, rather than attracting the flies to limited and particular sites where they are killed.

This inevitably results in much higher rates of insecticide application, and consequently environmental pollution. The advantage is that no artefacts have to be left in the field which can get damaged or stolen.

The technique is still practised in southern Africa in areas where the objective is eradication, and costs are similar to target operations conducted by government teams. It will probably continue to be used in areas where loss rates of traps or targets are unacceptably high or in epidemic situations to effect a more rapid reduction.

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Sequential aerosol technique (SAT)
In this technique insecticides are applied from fixed wing aircraft as a non-residual aerosol. The most commonly used insecticide is endosulfan, although synthetic pyrethroids have also been tested. The method has been developed for eradication and the aim is to kill all adult tsetse in the area with the first spray cycle. Spraying is then carried out at about two week intervals to kill all flies emerging from puparia before they can larviposit, usually this takes 15-20 days after emergence, and is continued until beyond the pupal period, calculated from the first treatment, so that no more pupae are available to emerge. This usually requires about five treatments.

Spraying must take place during hours when there are inversion conditions, so that the minute droplets of insecticide are drawn down into the canopy and not carried away on thermal updrafts; this means spraying in late evening, at night or for about 2 hours after dawn. Aircraft must be flown just above tree top level. Barriers, using targets or ground-spraying, may be used to prevent reinvasion if the area is not naturally isolated. Alternatively, as was the case before the development of targets, reinvaded areas can be re-treated in progressive seasons as operations are advanced.

This technique as currently developed is usually only economically viable for eradication, or to break transmission in a human sleeping sickness outbreak. The strategic use of aerial spraying for economic control has not been fully investigated. It has the advantages that very large areas can be covered in a short period of time, and the tsetse population is reduced very rapidly. The main disadvantage is that the technique relies on sophisticated equipment and expertise.

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Sterile insect technique (SIT)
In this method flies are mass-reared, and large numbers of sterilised male flies are released in the field. These mate with the wild females, which are then unable to reproduce. Thus this method works by reducing the birth rate of the population and not by increasing the death rate.

Pilot trials have shown that a high ratio of sterile to wild male flies must be achieved for the technique to be successful. Hence other control methods must be used first to reduce the population size, particularly because of the slow breeding rate of tsetse which restricts the availability of laboratory produced sterile flies.

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Chemotherapy and chemoprophylaxis
In much of Africa, chemotherapy, curative treatments of animals, and chemoprophylaxis, drug treatment to prevent the animal getting the disease, remain the main methods of dealing with animal trypanosomiasis. Similarly for human trypanosomiasis, passive screening, monitoring cases which report to hospitals for treatment, and active screening, searching for undetected cases in high risk areas, followed by treatment are still the main methods of disease control.

Such methods can be effective for a period under low to medium tsetse challenge, but resistance of trypanosomes to drugs is becoming a major problem. This is especially true in countries which rely heavily on this approach to deal with animal trypanosomiasis, rather than undertaking tsetse control. Development of resistance to chemoprophylactics necessitates an increase in both the frequency and level of dosage, to such an extent that they may reach toxic levels in the animals, or control may become uneconomic.

It has often been claimed that resistance results from uncontrolled use of drugs which often leads to underdosing. Whilst this may be true, it is only part of the story. Long term extensive use of a drug, even under strict veterinary control at the correct doses, will also result in resistance. Pronounced cases of resistance have also occurred on private and government ranches, where usage has been heaviest and no attempt has been made to reduce the fly numbers. Large scale introduction of cattle on chemoprophylaxis into an area may also play a part in extending the distribution of tsetse into previously uninfested areas, by providing an abundant food source. This would, however, not apply in areas where adequate wildlife was previously available.

The outlook for control of the disease with drugs alone is not promising. Not only is drug resistance increasing, but there is little likelihood of any new drugs being developed in the near future. This is because the economic returns from the sale of trypanosomiasis drugs are not sufficient to encourage commercial companies to make the very considerable investment needed for the development of new ones. Most research is now concentrated upon already available drugs produced for other diseases, or on novel combinations of existing drugs.

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Trypanotolerance
There has been great interest in recent years in utilising the natural tolerance of some breeds of cattle to trypanosomiasis. Cattle that have been exposed to tsetse challenge for hundreds of years have developed a degree of immunity to the disease, and are known as trypanotolerant.

This is most pronounced in the humpless or taurine breeds of cattle, which include the long-horned N'Dama cattle and the West African shorthorns such as the BaoulĖ. These breeds are found in the more humid zones of West and Central Africa, and have since been introduced to other countries such as Zaire. Although zebu cattle generally show less trypanotolerance, there is now clear evidence of trypanotolerance in some parts of East Africa, where zebus can tolerate low to medium tsetse challenge provided they are not put under nutritional or physical stress.

Although trypanotolerant cattle breeds are often smaller than improved breeds, extensive research has shown that they can be very productive under good management. They may not, however, survive trekking long distances as well as other breeds.

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Vector eradication
When the objective of a field operation is tsetse control and not eradication it is usual to select a single method of vector suppression which is appropriate and economically justified.

However, when the objective is eradication it is often the case that a single technique will not achieve this, especially when applied on a large scale. Contingency planning is needed to address such a situation and it is often necessary to combine different methods effectively to ensure the elimination of the last fly. To enable planners to do this they must be aware of the various options available.

(a) Selective ground spraying
In situations experiencing alternating long dry/cold and wet/hot seasons the persistent insecticide is normally applied during the winter when tsetse populations are under stress and relatively confined to restricted evergreen vegetation usually associated with all year water availability. They remain effective well into the rainy season when temperatures rise and force the recovering tsetse population into seeking sprayed mid-day refuges so enhancing the effectiveness of the campaign. Under such conditions it is normally anticipated that a single annual treatment will eradicate tsetse from some 65-80% of the area but that small residual foci will survive, particularly in remote and rugged areas where the efficiency of the spraying may be reduced. Further, if treatment is not taken to natural tsetse limits reinvasion can be expected to occur once the persistency of the insecticide is reduced over time.

Under such conditions a temporary barrier of targets could be positioned to counteract reinvasion whilst residual foci could be treated by the strategic positioning of targets or the insecticide treatment of cattle within the zone.

In areas where there is less seasonal contrast and rains are more frequent the persistency of insecticides can be much reduced and several applications may be needed. The ultimate effect would, however, be similar to that in drier areas and so the same combinations of methods could be used. However, the economics and environmental considerations may dictate that under such circumstances an alternative method should have been considered from the start.

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(b) Sequential aerial spraying
This technique has been developed to an advanced level to achieve eradication, its potential for more economical control has not been fully investigated. Normally applied over large areas, particularly for savanna species, eradication can be anticipated from 90% or more of the area. However, some surviving foci can be left, especially in protected areas where droplets fail to penetrate. The technique is also very vulnerable to reinvasion, even between cycles, as the insecticide has no persistence.

Surviving tsetse could be dealt with immediately using SIT or an attractive bait method. Barriers against reinvasion could be established by ground spraying or a target system. For maximum effect, whichever method is chosen, the barrier needs to be in place and fly pressure reduced, well before aerial spraying takes place.

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(c) Sterile Insect Technique (SIT)
This method is based on the mass release of laboratory reared and sterilised male flies which then compete with wild males in the fertilisation of females. It is, therefore, most effective and economic when applied in areas of low tsetse density. It is also species specific unlike other methods which have an effect of varying degree on all tsetse species present. The technique is best considered for the eradication of surviving tsetse populations to consolidate the achievement of other partially successful techniques. However, SIT could not be implemented simultaneously with methods aimed at killing the flies as this would also result in the indiscriminate killing of the sterile insects and jeopardise the objective.

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(d) Traps and targets
The concept is considered here in the variety of combinations of colours, odours, insecticides, etc. currently available. The majority of operations undertaken to date using these devices have been aimed at control and not eradication. However, there are a few examples of the latter achievement, particularly from southern Africa.

Their use as barriers and to support other eradication methods has already been described. When selected as the main method their effectiveness could be enhanced in problem areas by the use of insecticide treated livestock, providing the latter are available. It is, however more usual to monitor the progress of operations using attractive devices alone and to adjust their density and distribution according to the ongoing results.

A promising recent development has been the testing of synthetic juvenile growth hormones applied to trapping devices which inhibit the production of viable progeny by contaminated females. If developed sufficiently to allow large scale use the method has the potential to combine with and compliment most other techniques including SIT.

(e) Insecticide treated livestock
This method is a natural extension of artificial bait systems facilitated through the recent availability of suitable formulations of persistent insecticides. So far it has only been used on a limited scale in a small number of countries but even so has demonstrated the potential to replace artificial attractive devices in areas where sufficient livestock are available. Further development and field testing is required towards large scale and economic use. The indications are that the method may be used to eliminate residual fly foci following other techniques, to form reinvasion barriers and perhaps, also achieve eradication.

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Integrated tsetse and disease control
It is debatable whether there has been much integrated control of tsetse, although bush clearing in selected high-challenge areas combined with traps or targets comes closest to it. The aggregation of predators around non-insecticidal traps is also a form of integrated control. It is important to stress that a combination of methods that work in the same way may not be beneficial, since the use of several techniques involves greater logistical problems and costs.

Perhaps the greatest opportunities lie in integrated disease control, through the combination of a cheap method of tsetse control with much reduced chemotherapy. Since with control the aim is only to reduce tsetse to a very low level, chemotherapy will have to be used to treat the odd case of trypanosomiasis that will still occur.

This approach reduces the likelihood of drug resistance developing, since drug use is much less. Control with traps, targets or insecticide treated livestock can also be combined with the use of trypanotolerant animals, although they can survive in tsetse areas, productivity is decreased and tsetse control has been shown to be very beneficial.

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Conclusions
The new tsetse control techniques described in this volume offer great promise in reducing the problem of tsetse flies in Africa. In comparison with the older methods, they are generally cheaper and less polluting, and can also be combined with the more established techniques.

They may also provide a solution for control of tsetse and trypanosomiasis in areas where there has previously been sole reliance on drugs, or where there has been no tsetse or trypanosomiasis control at all. This may require not only the new control techniques, but also new management approaches, and these are discussed in the final chapter.

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