A.M. Jordan1
INTRODUCTION
Tsetse control can have both direct and indirect environmental and ecological consequences. Direct effects relate to the impact of the methods employed, whereas indirect effects relate to the consequences of removing or controlling tsetse flies, and hence trypanosomiasis, from or in an area. Direct effects have been relatively well studied and documented, but the importance and complexity of indirect effects are far from being fully understood. For these reasons limited attention is directed in this paper to the direct consequences of tsetse control techniques and rather more to a discussion of some of the issues that need to be taken into account when attempting to predict the indirect effects of removing or controlling tsetse flies under different ecological circumstances.
Direct Effects of Tsetse Control
Obviously, both the clearing of vegetation and the shooting of the wild animal hosts of tsetse flies have major environmental consequences, but as these methods are not deliberately employed today to control tsetse populations, these issues can be disregarded. Nevertheless, it is noteworthy that in some parts of Africa expanding human populations are removing both vegetation and wild animals and, coincidentally, tsetse populations depending on them for habitats and for food.
It is the use of insecticides for tsetse control which has caused the greatest controversy. Any use of insecticide is going to have some effects beyond those on the target organisms against which it is directed. Tsetse control workers have long recognised this and campaigns as early as the 1960's included environmental monitoring components, initiated by the tsetse control authorities themselves. Both chemical and biological monitoring have been carried out. Measurements have been made of chemical residues in abiotic components (air, water, soil) and biotic components (plant and animal tissues) of the environment and biological monitoring has included studies on the abundance and rates of mortality and morbidity of selected non-target organisms following the application of insecticide. Whereas these studies have had their limitations (e.g. there have been few long-term studies of prespray populations to which post-spray populations can be related), overall they have shown with very little doubt that although non-target organisms can be affected by anti-tsetse spraying and quantities of insecticide remain in the environment, these effects appear to be transitory, rarely lasting for more than a year.
In general, applications of residual formulations from helicopters are most polluting, ground spraying much less so and the application of aerosols from fixed-wing aircraft least polluting. To a very large extent these conclusions are academic as helicopters are now rarely, if ever, used to apply residual formulations, partly for environmental reasons, but also on the grounds of high cost. Ground spraying is rarely employed now, other than in Zimbabwe. Even aerial spraying of non-residual formulations, well-proven as environmentally acceptable (though not all would agree with this statement !), is not in widespread use as it is expensive, particularly when employed as a control, rather than eradication, strategy.
With the advent of traps and targets for use in tsetse control, direct effects of control on the environment has receded as a controversial issue. Even when impregnated with insecticide, these devices have negligible effects on non-target organisms. Although this is a clear advantage of such methods over ground and aerial insecticidal methods, the main advantage of traps and targets over other methods lies in their flexibility and comparatively low cost.
Indirect Effects of Tse-tse Control
The arguments associated with the use of insecticides for tsetse control have, to some extent, been counter-productive as they have diverted attention from the most important environmental and ecological issues associated with tsetse control that need to be addressed. Africa today is facing two closely-linked crises, a high rate of human population growth and environmental degradation - aggravated, in some countries, by political instability, which is outside the scope of this paper. Nevertheless, both of these crises, nothing to do with tsetse or trypanosomiasis control, have major implications for the development of appropriate disease control strategies.
The main factor affecting the ecology of Africa today is the rapidly increasing human population. In already densely populated countries this is causing the removal of tsetse habitats and hosts and the consequent disappearance of tsetse over large areas. The classic cases are Nigeria, The Gambia, Burkina Faso, Rwanda, Malawi and parts of Tanzania and Zimbabwe. The main effects of man are on the morsitans and fusca groups, but even the palpalis group is affected in drier areas. In sparsely populated countries, such as Mali, southern Sudan, Zaire, Angola, Mozambique and Zambia, tsetse are so far little affected by man and in some places are even invading new land.
It is against this background of varying demographic pressure on land (and tsetse), and much consequent land degradation, that control strategies need to be developed and assessed in terms of whether or not they are likely to exacerbate or reduce the risks of environmental degradation. In principle, effective tsetse control as a component of land use planning, gives a unique opportunity to allow the orderly development of land. In practice, this has often been a pipe-dream as, despite effective tsetse control and sound plans for the use of land, the plans have not been implemented and people have simply moved on to the cleared land and “developed” it in a random, and often non-sustainable, way. In the context of expanding populations, the prognosis for the future is not favourable. The environmental impact of tsetse control is likely to be insignificant against this background of human population growth, but this is no argument for complacency. When tsetse control is a component of other development activities, it is important to ensure that growth of livestock populations following reduced risk of trypanosomiasis does not exceed the carrying capacity of the land. Sustainability must be the objective, and different types of land will be able to sustain different intensities of land use.
When trying to predict the likely impact of tsetse control on the environment, it is important both to comprehend the role of cattle in Africa (most are kept primarily for a variety of social reasons and not for meat, milk or other ways in which they can contribute to a market economy) and to take account of existing livestock systems in the control area or, if non-existent, the type of system that is likely to develop following successful tsetse control. For example:
No Existing Use of Livestock
It is when successful tsetse control is practised over extensive densely-infested areas that the most profound subsequent changes in land use, and risks to the environment, are to be anticipated. It is under these circumstances that particular care has to be taken to prevent excessive pressure on what is already often a fragile eco-system. Perhaps the best documented example of a successful tsetse eradication campaign under such circumstances is in Nigeria where, in the short-term, the campaign released extensive new areas for cattle grazing and reduced pressure on the arid, naturally fly-free areas in the far north of the country. The short-term outcome was thus entirely environmentally-friendly. However, there are already indications that the occupation of new lands is only postponing a crisis as man continues to increase in numbers and yet more land is required. But surely it is this pressure of expanding human populations, and not tsetse control, which is to blame ?
National Parks and Reserves
Tsetse control in national Parks and Reserves, even by non-polluting methods, has been a particularly emotive issue. Those against would argue that the presence of the fly is a major factor preventing the encroachment of domestic livestock. Those in favour would argue that, whereas they are not advocating the take-over of reserved lands by livestock owners, they see the removal of the fly as essential to the protection of livestock owners around the periphery of reserves. Very often the basis of infestations affecting large numbers of domestic livestock is in reserved land. Clearly, if decisions to control tsetse are taken, then all possible precautions also must be taken to prevent encroachment of livestock into what are often areas of great economic as well as aesthetic value.
Pastoralism
Classically, pastoralists own cattle but do not own land. Classically, they also occupy the tsetse-free zones between the sub-humid zone and the great African deserts. The presence of tsetse flies has prevented the spread of cattle into the woodlands, other than seasonally when losses from disease are balanced against the necessity to locate herbage and water. For many years pastoralists resisted the efforts of Colonial authorities and, later, African governments to settle them, but in many areas today they are settling in response to the realities of life; grazing land is no longer limitless. Arable farmers are increasingly encroaching upon and settling on the traditional grazing grounds of pastoralists. Increasingly, pastoralists are often being pushed into closer contact with tsetse. Tsetse control can assist but there are attendant risks of subsequent uncontrolled increases in cattle numbers; and environmental degradation.
Traditional Agro-Pastoralism
This is an important from of land use in many countries, and becoming more so as pastoralists settle in response to pressures on their traditional grazing lands. It involves individually-farmed arable plots and communal grazing. One very important element of such systems is the increasing use of animals, especially cattle, for draught, there has been an enourmous expansion in the last 30-or-so years. In economic terms, draught functions of cattle can be more important than beef or milk offtake. Tsetse control can be much less controversial under such circumstances, it is protecting, and making more efficient, an existing agricultural system rather than permitting the opening up of new land.
Arable Farming
Today many cattle are owned by people whose chief concern is cultivation but where the two activities are less integrated than in 4. above. True mixed farming is uncommon, but many authorities consider that in some situations a move in this direction would assist in maintaining soil fertility and reducing land degradation. Trypanosomiasis is not usually a major constraint in areas of intensive arable farming where some domestic livestock are present, although there are exceptions, particularly in the presence of species of the palpalis group which are less affected by the activities of dense human population. Tsetse control under such circumstances can be helpful from an environmental point of view and is not generally a controversial issue.
Intensive Livestock Production Systems
Attempts have been made to manage range and other types of land for livestock production by setting up commercial or state-owned ranches. In some countries, such as Zimbabwe, these can be highly successful, but they make little contribution to the meat supply of most tsetse-infested countries. Tsetse control, especially by low-cost methods, can make a major contribution to increasing productivity but in such managed systems, where numbers of man and animals are generally kept within carrying capacity, environmental degradation is generally not an issue.
CONCLUSIONS
Although there is a continuing need to monitor the direct environmental and ecological effects of tsetse control, particularly when insecticides are employed, there is a much greater need to understand the complex issues related to the efficient use of land following successful tsetse control. The degree of ecological damage that can occur varies according to existing or potential livestock management practices which are, in turn, related to climatic, edaphic, social and other factors. Particularly rigorous precautions are necessary when previously unoccupied land is opened up to domestic livestock by effective tsetse control. The risks associated with tsetse control when carried out to protect well-established agricultural systems from trypanosomiasis are very much less. In between these two extremes are a range of intermediate situations.
The possibility that tsetse control could contribute to rapid and irreversible environmental degradation, even though expanding human populations may be the prime cause, is of great concern to all those associated with the funding and implementation of tsetse control programmes. Planners and those who have to ensure that plans for sustainable development are enforced, must walk a tightrope between reaping the undoubted potential benefits of tsetse control and preventing the after-effects of control from getting out of hand.
LUTTE CONTRE LA MOUCHE TSE-TSE:
FACTEURS ENVIRONNEMENTAUX ET ECOLOGIQUES
RESUME
A.M. Jordan
L'utilisation des insecticides pour combattre la mouche tsé-tsé est une question sensible et très controversée. Depuis de nombreuses années, la plupart des grandes campagnes de lutte utilisant les insecticides ont été suivies de près et les résultats indiquent que ces substances sont employées en si petites quantités ou de façon tellement sélective, qu'il n'y a aucune contamination notable de l'environnement. Malgré les preuves scientifiques, tout le monde n'est pas prêt à accepter cette conclusion. Il faut donc rester vigilant et continuer à contrôler l'utilisation massive d'insecticides.
Le débat sur l'utilisation des insecticides dans la lutte anti-tsé-tsé a, dans une certaine mesure, détourné l'attentions des véritables problèmes environnementaux et écologiques qu'il va falloir régler. Ces problèmes concernent les répercussions que peut avoir l'élimination d'un obstacle majeur à l'utilisation de grandes étendues de pâturages par le bétail. Dans de nombreux cas, l'élimination ou la réduction du risque de trypanosomiase - qui n'avait généralement rien à voir avec la lutte délibérée contre les tsé-tsé, mais résultait de l'élimination de la mouche par suite de la pression démographique - a amené à charger la terre en bétail au-delà de sa capacité, ce qui a entraîné surpâturage et dégradation. Le type de dégâts écologiques ainsi provoqués varie selon les méthodes d'élevage existantes ou potentielles qui dépendent à leur tour de facteurs climatiques, édaphiques et autres. Il est indispensable de bien comprendre ces question complexes si l'on veut exploiter judicieusement les terres qui auront été libérées de la mouche tsé-tsé.
