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Tsetse control — present and future
128
SEMINARON TRYPANOSOMIASIS - TSETSECONTROL - PRESENTAND FUTURE
Tsetse control
- present and future A. M. JORDAN
Tsetse Research Laboratory,
University of Bristol, School of Veterinary Science
Attention will be restricted to those methods of tsetse control which are in use at the present time or might be used in future; no reference will be made to methods which have been extensively practised in the past but which are now rarely employed, such as techniques for clearing vegetation, on which flies depend for shelter, and eradicating game animals, on which they depend for food. The aim of tsetse control is defined as the eradication of the genus within a prescribed area, although it is recognized that there are some arguments in favour of policies of management in which small residual populations of Glossina are retained (FORD, 1971). Some five or six species of Glossina may be considered of major economic importance; between them they occupy a wide range of habitats and even a single species can occur in a variety of ecological circumstances. A successful control technique in one part of a species distribution will not necessarily be effective elsewhere in the range of that same species.
Decca Doppler tracking aids, flew in echelon, some 115 m aoart and aoolied 7-9 litres/kmz of 2OY endosulfan for each of only four applications. The application of residual insecticides from the air is a relatively recent development. LE Roux and PLATT (1972) used a helicopter to apply a dieldrin invert emulsion to dense thicket habitats of G. pallidipes in Kenya. Much more extensive operations using helicopters, primarily against G. morsitans, have been undertaken in Nigeria (SPIELBERGER and ABDURRAHIM,1972; POLITZAR, H., personal communication). Dieldrin emulsion concentrate has been the most widely employed insecticide, and since 1971 it is estimated that some 4,790 km2 of country has been sprayed and that tsetse have probably been eradicated from more than 3,750 kmz. This work has been undertaken in the Northern Guinea Savannah vegetation zone where eradication can be achieved by limiting the application of insecticide to drainage lines, riverine forests and the outer edges of Isoberlinia woodland. I”
The present Application of insecticides from the ground
Ground spray techniques are particularly applicable against species of Glossina which have linear distributions. They have been extensively used against G. fuscipes in East Africa (GLOVER et al, 1960) and G. palpalis and G. tachinoides (KERNAGHAN, 1961) in West Africa. G. morsitans, usually considered as a Savannah species, is virtually a riverine species for much of the year under the harsh climatic conditions in the north of Nigeria where it has been eradicated from some 27,570 km2 of river system draining into Lake Chad, making some 68,740 km2 safe for cattle grazing (DAVIES, 1971). The insecticide used was DDT wettable powder, generally as a suspension of 24% active ingredient. During the campaign, from 1955 to 1970, following detailed studies of the resting sites of G. morsitans, progressively more selective techniques were used for the application of insecticide. Eventually no spray was applied above five feet and even then in only certain vegetation types. Operations are now in progress further south in Nigeria (DAVIES, H., personal communication) under climatic conditions much more favourable for G. morsitans. The flies are not so restricted in their resting sites, requiring the application of more, and more concentrated, insecticide in a less selective manner than was necessary further north. Application of insecticides from the air
Techniques for the aerial application of non-residual insecticides were originally developed at the Tropical Pesticide Research Unit, Tanzania (LEE, 1969). Eradication campaigns have been undertaken in a number of countries. In Zambia, PARK et al (1972) treated some 1,600 km2 with five applications of ultra-low-volume endosulfan and eradicated G. morsitans except near an unsatisfactory barrier. From 1970 to 1973 a further 14,500 km2 were sprayed (ASHTON, D. R., personal communication). Three Beechcraft Baron aircraft, using
The future Possible alternative methods of control
Insecticides will undoubtedly be the main method of tsetse control for many years to come, unless serious resistance problems develop. Other methods have been proposed and work is in progress on a number of possible alternatives to insecticides. The very low reproductive potential of Glossina makes it in theory particularly vulnerable to control by genetic methods. The sterile insect release method requires the establishment of large productive colonies (e.g. NASH et al, 1971) and the release of large numbers of sterilized male flies into the natural habitat of the same species (DAME and SCHMIDT. 1970). The disadvantages of the technique include its specificity (if more than one species of Glossina is to be controlled, more than one species will have to be colonized and released) and its high cost (particularly the costs of colonizing such a slow breeding insect). On the credit side, the technique has no harmful environmental effects and, unlike other methods of insect control, it is most effective when the density of the natural population is low. If field trials in Tanzania (WILLIAMSON, 1974) and Upper Volta (ITARD, 1974) prove the efficacy of the technique it may ultimately be possible, and desirable, to use it to “mop up” small residual populations after aerial applications of nonpersistent insecticides. Other genetic methods have been proposed, of which the most practical at the moment would seem to be the exploitation of the hybrid sterility resulting from some combinations of crosses between subspecies of G. morsifans (CURTIS, 1972). There is an urgent need to make an inventory of and characterize pathogens of Glossina before more practical steps to use biological control techniques can be taken (IDRC, 1974). Work is in progress in a number of laboratories on characterizing the effects of juvenile hormones, ecdysones
A. M. JORDAN and diuretic hormones, but the formulation of a hormonal insecticide for use against Glossina is only a distant prospect. Such insecticides are already in use against other pest species; they have a number of attractive features, including virtually no effect on mammals. The discovery of a powerful tsetse attractant would be of great value and the development of much more efficient catching devices, taking advantage of the natural attractiveness of host animals (VALE, 1974), is a major advance. The recent discovery of a sex pheromone in G. morsitans (LANGLEYet al, 1975) may have potential for development as an attractant for male flies in the field. It has been suggested that the sounds produced by tsetse flies may act as a means of communication (KOLBE, 1973); should this be so, it may ultimately be possible to devise an auditory attractant. Strategy for control campaigns Tsetse flies, especially the Savannah species, are acting
as conservation agents in that they are maintaining the status quo in Africa until man can put the land they
occupy to better use. Future tsetse control campaigns should be integrated with rational plans for land development to avoid the under- or over-exploitation of land cleared of Glossina. References Curtis, C. F. (1972). Sterility from crosses between subspecies of the tsetse fly Glossina morsitans. Acta tropica, 29,250-268. Dame, D. A. & Schmidt, C. H. (1970). The sterile-male technique against tsetse flies, Glossina spp. Bulletin of the Entomological Society of America, 16,24-30. Davies, H. (1971). Further eradication of tsetse in the Chad and Gongola river systems in north-eastern Nigeria. Journal of Applied Ecology, 8, 563-578. Ford, J. (1971). The Role of the Trypanosomiases in African Ecology. Oxford: Clarendon. Glover, P. E., Le Roux, J. G. & Parker, D. F. (1960). The extermination of Glossina palpalis on the KujaMigori river systems with the use of insecticides. International Scientific Committee for Trypanosomiasis Research. Brussels, 1958.331-342. IDRC (1974). Tsetse control: the role of pathogens, parasites and predators. Report of a Scientific Advisory
129
Group, St. John’s, 1974. Bard, J. (1974). The present state of colonies of Glossina at Maisons-Alfort (France). Proceedings of the Third International Congress of Parasitology. Munich, 1974. 940. Kernaghan, R. J. (1961). Insecticidal control of the vectors of human trypanosomiasis in Northern Nigeria. Journal of Tropical Medicine and Hygiene, 64, 303-309.
Kolbe, F. F. (1973). The modulated sounds made by the tsetse fly Glossina brevipalpis Newstead. Zoologica Africana, 8,241-258.
Langley, P. A., Pimley, R. W. & Carlson, D. A. (1975). Sex recognition pheromone in tsetse fly Glossina morsitans. Nature, 254, 51-53. Lee, C. W. (1969). Aerial applications of insecticides for tsetse fly control in East Africa. Bulletin of the World Health Organisation, 41, 261-268. Le Roux, J. G. & Platt, D. C. (1972). Application of a dieldrin invert emulsion by helicopter for tsetse control. International Scientific Council for Trypanosomiasis Research. Bangui, 1968.219-229. Nash. T. A. M.. Jordan. A. M. & Trewern. M. A. (1971). Mass rearing’of tsetse flies (Glossina spp.). In: Sterility principle for insect control or eradication. Vienna, IAEA, 1970.99-108. Park, P. O., Gledhill, J. A., Alsop, N. & Lee, C. W. (1972). A large-scale scheme for the eradication of Glossina morsitans morsitans Westw. in the Western Province of Zambia by aerial ultra-low-volume application of endosulfan. Bulletin of entomological Research, 61,373-384. Spielberger, U. & Abdurrahim, U. (1972). Pilot trial of discriminative aerial application of persistent dieldrin deposits to eradicate Glossina morsitans submorsitans in the Anchau and Ikara forest reserves, Nigeria, 1971. International Scientific Council for Trypanosomiasis Research, Lagos, 1971.271-281. Vale, G. A. (1974). New field methods for studying the responses of tsetse flies @iptera, Glossinidae) to hosts. Bulletin of entomological Research, 64, 199-208. Williamson, D. L. (1974). The biology and mass rearing of Glossina morsitans in Tanzania. Proceedings of the Third International Congress of Parasitology. Munich, 1974.941.
SEMINARON TRYPANOSOMIASIS - TSETSECONTROL - PRESENTAND FUTURE
Tsetse control
- present and future A. M. JORDAN
Tsetse Research Laboratory,
University of Bristol, School of Veterinary Science
Attention will be restricted to those methods of tsetse control which are in use at the present time or might be used in future; no reference will be made to methods which have been extensively practised in the past but which are now rarely employed, such as techniques for clearing vegetation, on which flies depend for shelter, and eradicating game animals, on which they depend for food. The aim of tsetse control is defined as the eradication of the genus within a prescribed area, although it is recognized that there are some arguments in favour of policies of management in which small residual populations of Glossina are retained (FORD, 1971). Some five or six species of Glossina may be considered of major economic importance; between them they occupy a wide range of habitats and even a single species can occur in a variety of ecological circumstances. A successful control technique in one part of a species distribution will not necessarily be effective elsewhere in the range of that same species.
Decca Doppler tracking aids, flew in echelon, some 115 m aoart and aoolied 7-9 litres/kmz of 2OY endosulfan for each of only four applications. The application of residual insecticides from the air is a relatively recent development. LE Roux and PLATT (1972) used a helicopter to apply a dieldrin invert emulsion to dense thicket habitats of G. pallidipes in Kenya. Much more extensive operations using helicopters, primarily against G. morsitans, have been undertaken in Nigeria (SPIELBERGER and ABDURRAHIM,1972; POLITZAR, H., personal communication). Dieldrin emulsion concentrate has been the most widely employed insecticide, and since 1971 it is estimated that some 4,790 km2 of country has been sprayed and that tsetse have probably been eradicated from more than 3,750 kmz. This work has been undertaken in the Northern Guinea Savannah vegetation zone where eradication can be achieved by limiting the application of insecticide to drainage lines, riverine forests and the outer edges of Isoberlinia woodland. I”
The present Application of insecticides from the ground
Ground spray techniques are particularly applicable against species of Glossina which have linear distributions. They have been extensively used against G. fuscipes in East Africa (GLOVER et al, 1960) and G. palpalis and G. tachinoides (KERNAGHAN, 1961) in West Africa. G. morsitans, usually considered as a Savannah species, is virtually a riverine species for much of the year under the harsh climatic conditions in the north of Nigeria where it has been eradicated from some 27,570 km2 of river system draining into Lake Chad, making some 68,740 km2 safe for cattle grazing (DAVIES, 1971). The insecticide used was DDT wettable powder, generally as a suspension of 24% active ingredient. During the campaign, from 1955 to 1970, following detailed studies of the resting sites of G. morsitans, progressively more selective techniques were used for the application of insecticide. Eventually no spray was applied above five feet and even then in only certain vegetation types. Operations are now in progress further south in Nigeria (DAVIES, H., personal communication) under climatic conditions much more favourable for G. morsitans. The flies are not so restricted in their resting sites, requiring the application of more, and more concentrated, insecticide in a less selective manner than was necessary further north. Application of insecticides from the air
Techniques for the aerial application of non-residual insecticides were originally developed at the Tropical Pesticide Research Unit, Tanzania (LEE, 1969). Eradication campaigns have been undertaken in a number of countries. In Zambia, PARK et al (1972) treated some 1,600 km2 with five applications of ultra-low-volume endosulfan and eradicated G. morsitans except near an unsatisfactory barrier. From 1970 to 1973 a further 14,500 km2 were sprayed (ASHTON, D. R., personal communication). Three Beechcraft Baron aircraft, using
The future Possible alternative methods of control
Insecticides will undoubtedly be the main method of tsetse control for many years to come, unless serious resistance problems develop. Other methods have been proposed and work is in progress on a number of possible alternatives to insecticides. The very low reproductive potential of Glossina makes it in theory particularly vulnerable to control by genetic methods. The sterile insect release method requires the establishment of large productive colonies (e.g. NASH et al, 1971) and the release of large numbers of sterilized male flies into the natural habitat of the same species (DAME and SCHMIDT. 1970). The disadvantages of the technique include its specificity (if more than one species of Glossina is to be controlled, more than one species will have to be colonized and released) and its high cost (particularly the costs of colonizing such a slow breeding insect). On the credit side, the technique has no harmful environmental effects and, unlike other methods of insect control, it is most effective when the density of the natural population is low. If field trials in Tanzania (WILLIAMSON, 1974) and Upper Volta (ITARD, 1974) prove the efficacy of the technique it may ultimately be possible, and desirable, to use it to “mop up” small residual populations after aerial applications of nonpersistent insecticides. Other genetic methods have been proposed, of which the most practical at the moment would seem to be the exploitation of the hybrid sterility resulting from some combinations of crosses between subspecies of G. morsifans (CURTIS, 1972). There is an urgent need to make an inventory of and characterize pathogens of Glossina before more practical steps to use biological control techniques can be taken (IDRC, 1974). Work is in progress in a number of laboratories on characterizing the effects of juvenile hormones, ecdysones
A. M. JORDAN and diuretic hormones, but the formulation of a hormonal insecticide for use against Glossina is only a distant prospect. Such insecticides are already in use against other pest species; they have a number of attractive features, including virtually no effect on mammals. The discovery of a powerful tsetse attractant would be of great value and the development of much more efficient catching devices, taking advantage of the natural attractiveness of host animals (VALE, 1974), is a major advance. The recent discovery of a sex pheromone in G. morsitans (LANGLEYet al, 1975) may have potential for development as an attractant for male flies in the field. It has been suggested that the sounds produced by tsetse flies may act as a means of communication (KOLBE, 1973); should this be so, it may ultimately be possible to devise an auditory attractant. Strategy for control campaigns Tsetse flies, especially the Savannah species, are acting
as conservation agents in that they are maintaining the status quo in Africa until man can put the land they
occupy to better use. Future tsetse control campaigns should be integrated with rational plans for land development to avoid the under- or over-exploitation of land cleared of Glossina. References Curtis, C. F. (1972). Sterility from crosses between subspecies of the tsetse fly Glossina morsitans. Acta tropica, 29,250-268. Dame, D. A. & Schmidt, C. H. (1970). The sterile-male technique against tsetse flies, Glossina spp. Bulletin of the Entomological Society of America, 16,24-30. Davies, H. (1971). Further eradication of tsetse in the Chad and Gongola river systems in north-eastern Nigeria. Journal of Applied Ecology, 8, 563-578. Ford, J. (1971). The Role of the Trypanosomiases in African Ecology. Oxford: Clarendon. Glover, P. E., Le Roux, J. G. & Parker, D. F. (1960). The extermination of Glossina palpalis on the KujaMigori river systems with the use of insecticides. International Scientific Committee for Trypanosomiasis Research. Brussels, 1958.331-342. IDRC (1974). Tsetse control: the role of pathogens, parasites and predators. Report of a Scientific Advisory
129
Group, St. John’s, 1974. Bard, J. (1974). The present state of colonies of Glossina at Maisons-Alfort (France). Proceedings of the Third International Congress of Parasitology. Munich, 1974. 940. Kernaghan, R. J. (1961). Insecticidal control of the vectors of human trypanosomiasis in Northern Nigeria. Journal of Tropical Medicine and Hygiene, 64, 303-309.
Kolbe, F. F. (1973). The modulated sounds made by the tsetse fly Glossina brevipalpis Newstead. Zoologica Africana, 8,241-258.
Langley, P. A., Pimley, R. W. & Carlson, D. A. (1975). Sex recognition pheromone in tsetse fly Glossina morsitans. Nature, 254, 51-53. Lee, C. W. (1969). Aerial applications of insecticides for tsetse fly control in East Africa. Bulletin of the World Health Organisation, 41, 261-268. Le Roux, J. G. & Platt, D. C. (1972). Application of a dieldrin invert emulsion by helicopter for tsetse control. International Scientific Council for Trypanosomiasis Research. Bangui, 1968.219-229. Nash. T. A. M.. Jordan. A. M. & Trewern. M. A. (1971). Mass rearing’of tsetse flies (Glossina spp.). In: Sterility principle for insect control or eradication. Vienna, IAEA, 1970.99-108. Park, P. O., Gledhill, J. A., Alsop, N. & Lee, C. W. (1972). A large-scale scheme for the eradication of Glossina morsitans morsitans Westw. in the Western Province of Zambia by aerial ultra-low-volume application of endosulfan. Bulletin of entomological Research, 61,373-384. Spielberger, U. & Abdurrahim, U. (1972). Pilot trial of discriminative aerial application of persistent dieldrin deposits to eradicate Glossina morsitans submorsitans in the Anchau and Ikara forest reserves, Nigeria, 1971. International Scientific Council for Trypanosomiasis Research, Lagos, 1971.271-281. Vale, G. A. (1974). New field methods for studying the responses of tsetse flies @iptera, Glossinidae) to hosts. Bulletin of entomological Research, 64, 199-208. Williamson, D. L. (1974). The biology and mass rearing of Glossina morsitans in Tanzania. Proceedings of the Third International Congress of Parasitology. Munich, 1974.941.