Baits, either live or artificial, are used to attract tsetse. Once the tsetse have been attracted they can be caught and hence give an indication of their presence and abundance in an area, as is done for monitoring purposes using surveys. The sensitivity of the survey method used can be measured by recording the level of trypanosome infection in a “sentinel” herd kept in the survey area.
Alternately, when the objective is vector control or suppression, the attracted tsetse can be caught in traps and later deliberately killed or killed directly by using insecticide impregnated targets, which do not incorporate a capture device.
The early work in this field was carried out by Harris who developed, in the 1920's, a trap designed to simulate an ungulate for the capture and control of tsetse. Although the trap had some success, later studies showed that it was not such a suitable control device, as there was considerable bias to the capture of female flies.
The trap was developed as an alternative to using human baits, involving hand nets or sticky screens worn on the back. These, though effective, could for practical reasons, only be used on a small scale. Live baits, such as cattle, also caught mainly males.
In the 1940's, following the advent of DDT, the use of cattle as live baits to achieve control by treating them with DDT was introduced but did not prove successful because the insecticide formulation used was not sufficiently persistent on cattle which, therefore, required spraying at frequent and impractical intervals.
Developments towards the reintroduction and refinement of baits systems started in the 1960's almost simultaneously by workers in West Africa, studying riverine tsetse, and those in Zimbabwe, studying savannah tsetse. These combined studies showed that tsetse responded to odour stimuli for distance attraction and visual stimuli for orientation and landing, and further indicated the preference of various species to a variety of colours and organic odour sources.
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In West Africa, Challier and Laveissiere developed a blue, black and white biconical trap that depended on visual attraction. This, and later, an insecticide impregnated blue square cloth, were effective in controlling restricted riverine tsetse. They were, however, too expensive to deploy against the much dispersed savannah tsetse.
Initial work in Zimbabwe, which later involved workers in UK and Kenya, showed that flies responded positively to an odour stimulus contained in the excretory component of ox breath and to ox and buffalo urine. To catch or kill the attracted tsetse, workers in Zimbabwe designed a suitable box trap for savannah tsetse, an electric net for entomological studies and an insecticide impregnated black cloth screen for control.
Studies were continued into the 1980's on more accurate determination of both visual and odour stimuli, on trap and screen designs and the application of these to survey and control operations. The progress was substantial and the findings are summarised below.
Table 1 shows the response of various groups of tsetse to black and blue, the colours to which tsetse show the greatest response (Green, 1989).
Table 1: Tsetse Visual Response
|COLOUR||PALPALIS GROUP||MORSITANS GROUP||FUSCA GROUP|
|(includes landing)||(needs net for landing)|
The Palpalis group respond only to the blue colour, the Morsitans group to both blue and black, except for G.longipennis which only responds to blue. Black is to be preferred as it induces a landing response to both G.pallidipes and G.morsitans. G.medicorum, a Fusca group fly, is attracted to blue but benefits from the addition of mosquito netting to induce landing.
All species of tsetse so far tested have been shown to be attracted to carbon dioxide, actone, butanone, octenol and phenols (Vale, 1988). Table 2 gives an indication of the tsetse response to phenolic odours (Spath & Kupper, 1989). Those of the Fusca group are attracted by the phenol components from bushbuck urine while the Morsitans group is attracted by the component from cattle or buffalo urine. However, in Somalia G.pallidipes does not seem to respond to phenols from these components. It is believed that other odour components remain to be discovered.
Table 2: Tsetse Olfactory Response
|ODOUR||PALPALIS GROUP||MORSITANS GROUP||FUSCA GROUP|
|(Bush buck urine)||(Cattle/buffalo urine)|
Trap and target design
Some notable advances made since the development of the biconical trap have been the introduction of F3 and epsilon traps in Zimbabwe and the Ngu trap in Kenya, both for monitoring and control of savannah tsetse. Work continues in Zimbabwe, in collaboration with UK and Kenya, in the development of monitoring systems involving “refuges”. For purely control purposes the black cloth/netting targets have remained standard. Instead of using cloth targets, use is being made of painting tree stumps or wrapping trees with blue cloth to make them attractive to tsetse (Vale, personal communication).
Investigations are also underway into the practical possibility of substituting insecticides by chemical sterilants and synthetic juvenile hormones which through a multiplier effect would induce sterility and block reproduction in wild populations.
Static baits, such as traps may also be replaced in the near future by cattle treated with suitable formulations of pyrethroid insecticides to kill tsetse. The current formulations in use, or on trail, are deltamethrin (Cooper, Wellcome), flumethrin (Bayer) and cypermethrin (Ciba Geigy).
Extensive use of traps for control has been made in West Africa against riverine tsetse and in Kenya (Dransfield et Al., 1989) against savannah species. Targets have been employed for control on a large scale in Zimbabwe (Vale, 1988) and to a limited extent in Kenya (Opiyo et al., 1988, 1989). The use of live bait cattle for control of tsetse has been successfully practiced in Zimbabwe and Zanzibar and promising trials have been carried out in Kenya.
FURTHER READING AND REFERENCES:
C.F. Swynnerton (1936): The tsetse flies of East africa. Trans. R. Ent. Soc. Lond., 84.
P.A. Buxton (1955). “The Natural History of Tsetse Flies”. Mem. Lond. Sch. Hyg. Trop. Med. No. 10.
Vale, G.A. (1988). The development of baits for survey and control of the savannah tsetse flies. In “Regional Development and implementation of tsetse control strategies for Eastern Africa with emphasis on targets and traps.”
All other references cited will be found in OAU/STRC publications No.s 114 and 115, the proceedings of the ISCTRC meetings in Lome (1987) and Mombasa (1989).
PROGRES RECENTS DANS LE DEVELOPPEMENT
TECHNIQUE DES APPATS POUR EVALUER LA PRESENCE
OU CONTROLER LES TSE-TSE
Les progrès en matière d'utilisation d'appâts pour repérer et contrôler les tsé-tsé ont été liés au développement:
des moyens pour attirer les tsé-tsé;
à l'augmentation de l'efficacité de l'attraction des tsé-tsé afin de les capturer, les exposer à des stérilisants ou des insecticides; et,
à l'amélioration dans la disposition des cibles.
Les mouches sont attirées vers les appâts par les odeurs provenant des animaux. Celles-ci proviennent soit d'octénol et de phénols synthétiques, soit d'urine de bovins. D'autres composés biologiques se sont montrés efficaces, mais n'ont pas été largement utilisés.
L'attraction visuelle des appâts dépend largement de la couleur des tissus. Glossina p. palpalis and G. tachinoides préfèrent la couleur bleue, tandis que le groupe morsitans préfère le noir, à l'exception de G. longipalpis qui préfère le bleu. L'efficacité des pièges est améliorée par l'addition de noir qui encourage les mouches à se poser.
De plus, des modifications sont apparues dans la forme des pièges et des écrans depuis le premier piège de Chailler jusqu'aux F3 ou NG2B (NGU) qui sont beaucoup plus efficaces. Un large succès dans le contrôle des mouches a été obtenu dans beaucoup de pays d'Afrique grâce aux pièges ou aux écrans pulvérisés à la deltaméthrine.
Le bétail vivant a récemment remplacé les cibles immobiles. Son odeur naturelle attire les mouches qui s'y posent et sont tuées par contact avec les insecticides. Ces derniers sont appliqués soit par “dipping” ou par application locale de formulations “pour-on”. Des essais fructueux grâce au bétail ont été réalisés au Zimbabwe et au Kenya en utilisant la deltaméthrine; au Burkina et au Kenya avec la fluméthrine. Des essais sont menés au kenya avec la cypermethrine.