A. Van der Vloedt1
INTRODUCTION
This working paper is intended to emphasize techniques for the control of tsetse other than the more conventional use of insecticides or the modern trapping and baited target technology. The term biological techniques is used hereinafter in its broadest sense, taking into account all possible options including:
The use of laboratory reared tsetse for autocidal control or for population monitoring purpose;
the deliberate management of natural enemies (parasites, predators) and microbial agents; and
the use of sex recognition substances and other attractants in combination with chemical compounds (chemosterilants, insect growth regulators, juvenile hormone analogues, etc.) for non-radiation based auto-treated, odour-baited targets.
Before trying to indicate ways in which some of these biological techniques might be combined into a sensible pattern for operational use, it would appear appropriate to assess their current status of development. In this respect, it has to be recognized that the interest in their prospective use and the support given to related research and development, has changed considerably following recent technical progress in the first place in ground and aerial applications of insecticide and more recently in the use of insecticide-treated, odourbaited targets.
In 1975, when the FAO Programme for the Control of African Animal Trypanosomiasis was launched, for an estimated maximum duration of 40 years but with good prospects that a significant part of the programme could be realized by the year 2000, a number of considerations related to the vector were made, some of which merit being requoted here, namely that:
progress in trypanosomiasis and tsetse control techniques now makes the implementation of large-scale operations possible;
each method available presents advantages and disadvantages according to the local conditions, the resources available and the objective;
there are no practical alternatives to the use of insecticides and aerial spraying of insecticides now appears to be the best techniques for large-scale control campaigns;
whatever the technique used, total elimination of tsetse is always difficult to achieve, the treated zone being vulnerable to repopulation by reinvasion or as a result of a few individuals having survived the insecticidal treatment;
biological control through predators, parasites and pathogens seems at the moment only a remote possibility, while genetic control, although promising, could only play a role as one component of integrated long-term control campaigns.
Now, almost two decades later, the points referred to above are still valid. However, tsetse control has entered a new era during which modern trapping methods and insecticideimpregnated odour-baited targets can be expected to continuously play a key role, as long tsetse flies do not start developing “defence mechanisms” such as tolerance to insecticides or changes in behaviour following continued exposure to stationary artificial targets. To what extent established biological techniques will become components of future tsetse control strategies in priority action areas, is still very uncertain. Prospects for their use, even when deemed necessary, will largely depend on more accurate knowledge of environmental biological balances, technical refinements, their acceptability when ready to be used, the availability of resource centres and reliable delivery systems, and not least positive benefit/cost analyses. Of all available biological control techniques, the Sterile Insect Tethnique (SIT) is undoubtedly the most advanced and its technical effectiveness, particularly under west african conditions, is no longer a matter of speculation. There is also justification for medium to large-scale use at least against those tsetse species that can now be massreared.
1. Tsetse Fly Rearing in Support of Control Programmes
Scientists engaged in entomological or parasitological research involving Glossina spp. have long recognized the importance of laboratory colonies. Tsetse flies certainly do not belong to the few easy-to-rear insects such as fruit flies, house flies, screw-worm flies and some stored-product insect pests. Since the successful establishment of the first autonomous tsetse colony during the mid-1960's, continuous advancements and particularly the shift from using living animals as hosts to artificial membrane feeding systems, have increased the prospects of a consistent and economical production of high-quality insects. Pioneering work at European centres (Lisbon, Bristol, Paris, Antwerp, Vienna) has made it possible to transfer a workable rearing technology to Africa.
The rationale for tsetse rearing on a defined production scale might be summarised as follows:
the maintenance of a small research colony, based on regular inputs of fly material from the field or supplied from other existing stock colonies, makes basic laboratory studies on aspects of genetics, nutrition, vector and parasite life cycle and the response of vectors to attractants possible;
an autonomous colony (self-perpetuating) with intermediate production can be important with regard to (i) training, (ii) bioassays for rapid screening of candidate insecticides and other compounds such as insect growth regulators and juvenile hormone analogues, (iii) field studies, including mark-release-recapture studies and the use of laboratory-bred pupae for determination of parasitism and predation in the natural habitat, (iv) population monitoring, for example, by using sexually sterilised female flies as “sentinel insects” and (v) further improvement of rearing procedures (automation etc.);
production colony with large distributable excess; to be used for genetic control with SIT or hybrid sterility.
Although tsetse rearing has become established at a few specialized centres in Africa, the level of using excess material is still very low. In most instances, progress has been slow because tsetse rearing is often regarded as non-technical labour and has not received adequate support. Moreover, facilities have been poorly designed and equipped. Full exploration of the artificial membrane feeding technique, based on the use of locally collected blood, is mostly hampered by lack of a suitable alternative method, other than radiation, for decontamination of blood before it is used for stock colony feeding. This particular aspect indicates the usefulness of the concept of centralized production at a few properly equipped and adequately staffed regional rearing facilities.
With regard to the concept of using laboratory-bred tsetse flies as “sentinel insects”, we believe that the classical mark-release-recapture approach for native population estimation can be effectively extended by releasing sexually sterile female flies (“released as virgin females”). Provided such females have normal receptivity and dispersal ability, as has already been demonstrated for G. p. palpalis in Nigeria and for G. austeni in Zanzibar, recapture data and information on their mating status would indirectly reflect the wild population density. Moreover, if such female fly releases were made before and also regularly, i.e. at least every 2–3 months, during control operations, it would serve as a baseline from which to compare the progress of the campaign. Indeed, the proportion of released females showing mating scars, for example the palpalis group species, and/or impregnated spermathecae should decrease as the wild population decreased. Of course, this approach is feasible only when there is access to fly material from stock colonies and to a radiation source. Once again, its potential for practical use would be highly enhanced providing flies from a central rearing facility could be made available to control programmes.
To complete the picture on the use of laboratory-reared flies, I should like to refer to the potential use of hybrid sterile males or “satyrs” as genetic control agents. The term satyrs is used to designate males that will mate with hetero-specific or hetero-subspecific females. Such matings result in no offspring (Ribeiro, 1988, Gooding, 1991). As recently pointed out by Gooding, the introduction of satyrs into territory of a species with which they will mate, has some potential features and offers further economic advantages: the maintenance of only one colony, i.e. the source of the satyrs, and the lack of need for the radiation treatment. Before evaluating the potential of satyrs one must know whether females of the target species can distinguish between sperm of their own taxon and that of satyrs when multiple mating occurs. From available information (Gooding, 1991, Vreysen & Van der Vloedt, 1990), it appears that the use of satyrs may be possible among palpalis group flies, notably for the palpalis/gambiensis model, but might be less effective for the morsitans group, where females appear to predominantly use conspecific sperm.
Other prospects for genetic manipulation are seen in those cases where the male of one species tends to puncture the body wall of the female of another species when mating, for example when G.p. palpalis males are brought in contact with G.f. fuscipes females. The consequences of this are fatal.
2. The Use of Biocontrol Agents
The increasing interest in the ecological approach to insect and pest control, together with well-documented successes in classical biological control projects since the 1960's, has prompted some workers to up-date existing inventory lists of natural enemies of tsetse (Buxton, 1955). Moreover, a Scientific Advisory Group meeting held at the Memorial University of Newfoundland, Canada in 1974, led to the publication of a very comprehensive review (Ed.M. Laird, 1977) on the role of pathogens, parasites and predators and the future of biological methods in integrated control. The most salient points made are worthy of being recapitulated here:
the lack of detailed knowledge on tsetse/natural enemy inter-relationships;
on present evidence, tsetse flies are not indicated as the prime natural host of known predators;
very few attempts have been made to study the biology, host-finding capabilities and preferences, ecological tolerances, and ways of breeding candidate biological organisms for tsetse control trials. Amongst the parasitoids, the hymenopteran Synthomosphyrum has been most throughly investigated. Release trials in East Africa had positive results, but after they ceased the level of parasitism in tsetse pupae soon fell to the natural level, i.e. less than 1%;
it also seems unlikely that ants, asilids and spiders can be manipulated to cause any substantial reduction of tsetse populations. From more recent experience, it is certainly important to realize that some of the predators, such as ants, may effectively contribute to the mortality of adult tsetse knocked-down following contact with insecticide-impregnated targets. Therefore, it is worthwhile to find out to what extent augmentation of such predators at tsetse hot spots, during control operations with targets, would be beneficial.
mutillid wasps, relatively common in East African tsetse biotopes but never recorded as tsetse parasitoids in West Africa, have been considered for possible region-to-region introduction. Unfortunately, there have been no serious attempts to develop laboratory breeding techniques.
However, it has been suggested (M. Laird & Simmonds, 1977), that these parasitoids might be economically employed by infecting surplus puparia at tsetse mass-production centres, and hand-seeding these in likely larviposition sites rather than simply releasing the adult mutillids. The same approach was also suggested for some of the parasitic bombyliids of the genus Exhyalanthrax.
With regard to the potential usefulness of micro-organisms such as Bacillus spp., microsporidian protozoans, fungi, viruses etc., it is also striking that very few sustained and co-ordinated efforts have been made to detect, isolate and possibly mass propagate Glossina pathogens (Kaaya & Okech, 1990). The potential of mermethid nematodes referred to by some authorities also remains to be confirmed under specific tsetse habitat conditions.
Of special interest are certainly the virus-like particles reported for various tsetse species. These virus-like particles cause hypertrophy of the salivary gland tissue and gonadal lesions which result in sterility in male flies (Jura et al., 1988; Odindo, 1988). There is evidence that females transmit the particles to their offspring. Moreover, the presence of masses of virus particles within the lumina of hypertrophied salivary glands suggests that affected tsetse transmit the virus to animals on which they feed and might be at the basis of horizontal transmission (Jura, 1989). The pathogenicity of the virus-like particles to various Glossina species also suggests their potential as microbial pathogens for biological control. However, it remains to be investigated as to how an appropriate mass cultivation procedure could be developed and contaminative infections spread amongst native tsetse populations.
3. The use of Female Reproductive Inhibitors
The use of female reproductive inhibitors, other than radiation and chemosterilants, may involve (i) systemic insecticides, (ii) insect growth regulators and (iii) juvenile hormone analogues.
The positive results of screening programmes of the systemic insecticide invermectin (MK-933) for antihelminthic activity as well as for control of ectoparasites (e.g. ticks, psoroptic mites and cattle grubs) (Drummond et al., 1981) have prompted some workers to test its effect on tsetse flies. Laboratory studies, during which invermectin was injected or orally applied to animals used for fly feeding, indicated a drastic effect on female fecundity. However, this effect only occurred when invermectin was used at concentrations much higher than the recommended clinical dose for treatment against endoparasites and other ectoparasites (Langley & Roe, 1984; Van den Abbeele et al., 1986; Mortelmans & Geerts, 1986). The possible use of this systemic insecticide on female populations might be considerably masked. This might be even more crucial, when the original tsetse population is low and has an intrinsic low input rate from emerging flies. Nevertheless, available experimental data on the stability of pyriproxyfen and the extreme sensitivity of tsetse larva to very small doses (Langley, 1991) suggests that the compound may have potential for use with odour-baited targets especially against savannah woodland species.
In conclusion, there are good reasons for intensifying field testing of attractants to be used in conjuction with new and safer reproductive inhibitors. Their potential to provide effective means of control or eradication and for suppressing tsetse populations prior to release of radiation-sterilized males merits consideration.
REFERENCES
Bursell, E. (1977) Trans. Rhod. scient. Ass. 58, 43–47
Buxton, P. (1955) The Natural History of Tsetse Flies. H.K. Lewis, London
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Dame, D. & Ford H. (1966) Bull. ent. Res. 58, 213-119
Dame, D. et al. (1975) Bull. ent. Res. 65, 453–457
Drummond, R. et al. (1981) J. Econ. Entomol. 74, 431–436
Gooding, R. (1991) Proceedings of FAO/IAEA seminar, Nairobi, 1991
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Hargrove, J. & Langley, P. (1990) Bull. ent. Res. 80, 397–403
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Langley, P. & Roe, J. (1984) Ent. exp. & appl. 36, 137–143
Langley, P. (1991) Proceedings of FAO/IAEA seminar, Nairobi, 1991
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Odindo, M. (1988) Insect Sci. Applic. 3, 59
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Vale, G. et al. (1986) Bull. ent. Res. 76, 179–193
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INTEGRATION DE TECHNIQUES BIOLOGIQUES DANS LES PROGRAMME
DE LUTTE CONTRE LES TSE-TSE
RESUME
A. Van der Vloedt
Le présent document de travail met l'accent sur des techniques de lutte autres que les méthodes classiques recourant aux insecticides ou les méthodes modernes de piégeage et d'utilisation de cibles appâtées. Le terme “techniques biologiques” est utilisé ici au sens le plus large, englobant toutes les options possibles, notamment:
l'utilisation de produits biologiques provenant de l'élevage des tsé-tsé pour la lutte autocide ou la surveillance continue des populations;
l'entretien délibéré de populations d'ennemis naturels (parasites, prédateurs), ainsi que d'agents microbiens;
l'utilisation de substances permettant de distinguer les sexes, en combinaison avec des composés chimiques régulateurs de la croissance des insectes, analogues des hormones juvéniles, etc. aux fins de l'autostérilisation des mouches dans leur habitat naturel sans avoir recours à l'irradiation.
Avant de tenter d'indiquer des moyens qui permettraient de combiner certaines de ces techniques biologiques dans un schéma opérationnel judicieux, il semblerait approprié d'évaluer leur utilisation et leur état de développement actuels dans le contexte des priorités établies pour la lutte contre les tsé-tsé, lorsque le “Programme de lutte contre la trypanosomiase animale africaine” a été entrepris en application de la Résolution No. XI de la Conférence mondiale de l'alimentation tenue en 1974. En outre, il faut bien se rendre compte que les perspectives d'utilisation de certaines de ces techniques, ainsi que l'intérêt accordé et le soutien apporté à la poursuite d'activités spécifiques de recherchedéveloppement, ont évidemment évolué avec les modifications des priorités résultant du progrès technique, tout d'abord en ce qui concerne les applications terrestres et aériennes d'insecticides, puis, plus récemment, l'utilisation de cibles traitées avec des insecticides.
En 1975, lorsque le “Programme de Lutte Contre la Trypanosomiase Animale Africaine” a été lancé (pour une durée estimative de 40 ans au maximum, mais avec de bonnes perspectives que le programme soit en grande partie réalisé en l'an 2000), un certain nombre de considérations ont été formulées en ce qui concerne le vecteur, dont quelques-unes méritent d'être rappelées ici, à savoir:
les progrès accomplis en ce qui concerne les techniques de lutte contre la trypanosomiase et contre les mouches tsé-tsé rendent maintenant possibles des opérations de lutte sur une grande échelle;
chacune des méthodes disponibles présente des avantages et des inconvénients qui doivent être appréciés en fonction des conditions locales, des ressources disponibles et de l'objectif à atteindre;
il n'existe pas dans la pratique de solutions permettant de remplacer l'utilisation d'insecticides et il semble maintenant que la pulvérisation aérienne d'insecticides soit la technique qui convient le mieux pour les campagnes de lutte de grande envergure;
quelle que soit la technique utilisée, l'élimination totale des tsé-tsé est toujours difficile à obtenir, soit parce que la zone traitée est facilement sujette à la réinvasion, soit parce que quelques sujets ont échappé au traitement insecticide;
la lutte biologique utilisant des prédateurs, parasites et agents pathogènes n'est pour le moment qu'une possibilité lointaine et la lutte génétique, qui semble certes prometteuse, ne pourra trouver sa place que comme un élément des campagnes de lutte intégrée à longue échéance.
Presque vingt ans plus tard et au moment de la présente réunion du Groupe d'experts sur les aspects écologigues et techniques du Programme de lutte contre la trypanosomiase animale africaine, les remarques précitées sont encore valables. Ainsi qu'on l'a rappelé à la précédente réunion du Groupe, tenue en 1988 à Accra, la lutte contre les tsé-tsé est heureusement entrée dans une ère nouvelle durant laquelle on peut escompter que les techniques de piégeage modernes et l'emploi de cibles munies d'appâts imprégnés d'insecticides joueront un rôle de premier plan, sous réserve que les mouches tsé-tsé ne commencent pas à développer des mécanismes de défence tels qu'une tolérance aux insecticides ou des modification du comportement pendant le contact continu avec des cibles artificielles stationnaires. La mesure dans laquelle les techniques biologiques bien établies et celles qui sont encore en cours d'élaboration trouveront place dans les stratégies futures de lutte anti-tsé-tsé dans des zones d'intervention prioritaire, dépendra en grande partie de l'enrichissement des connaissances sur environnement naturel, du perfectionnement des techniques en question, de leur acceptabilité, de l'accessibilité de centres de ressources et de systèmes fiables de livraison et, ce qui n'est pas le moins important, d'une analyse positive des coût/avantages. Il faut toutefois reconnaître que l'efficacité technique de quelques-unes des méthodes de remplacement, en particulier la technique de l'insecte stérile, n'est plus douteuse.
Un examen de l'état des connaissances sera présenté, en faisant ressortir les domaines dans lesquels il faudrait entreprendre de nouvelles recherches pour améliorer les techniques de lutte actuelles.
