to the design and execution of substantial antitsetse activities.
The natural limits of the tsetse population to be attacked should be determined before control operations are initiated. Surveys should be undertaken with care to determine population limits and maximum dispersal capabilities at the most favourable seasons of the year. Low density populations require the development of new sampling techniques (page 52).
Restricted application of residual insecticide requires prior knowledge of those plant communities with which a large proportion of the tsetse population is associated, especially at unfavourable seasons of the year (“discriminative” phase). Resting sites utilized by the flies throughout the day and night in these plant communities should be determined (“selective” phase). Resting sites can be confined to a small proportion of the overall habitat, and very restricted use of insecticide is possible under such circumstances, especially when applied by ground spray teams applying the principles of “discrimination” and “selection”. Findings from one area are not necessarily applicable elsewhere within the range of a species and such studies are necessary wherever control operations are to be undertaken.
Less detailed knowledge of the ecology of the species to be attacked is necessary when blanket applications of non-residual insecticide are to be employed. In order to determine the optimum number and interval between application it is, however, necessary to know the duration of the pupal period throughout the time when spraying is to be conducted; particular attention should be paid to the coolest breeding sites where the period will be most prolonged. It is the maximum period and not the mean which is most significant. For the same reason, it is also necessary to determine the length of time between the emergence of a female fly and the production of its first lava throughout the spray period, particularly at the shortest period.
At the conclusion of control operations, detailed surveys are again required to determine whether or not eradication has been achieved. The detection of tsetse flies at very low population densities is difficult and inefficient and the development of appropriate methods is perhaps the most pressing need in studies of Glossina ecology. Now efficient devices for sampling tsetse populations have been developed (104); the possibility of adapting these to detect low density populations requires urgent investigation.
If land is to be opened up for the purpose of providing food resources, then it must be recognized that the agricultural activities themselves which will ensue will bring about a far greater environmental change than initial applications of insecticides for tsetse control. Furthermore, the agricultural changes will require greater amounts of more persistent insecticides for crop protection than will be used in public health. Accordingly, planners of such large programmes should approach the problem of environmental modification from a realistic viewpoint. Governments which seek the long-term objective of increased agricultural yield cannot at the same time criticize too severely the short-term use of insecticides for vector control.
The alternatives to the use of insecticides for tsetse control are the direct killing of game and habitat clearance. The environmental effects of these operations are probably greater than the use of the insecticides. Although genetic control of Glossina is a distinct feasibility, a great deal of work will have to be done to determine its operational value. Despite recent publications on the possibilities of biological control of Glossina, it is considered to have a remote feasibility.
For tsetse control certain areas of wooded savannah, riverine forests and forests proper, including the ecotones between such systems, will be treated. The first step would be to map or classify the major ecosystems that will be covered by treatment and, based upon existing literature, to provide a preliminary listing and structure of the major animal groups in each. From this, a series of indicator organisms can be derived and their availability and relative abundance in each of those main ecosystems approximated and checked by spot surveys. Once spraying operations have been decided upon and the toxicity of the insecticide (s) selected determined for major groups, i.e. mammalian, avian, fish, etc., a sampling procedure for the indicator species population levels can be established. There are numerous adequate procedures for determining vertebrate population levels depending on availability of manpower and the precision desired. Other major groups to be sampled would be non-target forest insects, as well as soil organisms, especially Annalida and arthropods.
Physical measurements to determine the distribution and quantity of insecticides applied by aerial spraying in sample habitats should also be made. Initially, this should be done in a wooded area at a canopy level, at intermediate tree level, on ground vegetation and, ultimately, in the soil. Again, details can be determined for each of the habitat types.
Recent progress in the use of fixed-and rotary-wing aircraft and ground equipment for insecticide applications needs to be exploited in a wider variety of situations and new insecticide formulations require field evaluation. Thus, field studies to evaluate new techniques and chemicals should be extended to situations where treatments have been difficult or impossible in the past, as in certain riverine, moist savanna and forest zones. In some situations where control has been successful alternative methods are desirable as, for example, when economic or environmental aspects must be taken into account.
CORP is engaged in a continuing programme for the laboratory testing and evaluation of candidate insecticides for toxicity to tsetse flies in collaboration with WHO. Comparative toxicities of compounds supplied by industry, research institutes, universities and other sources are determined by a standard method of topical application of solutions to teneral G. austeni obtained from pupae*. The results are published from time to time (43) and indicate the most promising insecticides for space sprays. Other properties, such as volatility, solubility and stability are also measured to indicate suitability of compounds for residual activity.
A research programme on formulations of insecticides for residual activity on vegetation is also currently in progress. A spray tower has been constructed for the simulation of ULV conditions; that is, the application of concentrated solutions of 10% or more active ingredient at low volume rates and controlled droplet size. Observations are made on the physical state of deposits on leaves and of the persistence of the active ingredient on leaves. The persistence and availability of the active ingredient is measured by bioassay and chemically by GLC analysis.
Persistent chlorinated hydrocarbon insecticides, e.g. DDT, dieldrin and BHC are in current use for tsetse fly control. Insecticides are applied selectively to the known resting sites of flies, e.g. the trunks of trees and the undersides of certain branches or to leaves in the canopy and are effective when applied to riverine and savanna vegetation using ground spraying machines. Insecticides which are environmentally more acceptable and less toxic to cattle and humans would be evaluated for use by this method. Trials with these insecticides should include dieldrin as a reference treatment. Details of proposed trials are shown in Appendix II.
Hand carried equipment for applying aerosols for tsetse fly control has been used in various parts of Africa, but the method has never been developed to any extent. Equipment applying DDT as fogs has been tried but there is a need to evaluate alternative less persistent insecticides applied as ULV or as dilute formulations suitable for fogging. These methods are unlikely to lead to permanent eradication, but there are requirements to reduce fly numbers rapidly in emergency sleeping sickness situations, in areas of human habitation or of tourist interest. It would be necessary to study sequential applications of insecticide if long-term control was required. Applications of insecticide from boats would have to be considered for access to difficult riverine areas. Details are given in Appendix III.
Persistent chlorinated hydrocarbon insecticides, e.g. DDT, dieldrin and endosulfan have recently been shown to control tsetse flies effectively in West Africa by discriminative treatments of aerial sprays applied by helicopter to forest islands, drainage lines, ecotones and stretches of river where the vegetation is suitable. Since aerial applications are necessarily more diffuse than hand spraying, it is vital to evaluate alternative insecticides which are environmentally safer and less toxic to man and cattle. It is also important to examine closely the economics of this method and the possibility of using different formulations for greater efficiency. Details are given in Appendix IV.
The control of tsetse flies along large stretches of rivers where ground access is difficult has always presented problems. Fixed-wing aircraft do not have the manoeuverability to follow the course of the riverine vegetation and residual spraying from the air causes greater environmental side effects. There is, therefore, a need to devise a non-residual spraying technique using environmentally safer insecticides for the rapid reduction of fly, especially in areas where sleeping sickness is epidemic. Details are given in Appendix V.
The technique of applying aerosols from fixed-wing aircraft for tsetse fly control has been well tried and has been shown to be effective with a variety of insecticides in field experiments and in large airspray operations but requires additional development work. Endosulfan is currently used at rates of 6–14 g/ha per treatment. There may be situations where this insecticide is a hazard, e.g. around rivers or lakes, and it would be worthwhile assessing alternative insecticides. Details are given in Appendix VI.
