Candidate compounds for tsetse control from commercial companies, research institutes and other sources should continue to be assessed for insecticidal activity in laboratory tests as a basis for selection for field evaluation. This selection should take into consideration not only toxicity to tsetse flies, but also existence of a suitable formulation, availability and cost, toxicology and possible effects on non-target organisms. Some of these factors are listed (Appendix I) for new compounds already regarded as possible replacement chemicals on the basis of toxicity to flies. The availabilities mentioned are of the order of 1g to 100g for laboratory testing, 1 kg to 100 kg for field trials, and in normal commercial quantities for those which are readily available and apply to the situation as far as it is known at the present time.
Highest priority should be given to field evaluation of the synthetic pyrethroids, NRDC 143 and NRDC 161, which are outstanding in their toxicity to tsetse flies and their persistence in spray residues and could be used in both aerial and ground sprays. NRDC 161 is the most potent insecticide known against many insects but also has a high mammalian toxicity with a site of action which differs from other pyrethroids. It is important therefore that further information should be obtained as soon as possible on hazards to other animals, although it should also be noted that the very powerful insecticidal action means that it can be used at lower concentrations and dosages than the other candidates.
The organophosphorus compounds and carbamates listed have good to moderate toxicities to the flies as measured by topical application and can be divided into those which are likely to have a short term persistence only (for example, fenthion, naled, bromophos, propoxur) and those which could have a longer residual action (for example, azamethiphos, tetrachlorvinphos, jodfenphos, bendiocarb). Bromophos from the former group and tetrachlorvinphos from the latter have already been examined in field trials and found to be not very promising.
The susceptibility of different species and strains of tsetse to endosulfan and dieldrin and to representatives of other chemical classes that are likely to be used in field operations should be measured in different parts of Africa. This will provide guidance for variations in dosages required and also baseline data for a systematic monitoring programme for resistance. A simple standard method should be adopted; the most suitable is probably the WHO method for topical application (111), but field performance should also be studied by methods utilizing tarsal contact with weathered deposits on sprayed surfaces (e.g. bark) if persistence is to be investigated.
Solutions are not usually employed because the solvent is expensive compared with water, but they were used in early trials in East Africa with DDT (95) or more recently in Nigeria with dieldrin (73). The cheaper solvents, such as kerosene, which can be used at high volumes, evaporate rapidly from deposits and do not influence persistence or activity of the insecticide by their presence. They can modify the initial distribution of insecticide by moving it into bark or leaves before they evaporate.
Emulsions or suspensions are much commoner systems for insecticide dispersal, and the two insecticides, DDT and dieldrin, are invariably used in these forms in the larger control schemes, for example in Nigeria, Uganda, Tanzania, Kenya and Zambia. Suspensions are prepared from water dispersible powders which contain insecticide and inert diluent particles and surfactants and give dry deposits since the water evaporates very rapidly. Biological potency depends upon the particle size distribution of the active ingredient, its physical state and the surface which has been treated (44). Finer particles are picked up more readily by resting insects, while the adhesive properties of surfactant residues reduce pick up on the less porous surfaces. Those powders which meet WHO specifications have about the optimum particle size distribution.
Since the insecticide is in solution in emulsion concentrates, or in the emulsions prepared from these, the physical forms of the deposit after solvent and water have evaporated will be controlled by the crystallization properties of the insecticide and, again, the sprayed surface since this crystallization occurs more readily on some surfaces than others (5).
The persistence of the deposits resulting from the use of powders or emulsions will be controlled by the physical properties of the active material (e.g. dieldrin is more volatile than DDT), by climatic variables and by the surface sprayed. Thus, water-dispersible powder deposits have been found more effective in hot drier areas than those from emulsions, but the reverse is true in cooler areas with higher rainfall (63, 76). The powder residues are more easily washed away by rain than those from emulsions. Another important factor is the intrinsic toxicity of a compound to the flies. For example, dieldrin is more potent than DDT and deposits can be reduced by a larger amount but still be more effective.
All the candidates for replacement insecticides in tsetse control are available, or could be made available, as water-dispersible powders and emulsion concentrates. A few, such as pirimetaphos, azamethiphos, tetrachlorvinphos and bendiocarb, have a low solubility in the common solvents used to prepare emulsion concentrates, but this could be improved with known co-solvents. Conversely, their low solubilities mean that they usually make good water-dispersible powders.
This usually requires that the formulation should be a solution of high concentration, say 10% upwards, since it is important that a lethal dose is present in one or very few droplets. Although this criterion applies to most insecticides it is possible that with exceptionally toxic compounds, such as NRDC 161, the concentration can be reduced to something of the order of 1%. For these solutions the vital component is the solvent or, more often, solvents. Their properties control the initial atomization, the reduction in droplet size during passage to the target and the efficiency with which the insecticide passes into the insect, either from droplets on the cuticle or from droplets on surfaces on which the insect rests. They can also influence the persistence of insecticides in spray deposits when they are of low volatility themselves.
A discussion of all these interactions between insecticide, solvent, insect and surface would be too long for this report. Briefly, however, the best solvent system probably consists of a non-volatile component such as a vegetable oil, plasticizing ester or Dutrex-type petroleum extract combined with a volatile solvent which may be lost during application but ensures that the physical properties of the blend are appropriate for the atomization process. The non-volatile solvent is a less efficient carrier for insecticide
