Earlier papers in this series have described attempts to control tsetse flies by applying insecticides from aircraft. It was concluded that coarse aerosols, with mass median diameters of approximately 100 microns, were more effective than coarse sprays with mass median diameters of 350-700 microns, but that it would not be economically possible to control Glossina palpalis (R.-D.), the species associated with dense riverine vegetation, by applications from aircraft. Very promising results were obtained (Hocking & others, 1953) when coarse aerosols were applied to savannah woodland containing G. morsitans Westw. and G. swynnertoni Aust. In one experiment, where eight applications, each at a nominal dosage of 0-25 lb. of technical DDT (80 per cent. p,p'isomer) per acre, were made at intervals of two weeks, it is possible that G. morsitans, and even G. swynnertoni, would have been eradicated if the treated woodland had been isolated effectively enough to prevent immigration of flies. In another area, where a similar series of applications was made with BHC, the nominal dosage for each application being 0-25 lb. of crude BHC (12 per cent, y isomer) per acre, the reductions of fly populations were less spectacular, but these less favourable results were not attributed to a lesser effectiveness of the BHC, but to a combination of circumstances that led to a less effective series of applications.Several factors were considered to have reduced the effectiveness of the applications in the two experiments, quite apart from immigration which removed any possibility of eradication in the area treated with DDT. The aerosols were produced by allowing the insecticidal solutions to flow, under gravity, into modified exhaust systems of the engines of the aircraft; approximately 20 per cent, of the insecticide was thermally decomposed, and the emission rate fell considerably as the storage tanks emptied, so that wide variations in nominal dosages occurred during each sortie. It is also likely that much of the insecticide was contained in droplets that were too large to penetrate effectively into the resting places of the tsetse flies. Furthermore, the aerosols were not applied during the most suitable meteorological conditions of high temperature inversion and low wind speed (Yeo & Thompson, 1953, 1954, with the result that concentrations of aerosol near the ground were reduced. In the area treated with BHC an incomplete knowledge of the distribution of flies within the treated woodland, and a narrower overlap between successive sorties upon adjacent areas, also reduced the effectiveness of the applications.It was felt that the eradication of tsetse flies from a savannah habitat was possible, and could be achieved without any increase in the nominal dosage or the number of applications, if sufficient attention was paid to the factors mentioned above, and if there was no possibility of immigration of flies into the treated woodland. A new installation was therefore fitted to the aircraft, by which the aerosol was produced by emi...

In South Africa a considerable effort has been made to eradicate tsetse flies by applying insecticides from aircraft (see, for example, Fiedler, 1950). The experiments showed that coarse aerosols were more effective than sprays. A similar conclusion was reached from experiments carried out at Entebbe, Uganda ; sprays and aerosols were applied to densely forested islands in Lake Victoria, and significant kills of Glossina palpalis (R.-D.) were obtained only on the islands treated with aerosols. A further experiment (Hocking & others, 1953) showed that, in savannah woodland, a coarse spray of an oil solution containing DDT produced relatively low kills of G. swynnertoni Aust.It was consequently decided to investigate the effect of coarse aerosols upon populations of G. morsitans Westw. and G. swynnertoni, two very important species of tsetse flies in East Africa. Two blocks of typical savannah woodland, each of a few square miles, were treated, one with DDT and the other with BHC. The applications were made during the latter part of the long dry season, from the end of July to the beginning of November, 1949. During this period most of the trees were completely leafless, the ground was hard and dry and it was possible to move about in the area with ease.