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IntroductionSynergised pyrethrins topically applied are highly toxic to Glossina morsitans Westw. (Burnett, 1961) but have generally been considered too costly for large-scale aerial control measures. This paper describes an experiment run in 1967 to evaluate the economics and the eflFect of the aerial application of pyrethrum on a high density of tsetse fly, the objective being complete eradication.

An economical method of tsetse control aimed at reducing fly numbers to a low level prior to human settlement was tried in northern Tanganyika in 1964–65 in an area containing Glossina morsitans Westw., G. swynnertoni Aust. and small numbers of G. pallidipes Aust. The major central part of the infested area, about 7½ square miles, was sprayed four times at three-weekly intervals with endosulfan from an aircraft, and the remaining peripheral parts were sprayed from the ground. In the aerial applications, the volume of spray averaged 0·0121 gal./acre per application and the amount of endosulfan averaged 0·0242 Ib./acre (a total of about 1 Ib. per 10 acres for the whole operation). The results indicated that the tsetse population was probably eradicated in the part sprayed from the air. Though the ground spraying of the surrounding parts was not completely satisfactory, the tsetse population in the whole area a year later was only about ten per cent, of its original level. The cost of the air-spraying was £88 per sq. mile.

Two experiments were conducted simultaneously in 1962–63 at Chungai, in central Tanganyika, to test new equipment for disseminating insecticides from aircraft and a new insecticide, isobenzan (Telodrin), in the eradication of Glossina morsitans Westw. and G. pallidipes Aust. The new equipment consisted of an aerosol generator fitted to the exhaust of a Cessna 182E high-wing monoplane, and was used in both experiments. The isobenzan was compared with dieldrin, and the two insecticides were applied at rates inversely proportional to their toxicities to G. morsitans as previously determined in the laboratory.Two blocks of woodland, each 11 sq. miles in area, were treated, one with a 12·3 per cent, solution of dieldrin at the rate of 0·0254 gal. per acre, giving a dosage of 0·5 oz. (14 g.) toxicant per acre, and the other with a 10 per cent, solution of isobenzan at the rate of 0·0124 gal. per acre, giving a dosage of 0·2 oz. (6 g.) per acre. The former block received eight treatments with dieldrin at approximately 3-week intervals, the fourth treatment being incomplete; the latter block received six treatments with isobenzan at intervals ranging from 20 to 45 days. The effects were assessed by means of fly-catches along fixed paths which continued for one year after treatments had ceased.Both species of tsetse fly disappeared from the two blocks before the final treatments took place, and no more were caught until 11 months after spraying ended, when one example of G. morsitans was caught in each block; both were probably immigrants. None was found in the following month, and it is concluded that the flies were exterminated in both blocks. The fact that the blocks were unusually well isolated from sources of reinfestation probably contributed to the success of the operations.The cost per sq. mile was £224 using dieldrin and £190 using isobenzan. These were the basic costs, independent of the locality in which spraying took place. Additional costs were incurred which would vary with local conditions; for the present experiments they were £34 and £30, respectively. It is considered that there is good scope for further reductions in costs, particularly with dieldrin, and that these might make dieldrin economically competitive with isobenzan. Owing to the low fly density in the block treated with isobenzan, the efficacy of this insecticide cannot be regarded as conclusively proved until further experiments have been carried out. It may then merit serious consideration as a toxicant for aerial spraying against tsetse flies.

Earlier investigations on the resting sites of Glossina swynnertoni Aust. in Tanganyika showed that the most important of them was the underside of branches of trees; moreover, the favoured sites were on branches 4-9 ft. above ground, l^i in. in diameter, and at an angle of less than 35° from the horizontal (Chadwick, 1964). After reasonable assumptions about the behaviour of the flies had been made, it was considered that treatment of these sites alone with an insecticide persisting for at least 70 days should bring about the extermination of an isolated population of G. swynnertoni. The present paper describes the results of an experiment in which this hypothesis was tested directly in the field. Part of the chosen area was infested with G. pallidipes Aust., and a subsidiary experiment aimed at the eradication of the latter species was carried out simultaneously, using a method described by White (1963).