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Detailed data from a preselected release site in Tanzania were collected preparatory to testing the insect sterility concept against Glossina morsitans morsitans Westw. The site was at Mkwaja Ranch, a north-eastern coastal cattle-ranching enterprise, in which about 195 km 2 was encompassed by a 1-km-wide fly barrier constructed at an average cost of $37/ha. Weekly surveys of flyround transects spaced 1 km apart were conducted over a 14-month period. The estimated male density of G. m. morsitans in the release site was about 630/km 2 and for G. pallidipes Aust., 255/km 2 . The only other species of tsetse found was G. brevipalpis Newst., which was restricted to drainage areas during periods of low rainfall.

A field trial of the sterile insect technique was conducted in Tanzania using males of Glossina morsitans morsitans Westw. that were reared, irradiated and packaged at Tanga. The experimental site was a 195-km 2 area 110 km to the south. Following two applications of endosulfan as an aerosol at an interval of 28 days, irradiated puparia of G. m. morsitans were released twice weekly at stations distributed throughout the experimental site. Each of the 120 release stations was serviced every 14 days with puparia which produced sterile adults synchronously within 30 min after placement. Over the 15-month experimental period, an average of 81 % control of G. m. morsitans was obtained, whereas G. pallidipes Aust., against which no releases were carried out, recovered to prespray levels within five months. The released sterile insects were found to be highly competitive and to survive well in the field. Immigration of indigenous flies from outside the perimeter barrier surrounding the experimental plot provided a continuous influx of fertile flies that provided the nucleus of the 19% residual population. Under operational conditions, the prevention of migration should result in the elimination of^an indigenous G. m. morsitans population subjected to the combined stress of population reduction by two applications of endosulfan aerosols and sequential releases of sterile males.