The eradication of tsetse in the Chad river system of Northern Nigeria.
Rivers in Northern Nigeria flowing north-eastwards into Lake Chad traverse, as they pass through the Sudan Savannah zone, wide seasonal flood-plains that are infested by Glossina morsitans submorsitans Newst. and G. tachinoides Westw. An account is given of the course and results of campaigns to eradicate both species from two such flood-plains by the discriminative and selective application of insecticide deposits to the vegetation on which the flies rest. One flood-plain was treated progressively from 1955 to 1961 [cf. R.A.E., B 48 180, etc.] and the other from 1961 to 1964, treatment starting upstream in both cases. The insecticide used was DDT in a wettable-powder formulation, and it was applied from the ground. The strength of the spray was gradually reduced over the years from 5 to 2.5 per cent. [48 181; 51 224], except that at the end of each season 5 per cent. was used over the last mile along the whole front. The deposits remained fully effective for 13 weeks and effective for even longer [cf. 52 5], more than long enough to cover the pupal period of 3-7 weeks, and one treatment in the late dry season was found to be adequate. Treatment became progressively more discriminative (i.e., confined to certain patches or groups of vegetation) each year until, finally, only the fringing riverine vegetation, the forest islands, some of the larger thickets and some of the denser vegetation on the boundary between the plains and the upland were sprayed. It also became progressively more selective (i.e., confined to certain parts of trees and shrubs), until finally it was restricted to larger tree trunks in heavy shade and the shaded woody stems of trees, shrubs and climbers in the denser vegetation near water [cf. 48 181]; these are the sites in which the flies of both species are compelled to seek protection from the effects of the severe desiccating climate of the late dry season, the coolest conditions being found on the woody parts of the vegetation, especially the tree trunks, within a few feet of the ground [cf. 24 245], In these sites, spraying was confined to a height of 2 ft. from the ground, except that it was extended up to 5 ft. in areas heavily infested with G. morsitans [51 224]. By the treatment of 1, 212 sq. miles of country in the two flood-plains, an area of 7, 800 sq. miles was practically freed of fly and thus made safe for cattle. As few or no flies re-entered the reclaimed tracts between spraying seasons, it was not necessary to make costly cleared barriers to protect each year's work. The cost of the insecticides and labour fell from £100 per sq. mile sprayed in 1955-56 to £30 in 1963-64 [cf. 48 181], and the total cost over the whole period (1955-64), exclusive of costs of senior staff and depreciation of equipment, amounted to £14 per sq. mile (5.25 pence per acre), when related to the total area freed for grazing. The area is, however, threatened with re-infestation from the Kaduna-Zaria belt of G. morsitans to the south-west, which is advancing at the rate of about ten miles a year.