272 SCIENCE IN AFRICA 



mation of even the limited fertile tracts. The immediate problem 

 is therefore one of defence rather than of attack. It is not the 20,000 

 square miles of actual fly area with which the agricultural depart- 

 ment is concerned, so much as the 30,000 square miles of more 

 valuable country in the northern part of the territory, which the 

 pest has threatened to overrun. The fly front extends for about 

 600 miles from the Wankie district in the west to the Darwin 

 district in the east. Some 150 miles of this front are held to be pro- 

 tected by physical features, so that about 450 miles remain to be 

 defended. 



In view particularly of the ruin and dislodgment of European 

 settlers, prompt and effective action, of an extensive rather than 

 an intensive nature, had become imperative some years ago. The 

 only measure which seemed likely to achieve the object in view 

 within a sufficiently short period, and without involving excessive 

 expenditure, was to drive back the game by organized hunting, in 

 the hope that the fly would retire from country depleted of its 

 food. At the present day a cordon, in which the game is kept at a 

 minimum by controlled hunting, is maintained along practically 

 the whole of the 450 miles of open fly front. 



The experiment has been carried out in the face of criticism 

 both in Rhodesia and elsewhere, but the results, which are des- 

 cribed by Jack (1933, 34, 35a) and Chorley (1936) have been satis- 

 factory. Trypanosomiasis due to G. morsitans has now been practi- 

 cally eliminated from areas in European occupation; the advance of 

 the fly has been changed to retreat; over 2,000 square miles of 

 country have been freed from the pest, or at most are subject near 

 their limit to the intrusion of an occasional fly from the infested 

 country beyond; and cattle are now running freely in areas from 

 which they were eliminated by the encroaching fly comparatively 

 few years ago. 



A smaller problem of a different nature exists along a short 

 section of the Southern Rhodesian-Portuguese border in the Mel- 

 setter district. The Rhodesian side of the border has been subject 

 to incursions of fly, mostly G. pallidipes from Portuguese territory. 

 Some farms have been evacuated on account of the heavy mor- 

 tality of cattle. It was decided to try the effect of a forest clearing 

 along the border, an undertaking which was rendered feasible by 



