ENTOMOLOGY 259 



the flies may make inroads on inhabited areas along one frontier, 

 while on another they are themselves beaten back by the applica- 

 tion of scientific knowledge. The war may occasionally take on 

 calamitous proportions with outbreaks of sleeping sickness, but in 

 general the fly is not responsible, as locusts are, for great loss of 

 ground in the progress of civilization, because land occupied by 

 fly has never been utilized by man. There are, however, excep- 

 tions to this generalization. In Southern, and perhaps also in Nor- 

 thern Rhodesia, the fly retreated from wide areas at the time of the 

 big rinderpest epizootic in 1902-3 and subsequently recovered a 

 part of its former ground. Sir David Bruce held that the fly's 

 retreat was a direct result of the destruction of game, but not all 

 authorities now accept his view. In another part of Africa the 

 work of the Tanganyika Tsetse Department has recovered some 

 1,200 square miles of territory from the fly without any serious 

 game destruction, by means which are outlined below. 



The field represented by the rest of entomology is so huge that 

 it is useless to single out instances where progress is held back by 

 insect pests and where it has taken leaps forward as a result of 

 pests successfully controlled. There is one aspect of the question 

 which deserves stressing, however, and that is biological control 

 by parasites or predacious insects. Many people think of insects 

 in general as harmful creatures, but examples are growing in num- 

 ber year by year where pests, whether animal or plant in origin, 

 have been stamped out or put under control by the use of beneficial 

 insects, usually in the form of parasites. Biological control of pests 

 has not found wide application yet in Africa, except in South Africa, 

 where the eucalyptus snout beetle and other pests have been con- 

 trolled (see later). Many experiments have been made in other 

 parts of the continent, notably in relation to the coffee mealy bug 

 in Kenya, but the application of this method is limited since it is 

 only likely to be successful with introduced pests. Though not 

 usually included under the designation of biological control, plants 

 can often be used with effect in the control of pests; thus dangerous 

 species of mosquitoes can often be controlled by planting trees to 

 shade their breeding places. 



Another entomological problem, rather different from those con- 

 nected with pests of plants or vectors of disease, is that of damage 



