286 SCIENCE IN AFRICA 



the vectors of a virus disease. The breeding of resistant strains, 

 however, notably that known as U4, has reduced the importance 

 of these pests. 



Among other important pests, the cotton leaf-roller [Sylepta 

 derogata F.), for which trap-crops o^ Hibiscus may be useful, is very 

 widely distributed. Lygus simonyi Popp. causes considerable dam- 

 age in Uganda, and in the Sudan several species of Thysanoptera, 

 especially Hercothrips spp., retard the growth of the plants. 



Root crops 



Considerable attention is being given to the pests of root crops 

 for native food as well as to those of export crops. The work done 

 in Nigeria, described by O. B. Lean (1928), when the damage to 

 yams by a Dynastid beetle, Heterolygus claudius Klug, was so severe 

 during 1925-7 as to cause a famine, may be cited as an example of 

 such research. More recently a mosaic disease of cassava has been 

 the cause of serious loss to native food crops both in West, East, and 

 South Africa. The disease did not make its appearance in West 

 Africa until 1929 and is thought by some to have been introduced. 

 The insect vector has been proved in Nigeria, at Amani, and else- 

 where to be a species of whitefly (Bemisia). The most hopeful line 

 of control is the breeding of resistant varieties of cassava {see Chap- 

 ter XII) . The cultivation of native food crops usually differs from 

 that of export crops in that a mixture of crops is planted on one 

 plot. Mixed cropping may be a defence against serious outbreaks 

 of pests, so that many remain unnoticed or unimportant until the 

 system of agriculture is changed. 



Coffee 



Among the chief pests of coffee in East Africa are Pentatomid 

 bugs of the genus Antestia, especially lineaticollis Stol. Biological 

 control, even in Kenya, where the eggs of this bug appear to be 

 most heavily parasitized, is not sufficient to keep infestation within 

 bounds. A considerable amount of work on chemical control has 

 been done in that colony, where pyrethrum-kerosene sprays seem 

 to have been very effective (Le Pelley 1932). Bait-sprays con- 

 taining sodium arsenite and sugar are sometimes preferred, but it 

 is thought by some that their use is likely to lead to heavier infes- 



