ENTOMOLOGY 283 



also worked on the control of the aphids by predatory insects, 

 especially the Coccinellid beetle, Cydonia vicina Muls. (Vuillet 

 1934). There appears to be some doubt as to the possibility of 

 breeding resistant strains: some investigators have found some 

 strains more resistant than others, but resistant strains were not 

 observed in French West Africa during 1933, as noted by Bouffil 

 (1933). Strains resistant in one environment are not necessarily 

 resistant in others and thus the prospect of reducing damage by 

 the disease in this way is limited. Work on such lines must be done 

 in the territories concerned and probably in various regions. In 

 addition to Aphis laburni, it is suspected that the whitefly, Bemisia 

 gossypiperda var. mosaicivectura Ghesq., may also transmit rosette 

 disease. The virus apparently overwinters in diseased plants that 

 germinate in late autumn (Storey and Bottomley 1928), and one 

 of the control measures suggested by these researches is to destroy 

 all plants between growing seasons. In Nigeria, Uganda, and the 

 Gambia the close spacing of plants and controlled maintenance of 

 weed growth between the plants, so as to produce a complete 

 ground cover, have been proved markedly to reduce the incidence 

 of rosette disease. Possibly, though the fact is not yet established, 

 there is control of the insect vector in the damp atmosphere created 

 by close planting, by means of some entomogenous fungus. 



Cotton 



Bollworms are almost universally distributed throughout the 

 cotton-growing area of Africa, and in spite of considerable research 

 few sure means of control have been found. Attempts at biological 

 control have at best attained variable success. For instance, a 

 large percentage, generally higher on maize than on cotton, of 

 the American boll worm {Heliothis obsoleta F.) is parasitized by 

 Trichogramma luteum Gir. in Southern Rhodesia, but attempts to 

 establish this parasite in the Transvaal have failed. Similarly, an 

 effort to increase the effectiveness o^ Microbracon kirkpatricki Wilksn., 

 a parasite of the pink bollworm [Platyedra gossypiella Saund.) gave 

 doubtful results. In Uganda intensive work on possible parasites 

 and alternative hosts of this pest is in progress. Work with bacteria 

 against the pink bollworm (Metalnikov 1933) remains in the 

 experim.ental stage. Chemical measures are for the most part 



