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OBSERVATIONS ON COTTON THRIPS IN THE GEZIRA, BLUE NILE 



PROVINCE, SUDAN, IN 1918-19. 



. By G. H. CoRBETT, B.Sc. 



From conversations that the writer has had, it would appear that this thrips 

 {identified by Mr. R. S. Bagnall as Heliothrips indicus, Bagn.) has been known on 

 the Gezira cotton for about three years, and is popularly called " dry asal " in con- 

 tradistinction to the work of the " asal fly " {Aphis sorghi, Theo.) The natives have 

 been acquainted with this insect for some time. The reason suggested for the 

 enormous damage done by it is that the cotton is too scattered, and it is stated that 

 if cotton were cultivated in larger blocks, the thrips might be viewed with equanimity. 

 The cotton land at Barakat for this year (1919-20) has been, as far as possible, 

 prepared in larger areas, and it will be interesting to compare the result of this 

 arrangement with cultivation at Tayiba, where no alteration in grouping has taken 

 place. 



Towards the end of October 1918 the thrips attracted serious attention ; by the 

 middle of November the cotton looked " all withered up," but by the middle of 

 December it began to recover. It was not until the beginning of February that worlc 

 on this insect was commenced — most of it being done in the field whilst preparing a 

 laboratory to conduct experiments under more immediate control. At that time 

 the cotton had practically recovered, in spite of the fact that the thrips was present 

 in large numbers. 



In most cases the infested cotton was first noticed on the north side of a " hosa," 

 and this imphes that the thrips either came from weeds growing on fallow land on 

 that side, or was brought from a distance by the prevaihng wind, and finding cotton 

 and conditions favourable, propagated to such an extent that the immediate study 

 of the bionomics and control of this insect was considered necessary. 



The occurrence of this minute insect in enormous numbers caused, during the season 

 1918-19, a considerable reduction in the yield of cotton, and it must at present be 

 viewed as a great menace to cotton cultivation in the Sudan, though it may prove to 

 be quite local in its attack and occur only spasmodically. In any case, at a very 

 conservative estimate, an average loss of one and a half kantars of cotton per feddan 

 has occurred during the past season on well supervised and cultivated land. 



This is the most important pest with which the cultivators of cotton in the Gezira 

 have to contend. On account of its smaUness, the rapidity of its spread, and the 

 seeming suddenness of its attack in large numbers, the control of the insect is one 

 which rpjght present very great difficulties. If the thrips is found on cotton when 

 young, it is probably practicable as well as economical to spray ; but on the other 

 hand, if it comes suddenly when the cotton plants are -about five months old, the 

 writer considers the policy of spraying doubtful. 



The injury caused to the cotton plant is due to the larvae and adults feeding on 



both the upper and lower surfaces of the leaves. By piercing the epidermis of the 



leaf and removing the sap, they lower the vitahty of the plant. At the points of 



attack spots are formed which, often running together, produce distinct white streaks 



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