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SCALE INSECTS : REMEDIAL MEASURES AND 

 INSECTICIDES. — II. 



By E. E. Green, F.E.S. * 



general list of substances and processes employed im the treat- 

 ment of scale insects : 



The Gas Treatment. 



Hydrocyanic acid gas is the material employed in this process. It is 

 generated by the admixture of cyanide of potassium, sulphuric acid and 

 water. For the details of treatment I cannot do better than quote in 

 extenso from the admirable paper on ' Gas Treatment for Scale Insects,' 

 complied by Mr. C. P. Lounsbury from his personal experience as 

 Government Entomologist at the Cape of Good Hope. The process 

 described was principally employed against Aspidiotus aurantii, an insect 

 that appears to be distinctly on the increase in Ceylon. I may add 

 that I have followed Mr. Lounsbury's directions in my own experi- 

 ments with most satisfactory results. 



'Generation of the Gas. — Hydrocyanic acid gas is generated by the 

 action of sulphuric aoid on potassium cyanide in the presence of water. 

 The required quantities of the cyanide and water are first placed in the 

 generating vessel, the cyanide being broken into small pieces not above 

 the size of lump sugar. The tree is then covered with the tent or sheet 

 and the vessel slipped under almost to the base of the tree ; reaching in, 

 the operator then adds the acid, pouring it slowly into the vessel so as 

 to avoid its splashing and thus burning his hand or the cloth. He imme- 

 diately withdraws and the men shovel a little soil on the edges of the 

 cloth all around, to more thoroughly prevent the escape of the gas. 



4 The rapidity of the evolution of the gas depends largely upon 

 the size of the pieces of cyanide. If these are like powder, the 

 reaction is violent and immediate ; but if in lumps the reaction takes 

 place more slowly and continues for a minute or longer. The slow reac- 

 tion is desired partly because less injury results to the foliage immedi- 

 ately above the vessel. But the lumps must not be too large, for then 

 the reaction is liable to be imperfect owin^ to a black coating (carbon ?) 

 forming over the lumps and preventing further decomposition by the 

 acid. The water should not be added too soon or part of the cyanide 

 becomes dissolved and gives a violent reaction. The residue which re- 

 mains in the dishes is buried; and the dishes are washed in clean water 

 before being again used. 



' Time necessary for treatment. — The cover is left over the tree for 

 thirty minutes in the case of small trees, and forty-five in the case of 

 those over twelve feet in height. At the expiration of this period the 

 generating vessel is removed, and the residue buried in the soil. 



i A number of trees are fumigated together, the endeavour being 

 to treat as many at time as can be covered and uncovered during 



* From « The Coccidae of Ceylon," Pt. II., Dulau & Co., 1899. 



