Cyanide Gas Remedy for Scale Insects. 437 



CYANIDE GAS REMEDY FOR SCALE INSECTS. 



By C. P. LouxsBURY. Chief, Division of Entomology. 



(hifluic of Treatment . — Fumigation with liydrocyanie acid ga.s 

 is by far the most eflfieient and, in the end, the cheapest remedy for 

 the suppression of Red Scale and other " hard " scale insects in 

 commercial plantings of citrus trees. The operation of fumigating 

 trees is very simple. The tree to be treated is covered with an approxi- 

 mately air-tight cloth, and gas then generated within the enclosed 

 space by the action of dilute sulphuric acid on potassium cyanide or 

 sodium cyanide in an acid-proof vessel placed near the foot of the 

 tree. The gas diffuses throughout the space under cover, and thus the 

 whole surface of the tree becomes enveloped in one of the most deadly 

 of poisonous substances. An exposure of forty-five minutes is con- 

 sidered ample for the destruction of the common scale insects, and 

 by the expiration of this period most of the gas has disappeared by 

 leakage through the cover and in other ways. The cover is then 

 removed, and may be at once employed for enclosing* another 

 tree. It takes only a few minutes to cover a tree and start the gas. 

 Hence as many trees can be done at a time as there are covers 

 available and as can be covered and uncovered in the exposure period. 

 Experienced fumigators in California working under favourable 

 conditions on trees about 12 to 14 feet high can operate thirty to 

 forty covers and do three hundred to four hundred trees in one night: 

 but in South Africa it is highly excei)tional to have over twenty 

 covers to work, and many private " outfits " consist of less than 

 half a dozen and some consist of single covers. 



Cyanide Details. — The cyanide used is the same chemical that 

 is employed in enormous quantities for gold extraction. Either 

 potassium cyanide or sodium cyanide may be used for fumigation, 

 but because it is now cheaper and more easily procurable the latter is 

 generally employed. Pure sodium cyonide contains about one-third 

 more cyanogen than pure potassium cyanide; and it is a somewhat 

 unfortunate common trade custom to quote the grade of a sodium 

 cyanide on the basis of the cyanogen in it compared with pure 

 potassium cyanide. While high-grade potassium cyanide is termed 

 96-99 per cent., the similar grade of sodium cyanide is often termed 

 126-130 per cent. One has to be careful to avoid accepting an impure 

 mixed cyanide, ('ontaining the amount of cyanogen there should be 

 in commercially pure potassium cyanide, under an impression that he 

 is getting cyanide that is 96-99 per cent, pure; much such impure 

 mixed cyanide has been sold as potassium cyanide. The Californian 

 fumigation of citrus trees (by the method used in South Africa) is 

 done chiefly with " Cyanegg." This is a sodium cyanide said to be 

 of 96-98 per cent, purity and therefore equivalent in purity to what 

 in the South African trade is called 126-130 per cent, cyanide. 

 Cyanegg is manufactured by the Roessler and Hasslarher Chemical 



