INSECTS ON GREENHOUSE PLANTS 221 



experimentation. Different strengths also have their effects in 

 destroying the protoplasmic content of the plant. Still another 

 problem which confronts the flower-grower is to determine the 

 proper time of day for liberating this gas, for it is known that 

 plants are more susceptible to injury when the vital processes 

 of the plant are most active. Experimentation proves that 

 early evening is the best time for this work. The gases are al- 

 lowed to remain in the houses overnight. 



260. The use of hydrocyanic acid gas as a fumigant. 

 Hydrocyanic acid gas is now quite generally used for the de- 

 struction of white flies, thrips and other insects difficult to ex- 

 terminate with tobacco fumes. Care must be taken to use 

 proper amounts of this gas, for if large amounts be liberated, 

 the plants may be seriously injured. The amount most gener- 

 ally used is one ounce of potassium cyanide for every thousand 

 cubic feet of greenhouse space. A special formula for houses 

 in which such tender plants as coleus, antirrhinum and adiantums 

 are grown, is one ounce of potassium cyanide to two thousand 

 cubic feet of greenhouse space. With this amount of cyanide, 

 one ounce by measure of sulfuric acid and three ounces by meas- 

 ure of water, should be used. This should be prepared as recom- 

 mended by Fernald. Light fumigations once in from four to 

 seven days are preferable to heavy fumigations. 



In a paper on fumigation 1 prepared by H. T. Fernald of 

 Massachusetts Agricultural College, Amherst, Massachusetts, 

 he says, " Hydrocyanic acid gas is perhaps the most poisonous 

 gas known at least, at all well known. It can be produced 

 from various chemicals, but until recently it has generally been 

 obtained by adding sulphuric acid to potassium cyanide. It has 

 been used experimentally, and by comparing results a general 

 rule for its production and application has been outlined and 

 published, which has come into quite general use for greenhouse 

 work. Strangely enough, however, just what amounts of the 

 various materials should be taken, and the best grade of these 

 for the purpose, have only recently been carefully investigated, 

 and the results of this work have not as yet become widely known. 

 Let us consider the materials and methods of cyanide fumigation, 

 then, in some detail. 



" There must be a standard established in the production of 

 the gas, if results are to be compared and conclusions drawn with 

 any safety. The man who fumigates at a supposed certain 



1 Abstract from a paper read by H. T. Fernald, during Farm- 

 ers' Week, February 16, 1912. 



