386 



INSECTS AFFECTING VEGETATION 



recent use for this gas is in disinfecting houses of insect pests and 

 vermin. In all work with hydrocyanic-acid gas, its extremely poi- 

 sonous nature must be constantly kept in mind and the greatest pre- 

 cautions must be taken to avoid inhaling it. 



Fumigation of Nursery Stock. For the fumigation of nursery 

 stock or imported plant material in a dormant or semidormant 

 condition, a building or room should be provided, which can be 

 closed practically air-tight, and it should be fitted with means of ven- 

 tilation above and at the side, operated from without, so that the 

 poisonous gas can be allowed to escape without the necessity of any- 

 one entering the chamber. The gas is generated by combining po- 

 tassium cyanide, sulphuric acid, and water. The proportions of the 

 chemicals are as follows: Refined potassium cyanide (98 per cent), 

 1 ounce ; commercial sulphuric acid, 1 ounce ; water, 3 fluid ounces to 

 every hundred cubic feet of space in the fumigating room. For 

 comparatively green or tender material the same amounts may be 

 used to 150 cubic feet of space; 



The generator of the gas may be any glazed earthenware vessel 

 of 1 or 2 gallons capacity and should be placed on the floor of the 

 fumigating room, and the water and acid necessary to generate the 

 gas added to it in the order named. The cyanide should be added 

 last, preferably in lumps the size of a walnut, and the premises 

 promptly vacated and the door made fast. Treatment should con- 

 tinue forty minutes. 



ORCHARD FUMIGATION. 



Amounts of Chemicals to Use. The amounts of chemicals used 

 vary with the size of the tree and, as now employed in California, 

 are considerably in excess of the amounts recommended as recently 

 as 1898. The gas treatment was first chiefly used against the black 

 scale and at a season of the year when these scales were all in a 

 young stage and easily killed. The effort is now made not only 

 to kill the black scale, but also the red and purple scales, and to do 

 more effective work than formerly against the black scale. The 

 amounts of chemicals ordinarily advised and commonly employed 

 in Los Angeles, Orange, and some other counties in southern Cali- 

 fornia are indicated in the subjoined table, published by the horti- 

 cultural commissioners of Riverside County, Cal. 



Amounts of Chemicals and Water Ordinarily Used for Trees of 



Different Sizes. 



