86 CONTROLLING INSECTS AND DISEASES 



forming a deadly gas that is heavier than air and permeates 

 throughout the mass of seeds. The receptacle containing the seed 

 should be left closed for several hours after the treatment is started 

 in order to insure thorough action upon the insects within the 

 seeds. The gas given off from the carbon bisulfide is very in- 

 flanmiable, and hence no fire of any kind should be in the vicinity 

 while the treatment of the seed is in progress. The carbon bisul- 

 fide is used at the rate of one pint per 1000 cubic feet of space. 

 For most effective results the treatment of beans and peas should 

 take place as soon as possible after harvesting. 



In greenhouses, fumigation is the most common means of con- 

 trolling insects. Some form of tobacco or tobacco product is 

 ordinarily used for the control of plant lice and similar insects. 

 Formerly tobacco stems were burned to make a smudge in the 

 house, but a more modern method is to apply a concentrated 

 hquid tobacco product to the heating pipes, the heat from which 

 causes the poisonous gases to be given off. Some greenhouse 

 pests are harder to kill than plant lice, and necessitate the use of 

 a more deadly material than tobacco compounds. For the white 

 fly, hydrocyanic acid gas is sometimes used. This is generated 

 by bringing potassium cyanide into contact with a mixture of 

 sulfuric acid and water. It is a deadly poison, and extreme care 

 must be exercised to avoid breathing any of the fumes. 



Fumigation is also effective in controlling certain fungous 

 diseases in greenhouses. The material most commonly used for 

 controlling fungous diseases in greenhouses is sulfur. When the 

 houses are empty in summer, they may be given a thorough 

 fumigation by burning the sulfur in quite large quantities, as a 

 precautionary measure. When plants growing in the houses be- 

 come affected with mildew, a milder treatment must be employed, 

 since the actual burning of sulfur would injure the plants. How- 

 ever, the slow evaporation of the sulfur by means of moderate 

 heat will check the mildew without injuring the plants. A common 

 practice is to sprinkle powdered sulfur on the heating pipes, or 

 to paint the pipes with a paste made of sulfur, lime and water. 



SPRAYING 



While the various methods of controlling insects and fungous 

 diseases mentioned above are applicable to particular cases, they 

 do not constitute the chief means of warfare against crop pests 

 under outdoor conditions of culture. Spraying is the great modern 



