170 Principles of Plant Culture. 



jar containing three fluid ounces of water. The jar is 

 covered with burlaps to prevent the rapid escape of the 

 gas. The tent is left over the tree fifteen minutes to one 

 hour. It is advisable to apply this treatment during 

 the dormant season and in a cool season. Laws in some 

 localities now requires nursery stock to be treated with 

 hydrocyanic gas before shipment, to prevent the 

 dissemination of dangerous scale insects. Hydrocyanic 

 acid gas is a deadly poison, only small amounts being 

 necessary to cause death, therefore its use is only rec- 

 ommended in the hands of experienced persons. 



298. Fir- Tree Oil is considerably used in greenhouses 

 and conservatories for destroying scale insects and the 

 mealy bug.* It is mixed with warm soft water at the 

 rate of a tablespoonful of oil to a pint, and applied 

 with a syringe; or the plants are dipped into the 

 mixture. 



299. Hot Water may also be used for destroying the 

 above named insects (298) and plant lice (aphidas). In- 

 fested pot-plants are inverted and immersed five or six 

 seconds in a vessel containing water at 120° F. This 

 treatment must be used with caution. 



Forcible syringing of plants with water is also an 

 excellent method of ridding greenhouse plants from 

 insects. 



300. Insect Attacks Sometimes Become Formidable 

 from the vast number of the individuals. The chinch- 

 bug,! the army-wormj and various species of locusts or 

 grasshoppers sometimes devastate large tracts of coun- 



* Dactylopiua. + Blissus levcopterus. t Leucania unipuncta. 



