162 PRINCIPLES OF PLANT CULTURE 



covered with burlap 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 

 states now require that nursery stock be treated with 

 hydrocyanic gas before shipment, to prevent the dis- 

 semination of dangerous scale insects. Hydrocyanic acid 

 gas is a deadly poison, only small amounts being necessary 

 to cause death, therefore its use is only recommended 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 (Dactylopius) . 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 (aphids). 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 of in- 

 sects. 



300. Numbers of insects. — Insect attacks sometimes 

 become formidable from the vast number of the individ- 

 uals. The chinch-bug (Blissus leucopierus) , the army- 

 worm (Leucania unipunda) and various species of locusts 

 or grasshoppers sometimes devastate large tracts of coun- 

 try. For the destruction of these insects, special means 

 must be employed. 



301. The chinch-bug may be controlled, in a meas- 

 ure, by burning over all grass land early in spring, in 



