Plants as Affected by Animal Parasites. 167 



powder may be used with water at the rate of half a 

 pound to three gallons. 



The pyrethrum plant is comparatively hardy and has 

 been successfully grown in northern United States. It 

 is said that a decoction of the unopened flowers possesses 

 the insecticide properties of the commercial product. 



291. Hellebore and Pyrethrum Powders should be 

 Kept in Close Vessels, since their poisonous properties 

 are volatile. In purchasing, only fresh samples should 

 be accepted. If fresh and pure, these powders produce 

 a tingling sensation when applied to the nostrils. 



292. Tobacco Smoke is much used for destroying 

 "lice" or "green fly" (aphids) on plants under glass. 

 For this purpose, the partially-dry stems or leaves are 

 burned upon pans or bricks, or in small sheet-iron 

 stoves. Many delicate flowers are, however, injured by 

 tobacco smoke. 



Stems or leaves of tobacco, strewn abundantly be- 

 neath gi^eenhouse benches, tend to prevent the multipli- 

 cation of aphidse. 



Several semifluid extracts of tobacco are sold which 

 may be evaporated in the greenhouse oyer an oil stove, 

 or preferably by steam under pressure. Some of these 

 are very efficient for destroying insects and do not injure 

 flowers. 



293. A Strong Decoction of Tobacco is often used 

 for destroying aphidse on plants in rooms where tobacco 

 smoke would be objectionable. The plants are im- 

 mersed in, or washed with the decoction. It is 

 often effectually used on young plants of cabbage, cau- 



