488 FRUIT INSECTS 



fairly effective when diluted with not more than 15 parts of 

 water. To lessen danger of injury to the trees applications 

 should not be made when the temperature is below freezing, 

 nor when the trees are wet with snow or rain. Trees are less 

 susceptible to injury just before the buds start in the spring. 

 Methods have been devised for preparing these concentrated 

 emulsions at home, but as there is considerable danger attend- 

 ing the process, it is better to buy them ready-made. 



Tobacco 



Tobacco is one of our most useful insecticides. The poison- 

 ous principle in tobacco is an alkaloid, nicotine, which in the 

 pure state is a colorless oily fluid, slightly heavier than water, 

 of little smell when cold and with an exceedingly acrid burn- 

 ing taste even when largely diluted. It is soluble in water and 

 entirely volatile. It is one of the most virulent poisons known ; 

 a single drop is sufficient to kill a dog. 



Commercial tobacco preparations have been on the market 

 for many years. The most important of these are Black Leaf, 

 *' Black Leaf 40" and Nicofume. 



Black Leaf was formerly the most widely used tobacco ex- 

 tract. It contains only 2.7 per cent nicotine and has now been 

 replaced by the more concentrated extracts. It is used for 

 plant-Uce at the rate of 1 gallon to 65 gallons of water. 



"Black Leaf 40" is a concentrated tobacco extract containing 

 40 per cent nicotine sulfate. Its specific gravity is about 1.25. 

 In this preparation the nicotine is in a non-volatile form, it 

 having been treated with sulfuric acid to form the sulfate. 

 "Black Leaf 40" is used at strengths varying from 1 part in 

 800 parts of water to 1 part in 1600 parts. It can be satisfac- 

 torily combined with other sprays, as, for instance, lime- 

 sulfur solution, arsenate of lead and the various soap solutions. 

 When used with water about 4 pounds of soap should be added 



