228 HORTICULTURE FOR SCHOOLS 



insect pests. It is, however, not adapted to the control of 

 those insects which have the ability to move about. Fur- 

 thermore, because of the deadly nature of the gas, its use is 

 always attended with danger, and should not be undertaken 

 by a person who is not familiar with it. The gas is obtained 

 by treating sodium cyanide with sulfuric acid, when a chem- 

 ical reaction takes place in which hydrogen cyanide is liber- 

 ated into the air. 



339. Importance of a study of life history. A complete 

 account of the metamorphosis of any given insect, the condi- 

 tions under which the changes take place, times of year at 

 which they occur, food on which the insect lives during the 



FIG. 116. The codlin-moth, showing stages in life history, a, egg; b, larva; c, pupa; 

 d, adult. (Greatly magnified.) 



different stages and all other matters connected with these 

 changes is known as the life history of the insect. A knowl- 

 edge of this history is the first essential in devising methods 

 of control, for such a study reveals the insect's weakest point 

 and this suggests the method of attack most likely to result 

 in effectual control. 



An interesting case in this connection is that of the codlin- 

 moth (Fig. 116). The larva of this moth leaves the fruit in 

 the fall and spins a cocoon in which it lives during the winter. 

 These cocoons are commonly found underneath loose scales 

 or bark of the trees where the larvae have been feeding. In 

 the spring of the year, about the time that active growth 

 begins again, these larvse pupate in the same cocoons in 

 which they spent the winter. After two or three weeks as 



