INSECT CONTROL 327 



(h) Trap Crops. It sometimes happens that one plant 

 or variety is preferred by an insect and can be used 

 for attracting it from the plants to be protected. For in- 

 stance, the cotton bollworm prefers to lay its eggs on corn. 

 If a few rows of corn be planted here and there through the 

 cotton field so as to come into silk about the time moths 

 which normally lay on cotton are flying, the eggs will be laid 

 on the corn, which can then be cut and fed to stock, and the 

 cotton will be protected. In a similar manner radishes are 

 sometimes used as a catch crop for the maggots affecting 

 cabbage and onions, while kale makes an excellent catch 

 crop for the harlequin cabbage bug. 



164. Insecticides. Substances which destroy insects are 

 commonly called insecticides and may be divided into four 

 classes: 



1. Poisons kill by being eaten and are usually composed 

 of some form of arsenic and are, therefore, called arsenicals. 



2. Contact Insecticides kill either by clogging up the spir- 

 acles, the openings of the respiratory system, or by entering 

 the trachea, and thus causing suffocation, or by their cor- 

 rosive action on the skin. 



3. Gases are used for fumigating buildings, stored 

 products, greenhouses and similar structures infested with 

 insects where other means are not practicable. 



4. A fourth class of substances used against insects may 

 be known as repellents. They are not real insecticides, for 

 they do not kill the insects but merely prevent them from 

 attacking the plant or animal to which they are applied. 



1. Poisons. Poisons are applied to the food of the insect 

 and must be eaten to be effective. It is evident, therefore, 

 that they are effective only against biting insects, or those 

 which lap up their food from the surface, and that they can 

 be of no use against the sucking insects, such as the true 



