INSECTS AND THEIR CONTROL 227 



This difference as regards method of securing food has a 

 very intimate relation to the whole question of insect control. 

 In the case of insects with biting mouths, it is possible to kill 

 them by placing poison on the surface of the plant ; for they 

 eat the leaves and tender parts of the plant in their entirety. 

 Many of the leaf -eating beetles and other insects of this type 

 are controlled entirely in this way. When insects subsist by 

 sucking, this poison method is, of course, of no avail. Since 

 the insect does not eat any of the surface tissue of the plant, 

 other means have to be resorted to. The so-called contact 

 sprays which kill because of their action on the body of the 

 insect are employed here. 



The active agent in most contact sprays is kerosene or 

 distillate. It is, of course, entirely out of the question to 

 apply these agents in undiluted form, as they would kill 

 any plant with which they come in contact, to say nothing 

 of the expense of such a procedure. It is also impossible, as 

 is well known, to mix them with water. However, by first 

 stirring them vigorously with soap, and then adding this 

 soap to water, there is formed what is known as an emulsion. 

 The oil exists in a very finely divided condition throughout 

 the liquid, very much in the same way that small particles 

 of butter-fat are present in newly-drawn milk. In this form 

 the oil is not injurious to the tree, and is sufficiently strong to 

 kill the insect. Great care has to be exercised in applying 

 these emulsions to plants with tender foliage. They are 

 usually applied in the dormant season and if it is necessary 

 to use them on evergreen trees they are diluted to such an 

 extent as to minimize the possibility of injury. 



In the case of citrus trees in southern and central Cali- 

 fornia, a very effective method, known as fumigation, has 

 been perfected. In this process trees are covered with canvas 

 tents, and a definite quantity of hydrocyanic acid gas is then 

 introduced into these tents. The gas is very deadly, and the 

 method is exceedingly effective for the control of scale and 



