CHAPTER XII 



Insecticides and their Application 



It is well known that there are two principal ways in 

 which plant-feeding insects get their food. Some insects 

 bite or chew the parts of the plants upon which they feed, 

 while others are provided with a hollow beak that they 

 push into the tissues of the plant to suck the sap. All the 

 leaf-eating caterpillars are examples of the biting insects, 

 while the plant lice, squash bug, and many related species 

 are examples of the sucking insects. The picture below 

 shows the way in which the sucking mouth parts of the 

 plant louse are inserted among the cells of plant tissues. 



This essential difference in the feeding habits of insects 

 must be considered when we try to kill them by the appli- 

 cation of insecticides. In the case of a 

 biting insect, we may hope to poison it 

 by placing fine particles of arsenic in some 

 form upon the leaves of the food plant. 

 As the insect bites the leaves, it will be 

 likely to eat some of these particles and 

 be killed. But in the case of a sucking 

 insect, we must use some insecticide that 

 will kill by coming in contact with the 

 insect itself. Such insecticides commonly 

 act by reaching the interior of the insect 

 through the breathing pores, or trachece, 

 and so they are sometimes called tracheal poisons to dis- 

 tinguish them from the ordinary internal poisons. Kero- 



I5S 



Head of Aphis 



SHOWING Sucking 



tongite among 



Plant Cells 



Magnified 



