IQ2 PRINCIPLES OF FARM PRACTICE 



times the leaves of this plant, will not do much if any damage 

 to some varieties of the grape. A rapidly maturing variety 

 of wheat would be likely to sustain less damage from the 

 jointworm than a slowly maturing variety. 



Natural enemies of insects. Injurious insects, in spite 

 of man's effort to control them, would soon become so numer- 

 ous as to make agricultural production very unprofitable were 

 it not for their natural enemies. These enemies are bacterial 

 and fungal diseases; parasitic insects; birds; and mammals. 



Insect diseases. Insects like other forms of life are 

 subject to disease, and when they have become diseased 

 they are unable to cause further injury to growing crops. A 

 few examples will illustrate insect diseases. A germ (bac- 

 terial) disease of the cabbage worm is not uncommon. In 

 the course of the disease the worm ceases to eat and soon 

 dies. The dead remains become soft and dark in color, and 

 finally decay entirely. In late autumn, house flies may be 

 seen attached to walls or window panes by fine threads of a 

 mold-like substance. These flies have been attacked by a 

 fungus. A similar disease sometimes attacks chinch bugs, 

 destroying them in great numbers. 



Parasitic insects. An organism living on the body of 

 another is called a parasite. The organism invaded by the 

 parasite is known as the host. Nearly all insects are subject 

 to injury by parasitic insects. The parasite usually lays its 

 eggs on the body of the host. The eggs hatch into little 

 grubs which enter the body of the attacked insect. Here 

 they live until they find their way to the outside and form 

 pupae. For example, tomato worms are often seen covered 

 with the white oblong cocoons of parasites that have been 

 living inside the worms. At this stage the worm is either 

 dead or about ready to die. 



