INSECT ENEMIES — PACKARD 337 



hessian fly (pi. 2) ; varieties of corn resistant to the European corn 

 borer, earworm, chinch bug (pi. 12), or corn leaf aphid; sorghums 

 resistant to the chinch bug; and barleys, oats, and wheats resistant to 

 the chinch bug or to the green bug, have all been discovered through 

 systematic tests conducted in recent years. Breeding operations have 

 also shown that by controlled crossing and selection many of these 

 insect-resistant qualities can be bred into other varieties more suitable 

 for commercial production, and that these qualities are not linked with 

 undesirable characters or incompatible with other desirable ones such 

 as high yield, quality, and disease resistance. Commercially satisfac- 

 tory varieties or lines of wheat, oats, barley, and corn resistant to one 

 or more of the insects just mentioned have already been produced or, 

 through the application of modern methods of plant breeding, are well 

 advanced toward commercial availability. 



Insect control through the use of resistant varieties would have 

 several advantages over other methods of control. Once the seed of 

 such varieties becomes available in quantity and comes into general 

 use the farmers will be automatically relieved of the expense and 

 trouble of applying other control methods. More latitude in cultural 

 procedures, time of planting, and crop rotations would be gained. 

 The frequent, severe, and general outbreaks of certain species that 

 confine their attack to only one or two kinds of grain crops might 

 even be prevented. The development of cereal crop varieties resistant 

 to insects has now progressed far enough to warrant the definite ex- 

 pectation of extremely valuable results in this field. 



BIOLOGICAL CONTROL 



Several writers have called attention to the incessant contest between 

 the insects and man for supremacy. The insects probably would 

 have won long ago if it were not for the natural agencies that hold 

 them in check and for the fact that most of the noxious species are 

 much less readily adaptable than man to variations in environment. 

 Many of those that attack cereal crops, such as the corn rootworm, 

 corn leaf aphid, and wheat jointworm, can breed successfully only 

 on one or a very few species of host plants. Others, such as the grass- 

 hoppers, chinch bug, hessian fly, and green bug, although extremely 

 prolific, are highly susceptible to unfavorable weather conditions at 

 certain times in their life history and often suffer violent reductions 

 in numbers as a result. Still others, armyworms and cutworms for 

 example, have many parasitic and predaceous insect enemies that at 

 times almost completely exterminate them in localities where they 

 occur in outbreak numbers. Some cereal insect pests, armyworms, 

 chinch bugs, and grasshoppers for instance, are also subject to bacterial 

 and fungous diseases that occasionally become epidemic under favor- 



