PLANT LICE INJURING THE FOLIAGE 



197 



grown and thus have acquired tougher leaves, the hoe develop a winged 

 generation, leave the section where at work, and appear as suddenly in 

 other regions farther north, where conditions are favorable to further 

 multiplication. 



Late in summer, as the grain fields mature, the lice leave them and 

 take up their residence on grasses, such as common bluegrass or on corn. 

 In the fall, when winter grain has sprouted, many move back to the new 

 feeding ground. With the coming of cold weather, eggs are laid 

 among grain plants. In the more Southern states there are living 



Fig. 245. — The 

 Winged adult, 

 size. Original. 



Spring Grain-aphis. 

 Enlarged and natural 



Fig. 246.— The Spring 

 Grain-aphis. Wing- 

 less form. Enlarged 

 and natural size. Orig- 

 inal. 



females in existence all winter. The insect is able to breed at unusu- 

 ally low temperatures. 



Oats and wheat are the favorite food plants, but the list includes rye, 

 barley, corn, and several species of wild grasses, especially orchard grass. 



Normally this insect is held in check by its natural enemies, especially 

 by a four-winged parasite which attacks the lice, laying its eggs within 

 their bodies. Severe attacks follow warm winters combined with cold 

 springs — a condition unfavorable to the parasite, but favorable to the 

 aphis. 



Direct control is not feasible. Cleaning up volunteer oats and other 

 food plants in the fall will greatly reduce the numbers the following 

 spring. Attempts have been made artificially to introduce the parasite, 

 but without entire success. 



