184 JOURNAL OF ECONOMIC ENTOMOLOGY [Vol. 9 



Wheat aphis injury is more commonly found in circular areas from 

 10 to 50 feet in diameter, distributed in an irregular manner about 

 the field. In May these spots are noticeable because of the stunted 

 and peculiar appearance of the small plants; in June many of the plants 

 are wilting and yellowing, while a few are sending up curled stems 

 bearing deformed heads; by midsummer practically all the wheat 

 plants are dead and the area is covered with a growth of weeds. In 

 some cases entire fields of wheat have been completely destroyed; 

 such instances are not common, but are occurring more frequently 

 each year. 



Other Host Plants 



Barley is the only grain crop besides wheat which thus far is known 

 to be injured by the wheat aphis. Oats seem to be disliked and are 

 not harmed even when growing in the midst of infested wheat. 



Blue joint grass {Agropyron occidentale Scribn.), when growing in 

 and around the edges of infested wheat fields, is always heavily in- 

 fested and is probably the native host plant. However, an extended 

 search for the wheat aphis upon blue joint grass some distance from 

 infested grain fields has not as yet revealed it in such localities. Near 

 infested wheat, the wheat aphis has also beenfound upon cheat (Bromus 

 secalinus L.), spear grass (Stipa coinata F. & R.) and timothy {Phlewn 

 pratense'L.). 



Seasonal History and Habits 

 The winter is passed in the egg stage upon fall seeded wheat, vol- 

 unteer grain and grasses. The eggs hatch early in April and the re- 

 sulting stem mothers become adults and begin producing young in 

 about two weeks. About June 1, a few winged forms appear and fly 

 to new host plants. By June 15, winged migrants are very plentiful 

 and continue in increasing numbers until about July 1, when they begin 

 to decrease in number. In Montana no winged forms have been seen 

 after August 15, and it appears that the period of greatest activity in 

 migration in normal seasons occurs from about June 15 to July 1. 

 This period is of particular importance in relation to control measures. 

 The winged migrants do not settle upon large plants even though they 

 are green and succulent, but are apparently prompted by instinct to 

 search out smaller plants which will afford food foi; a longer period. 

 Summer fallowed fields plowed previous to June 15 and then allowed 

 to grow up to grasses and volunteer grain offer the ideal conditions 

 which the migrants are seeking. Scattering plants spring up and, 

 having a large area from which to draw moisture, remain green and 

 succulent until killed by heavj^ frosts in the fall. Upon such plants^ 

 wheat and blue stem primarily, the wheat aphis passes the summer. 



