February, '16] PARKER: WESTERN wheat aphis 183 



short. The peculiar long and narrow body with its covering of white 

 powder will easily distinguish this form from other aphids commonly 

 found upon wheat. 



In the winged vi\iparous female the head, antennsG and thorax are 

 black, while the abdomen is light green. The body is powdered with a 

 white secretion, but to a less extent than in the wingless female. The 

 body is also shorter, the length averaging about 1.5 millimeters. The 

 cubital vein of the wing is twice forked. This form may be distin- 

 guished from other grain aphids by the cornicles, which are greatly 

 reduced and appear only as mere rings slightly raised above the surface 

 of the body. 



The oviparous female is similar in appearance to the wingless vi- 

 viparous female. 



The male is wingless, only about half as long as the female, less 

 densely clothed with white powder and is somewhat lighter in color. 



The eggs are pale yellow when first laid, but soon change to green 

 and after a few hours become shining black. 



Character and Extent of Injury 



Wheat plants infested with the wheat aphis have a characteristic 

 appearance which is easily recognized after having once been seen. 

 On small plants the first indication is a thickening and broadening 

 of the leaf blade. Soon longitudinal, whitish stripes appear and these 

 frequently become pinkish in the later stages. Leaves of infested plants 

 in late fall and early spring have a fleshy and whitened appearance 

 as compared with the slender dark green leaves of uninfested plants. 

 One or two aphids hidden from sight and feeding in the yet unex- 

 panded basal portion of the leaves are sufficient to bring about the 

 characteristic appearance described above. These soon increase, 

 however, and the entire upper surface of the leaves is frequently 

 covered with lice. No curling of the leaves has been observed in the 

 fall or early spring. Heavily infested plants make little or no growth 

 and by midsummer are generally dead. Plants less heavily infested 

 sometimes stool out and eventually send up a twisted central stem 

 bearing a deformed head. A short time previous to heading, the leaves 

 surrounding the central stem generally become tightly curled and as 

 a result the head has difficulty in pushing up through them and very 

 frequently forces its way through the side of the boot or sheath. The 

 peculiar twisting of the terminal leaves which occurs in this stage is 

 quite characteristic, the leaf twisting spirally in one direction for a 

 short distance and then curling in the opposite direction. The de- 

 formed heads seldom reach a height of over ten inches from the ground 

 and, as a rule, do not ripen any grain. 



