BULLETIN 112, U. S. DEPAETMENT OF AGBICULTUBE. 



bug," although occasionally it may be found in conspicuous and alarm- 

 ing numbers, but it is ever present on wheat ; and, especially in the fall, 

 when it occurs at the base of the plant and on the roots, it is easily 

 overlooked by the casual observer. However, there is no doubt that 

 these plant-lice, even though they may not be conspicuous and 

 apparent, weaken the plants and decrease the yield. This de- 

 crease in yield is presumably general, but may not as a rule be 

 locally conspicuous as in the case of the " green bug," that is, not 



enough to be recognizable. On the 

 whole, however, it can hardly be doubt- 

 ed that these little insects are respon- 

 sible for the loss in this country of thou- 

 sands of bushels of wheat annually. 



DISTRIBUTION. 



The oat aphis is almost cosmopoli- 

 tan in its distribution, and irt this 

 respect rivals such well-known plant- 

 lice as M acrosiphwn, pisi Kalt., M. 

 granarium, and T oxoptera graminum. 

 It has been found in all parts of 

 Europe, as well as in most of the 

 States of the United States. Quite 

 likely it will also be found to occur 

 in Asia, and probably in Africa, 

 although we believe it has never 

 been reported in literature from 

 these countries up to the present 

 time. 



The map (fig. 1), compiled from rec- 

 ords made by assistants in the Cereal 

 and Forage-Crop Insect Investigations and from authentic pub- 

 lished records, indicates the present known distribution in the United 

 States. It will be observed that the species has not been found in the 

 Gulf region. 



DESCRIPTIVE. 



Fig. 2. — The oat aphis {Aphis 

 avenw) : Wingless viviparous 

 female, much enlarged. a, 

 Antenna of same, still more en- 

 larged. (Original.) 



On grain two forms of the oat aphis are found — the winged and 

 wingless viviparous females. As will be explained later, it occurs 

 on the apple where there are to be found, in addition, the sexual 

 forms, namely, the wingless oviparous female, the winged male, and 

 eggs. (See fig. 5.) 



The wingless viviparous female (fig. 2) is yellowish green to 

 olive green, often somewhat mottled. The stem mothers on apple 



