THE OAT APHIS. 



13 



it alternately froze and thawed until March, 1910, the weather 

 being so severe that 50 to 75 per cent of the wheat in that vicinity 

 was killed by the cold. 



Sometimes these winter root forms are attended by ants, as has 

 been observed by Prof. Webster and the writer. The forms which 

 go to apple migrate early in October in the latitude of La Fayette, 

 Ind., and usually fully a month later in the latitude of northern 

 Oklahoma. In the rearing cages it has never been possible to get 

 the forms from wheat to migrate to apple, the failure doubtless re- 

 sulting from the use of too small cages. On the other hand, there 

 was no difficulty in getting the spring migrants to go to wheat and 

 there continue to reproduce throughout the summer from apple 

 shoots, even in small lantern globe cages. 



Fig. 6. — Aphidius testaceipes ovipositing in the body of the 

 spring grain-aphis. Enlarged. (From Webster.) 



NATURAL CHECKS. 



Like most plant-lice of the genus Aphis, avenw is freely attacked 

 by various parasitic and predaceous animals, principally insects, and 

 doubtless these 

 are responsible for 

 the usual control 

 of this pest. 



Among the in- 

 ternal parasites, 

 Fitch x has r e - 

 corded Toxares 

 triticaphis Fitch, 

 (Praon) Aphidius 

 avenaphis Fitch, and AUotria tritici Fitch, but it is probable that he 

 reared these from Macrosiphum granarium rather than from Aphis 

 avence as was supposed by Mr. Pergande. 2 In 1894 F. M. Webster * 

 reports rearing P achy neuron micans Howard and (Lysiphlebus) 

 Aphidius testaceipes Cresson {tritici Ashmead). The latter species 

 (figs. 6 and 7) is the one which ordinarily holds the spring grain- 

 aphis {Toxoptera graminum) in check, and doubtless is likewise 

 beneficial in preventing undue multiplication in avence. Mr. Theo. 

 Pergande 4 reared another species of Aphidius {A. nigriceps Ash- 

 mead) in considerable numbers from this aphis. 



Among the predaceous insects Pergande 4 has reared a common 

 syrphid fly (Syrphus americanus Wiedemann) (fig. 8) ; the writer 

 has reared a species of Aphidoletes from larvae feeding on Aphis 



1 Sixth Rpt. on the noxious and other insects of the State of N. Y.. 1865, pp. 98-112. 



2 U. S. Dept. Agr., Div. Ent., Bui. 44, 1904, p. 13. 



3 Ohio Agr. Expt. Sta., Bui. 51, 1894, p. 117. 

 * Op. cit. 



