THE OAT APHIS. 9 



ton and H. E. Smith, of this bureau, under directions from Prof. 

 Webster. They found very few of the " green bug," while the oat 

 aphis was present in considerable numbers. After a careful examina- 

 tion of the fields, the conclusions reached were that the injuries 

 Avere due to one or more of three causes, namely, attacks by the oat 

 aphis, impoverished soils, and weather conditions, particularly ex- 

 cessive rains during the late fall and early winter. Of these, weather 

 conditions seem to have been the cause of the greatest amount of 

 injury, although in certain areas the damage was more probably 

 the result of attacks of the oat aphis. However, the parasites were 

 in noticeable evidence everywhere, so that with normally late winter 

 and spring weather they should prevent the aphides from becoming 

 injuriously abundant. 



As described by Prof. Webster in the foregoing quotation, the 

 infested plants take on a yellowish or greenish yellow color, appear 

 sickly, and cease to make any apparent growth, and since the insect 

 works on the lower parts of the plant and is not always easily 

 detected, the cause of the injury may sometimes be overlooked. 

 During the summer this aphis usually feeds on the under surface of 

 the leaves, on the stems, and in the axils of the leaves — seldom in 

 the grain heads, as does Macrosiphwm granarium. 



CAUSES OF OCCASIONAL OUTBREAKS. 



Prof. Webster x has made clear the reason for periodic outbreaks of 

 the spring grain-aphis (Toxoptera graminum), and the usual abun- 

 dance of the oat aphis in certain years may be attributed to the same 

 cause. As in the case of the spring grain-aphis, the oat aphis breeds 

 and multiplies at a temperature of about 40° F., or above, while the 

 common parasite of these and many other aphides, Aphidius testa- 

 ceipes Cress., is hardly active at a temperature less than 56° F. Con- 

 sequently, mild winters and cool springs, when the temperature 

 fluctuates between 40° and 56° F., permit the aphis to multiply, unin- 

 terrupted by attacks from their common natural enemy. 



LIFE HISTORY OF THE INSECT. 



The oat aphis occurs on grains and grasses throughout the sum- 

 mer, the spring colonies originating either from viviparous females 

 which passed the winter on wheat, grasses, etc., or from spring mi- 

 grants from apple and related trees — that is, the progeny of aphides 

 hatching from eggs laid the previous fall on such trees. The plant- 

 lice usually become more abundant toward fall, and as the weather 

 becomes cooler they seek the lower parts or roots of wheat and other 



1 U. S. Dept. Agr., Bur. Ent., Circ. 85, Mar. 29, 1907, and U. S. Dept. Agr., Bur. Ent., 

 Bui. 110, Sept. 6, 193 2. 



