INSECTS INJURIOUS TO SMALL GRAINS 137 



a third longer than the body, and are of the same general colora- 

 tion except that lobes of the thorax are brown or blackish, and 

 the abdomen is marked with four or five transverse blackish 

 spots in front of the nectaries. 



Like the other aphids affecting small grains, this species is 

 held in check by parasitic insects, aided by predaceous insects and 

 fungous diseases. Injury by the aphids is usually due to the 

 parasites having been killed off, thus giving the aphids oppor- 

 tunity to multiply unchecked. Among the most abundant parasites 

 are species of the genus Aphidius (family Braconidce), one of which 

 is shown in Fig. 117, greatly enlarged. Cold, wet weather in 

 spring greatly retards the development of these parasites, 

 so that the aphids are always more numerous in such sea- 

 sons. It has also been observed that an outbreak is often pre- 

 ceded by several dry seasons, which may be due to the fact that 

 such dry seasons check the development of fungous diseases which 

 kill off large numbers of the aphids and which do not propagate 

 in hot dry weather. Thus weather conditions are very intimately 

 associated with the abundance of the pest. When the parasites 

 become abundant they will often completely rid a field of the 

 aphids within a few days. All of the common ladybird -beetles 

 (Coccinellidce), Syrphus-fly larvae, and lace-winged fly larvae 

 (Chrysopidce) are commonly found feeding upon the aphids. 



Control. No practical remedy for this species is known nor 

 are means of control easily suggested. The suppression of 

 volunteer wheat and oats in early fall will prevent the multiplica- 

 tion of the pest before fall-sown wheat is available, and the late 

 sowing of wheat in the fall will reduce the numbers entering 

 hibernation. A wise rotation and the thorough preparation 

 of the seed-bed and liberal fertilization will be of value in avoid- 

 ing injury in the same way as has been described for other pests 

 of small grains. Fortunately this species rarely does very wide- 

 spread injury and its parasites usually soon bring it under control. 



The Spring Grain-aphis or Green Bug * 



Though long known as a serious pest of small grains in Europe, 

 this aphis has done widespread injury in this country only during 

 the past ten years. Though it occurs throughout the territory 

 * Toxoptera giaminum Rond. Family Aphididce. 



