INSECTS INJURIOUS TO SMALL GRAINS 



149 



the abdomen is marked with four or five transverse blackish 



spots in front of the nectaries. 



Like the other aphides affecting small grains, this species is 



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



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



parasites having been killed off, thus 

 giving the aphides opportunity to 

 multiply unchecked. Among the most 

 abundant parasites are species of the 

 genus Aphidius (family Braconidce), 



\ 



FIG. 108. Grain aphides 

 clustered on wheat head, 

 greatly enlarged. (After 

 Weed.) 



FIG. 100. Wheat-louse parasite (Aphiditis 

 avenaphis Fitch), and parasitized louse 

 from which it has issued. (Copied from 

 J. B. Smith.) 



one of which is shown in Fig. 109, greatly enlarged. Cold, wet 

 weather in spring greatly retards the development of these para- 

 sites, so that the aphides are always more numerous in such 

 seasons. It has also been observed that an outbreak is often 

 preceded 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 aphides and which do not propagate 

 in hot dry weather. Thus weather conditions are very intimately 



