INSECTS INJURIOUS TO SMALL GRAINS 



153 



observed, but strangely enough they have only been secured in 

 small numbers by being reared in the laboratory, and have not 

 been observed in the field, so that although these females laid eggs 

 freely on the leaves of grain, we do not know whether they are 

 essential or not to the life history of the insect in the field, for 

 while the eggs are being produced other females continue to give 

 birth to live young until the cold of winter, and they have been 



FIG. 112. Toxoptera graminum: a, newly born, and b, adult wingless green 

 bug, greatly enlarged. (After S. J. Hunter.) 



observed to reproduce with a daily mean temperature barely 

 above freezing. 



Natural Control. The natural control of this most destructive 

 pest involves a most interesting relationship between temperature 

 and the development of the parasites which check its development. 

 "The 'green bug' in normal years that is, when its breeding 

 begins in spring is effectively held in check by its natural ene- 

 mies, and notably by a minute, black wasp-like insect, Lysiphlebus 

 testaceipes Cress. (Fig. 113), that deposits eggs singly in the 'green 



