FIELD CROP INSECTS 249 



female is from one-twenty-fifth to one-fourteenth inch long, 

 yellowish-green, with a median line slightly darker, and eyes 

 and most of the antennae black. The winged female is 

 slightly larger and of the same coloration except that the 

 head is brownish-yellow and the lobes of the thorax are 

 blackish. The agamic females multiply rapidly in summer, 

 for during its life of about a month each female will give 

 birth to fifty or sixty young, which commence to reproduce in 

 the same manner when about a week old. Reproduction is 

 slower in winter, but in an open winter a few individuals will 

 soon give rise to 

 infested spots 

 from which 

 countless indi- 

 viduals will 

 spread over the 

 field and en- 

 tirely ruin it by 

 the middle of FIG. 175. Lysiphlebus parasite in act of depositing 



A *il in north eggs in the body of a grain-aphis much enlarged. 

 April in north- (After Webster> n g Dept Agr } 



ern Texas. As 



the food supply disappears almost all the young develop 

 wings and immense clouds of winged females are carried 

 northward by the winds, so that an outbreak in early 

 spring in the South leads to infestation further north. As 

 soon as they multiply they again spread northward. Pro- 

 gressing thus in 1907, they reached southern Minnesota 

 by July. 



Grain aphides are prevented from becoming so over- 

 abundant as to cause frequent injury by the attacks of small 

 wasp-like parasites. These little parasites lay their eggs in 

 the aphides, which are soon killed by the growing larvae. 

 These parasites reproduce even more rapidly than the 



