CLASSIFICATION AND DESCRIPTION OF COMMON INSECTS 141 



more substance than 500 million stout men; the latter computed that 

 in 300 days, or 15 generations of 20 each, there would be 20"" indi- 

 viduals, equal in weight to 1,638,400,000 men, supposing that 1000 

 aphids weigh i grain and i man weighs 2,000,000 grains. Another 

 computer makes the possible number of individuals very much higher, 

 viz., 210^^. It is believed that in the case of the Spring Grain Aphis 

 or "Green Bug" (Toxoptera graminum) reproduction would not fall 

 short of the figures given above as the average number of young pro- 

 duced in Indiana by each viviparous female for 1907-8-9 was 28.2. 

 It is conceivable, therefore, how this pest is able to devastate vast 

 areas of grain fields within a few weeks. 



Professor Forbes of Illinois (Bulletin 130) also computes the 

 fecundity of the Corn Root Aphis on the basis of a generation of 12 

 to 15 young in 2 weeks. If all the progeny lived and multiplied at 

 this rate for a season they would reach 9,500,000,000. 



In the case of the Green Apple Aphis {Aphis pomi) A. C. Baker 

 reports the average reproduction per insect for the entire season as 

 about 40. As the stem-mother becomes mature about May ist in Vir- 

 ginia, and there are from 9 to 17 generations, or an average of 13, re- 

 production is very rapid. The potential number of green apple aphids 

 in September from a single stem-mother would be 40^^ = 670,088,- 

 640,000,000,000,000. 



J. J. Davis reports for the Oat Aphis {Aphis avence) 15 generations 

 for Indiana with an average of 30 young for each female. The progeny 

 of one stem-mother at the end of the season would be 30^^. 



The same writer gives for the Pea Aphis {Macrosiphum pisi) the 

 average number of young borne by female of 16 consecutive genera- 

 tions as 65. The progeny of a stem-mother at the end of the season 

 would in this case be 65^^. 



Natural Enemies. — ^Lady-bird beetles, syrphid maggots, lace-wing 

 larvae, many parasitic hymenoptera, nymphs of tree-crickets, harvest- 

 spiders, birds. 



Control. — Spray with kerosene emulsion, whale-oil soap, tobacco 

 extract, etc. 



Chief Economic Species 

 COMMON CEREAL AND FARM CROP APHIDS 



Apple Bud or Oat Aphis {Aphis avencB Fab.). — Sometimes called 

 the European Grain Aphis; is often injurious to apple buds and appears 



