80 PROTECTION OF PLANTS, 1917-18 



grape phylloxera gall, the cockscomb gall on the elm, the cottonwood gall, the 

 poplar gall, etc. 



Most plant-lice excrete a sweet liquid called "honey-dew" or "nectar," 

 which is attractive to ants, bees, wasps and other insects. On account of this 

 honey-dew aphids are often attended by ants who guard them. Forbes has shown 

 that the little brown ant {Lasius niger) has domesticated the Corn Root Aphis, 

 which is cared for and controlled in all stages of its development. 



Fecundity. 



Regarding" the powers of reproduction of aphids Webster and Phillips (Bui. 

 1 10, U. S. Bur. of Entom.) cite the estimates of Huxley and Buckton. The former 

 estimated that the tenth generation alone of a single rose aphis, were there no 

 deaths, would contain more substance than 500 millions stout men; the latter 

 computed that in 300 days, or 15 generations of 20 each, there would be 20^^ 

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

 weigh 1 grain and 1 man weighs 2,000,000 grains. Another computer makes the 

 possible number of individuals very much higher, viz., 210^M It is believed that 

 in the case of the spring grain aphis or "green bug" {Toxoptera graminum) re- 

 production would not fall short of the figures given above as the average number 

 of young produced 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 1st May in Virginia, and there are from 9 to 17 

 generations, or an average of 13, reproduction is very rapid. The potential number 

 of green apple aphids in September from a single stem-mother would be 40^^ = 

 67 1 ,088,640,000,000,000,000 ! 



J. J. Davis reports for the Oat Aphis {Aphis avenae) 15 generations for In- 

 diana 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 {Macrosiphiim pisi) the average 

 number of young borne by female of 16 consecutive generations as 65. The 

 progeny of a stem-mother at the end of the season would in this case be 65^®. 



