50 BULLETIN 276, U. S. DEPARTMENT OF AGRICULTURE. 



to be about 7 young per day, or even more. In our experiments indi- 

 vidual females bore as many as 124 young, and Dr. J. W. Folsom 

 (1909) reports a case where a single female produced 147 young. In 

 Mr. R. L. Webster's experiment conducted at Urbana, 111., in 1905 

 (Folsom, 1909) the average number of young borne by females of 

 16 consecutive generations was 46, while the average number of 

 young produced by individual females in all our experiments con- 

 ducted at La Fayette, Ind. (53 examples), excepting a few which 

 were accidentally or prematurely killed, was 68.3 + . 



From these figures it is not difficult to see why this insect becomes 

 so remarkably abundant, apparently within a few days, on its 

 various hosts, and why it is able, collectively, to ravage and com- 

 pletely destroy crops almost before they become apparent to the 



casual observer. 



SEXUAL FORMS. 



Sexual forms may occur in the fall of the year, but there seems to 

 be no uniformity in their production as is the case of certain other 

 plant-lice. For instance, oviparous females may be produced by 

 either wingless or winged females and the same female may produce 

 both viviparous and sexual forms alternately; for example, in one of 

 the experiment cages of 1912 a wingless female gave birth to her first 

 young on October 10 and these proved to be oviparous females; 

 later she gave birth to young which became viviparous females, and 

 still later again bore oviparous females. A number of instances 

 where females gave birth to viviparous and sexual forms alternately 

 were observed in 1912 and 1913. Our earliest record of the birth 

 of individuals of the sexual generation was October 10 in 1912 and 

 October 14 in 1913. Dr. Folsom (1909) found the males in the field 

 as early as October 10 in Illinois, and in one instance, hi an experi- 

 ment cage, an oviparous female was born as early as September 22. 

 At Funkstown, Md., Mr. J. A. Hyslop observed the sexes of this species 

 swarming on an alfalfa field November 12, 1912, the males and ovi- 

 parous females predominating, although some viviparous females 

 and young were observed. This observation was repeated at fclie 

 same place by Mr. C. M. Packard October 28, 1913. 



From these observations it is impossible to attribute the pro- 

 duction of sexes to any particular cause. Certain aphids, notably 

 Aphis maidi-radicis, Siplm flava, CaUipterus trifolii, Chmto'phorus 

 negundinis, EulacJinus rileyi, etc., invariably produce the egg-laying 

 forms toward winter in this latitude, and this may be attributed 

 largely to the weather conditions; but in the case of pisi, both vivi- 

 parous and oviparous forms are commonly borne of the same mother, 

 and in the same fine of general ions, conducted under exactly identical 

 conditions, reproduction may continue viviparously throughoul the 

 winter, while parts of one or more of these generations may become 

 sexual forms. It is noted, however, thai sexes are never produced 



