6 BULLETIN 11-i, IT. S. DEPARTMENT OF AGRICULTURE. 



hosts, the aphids being retarded on weaker trees. The instars occu- 

 pied, on the average, equal periods of time, except that the fourth was 

 slightly shorter than any one of the others. 



Forty-five individuals of the second generation were observed dur- 

 ing the period March 27-May 4, the majority developing during 

 the period March 30- April 22. The average growing period was 12.6 

 days, the maximum 18, and the minimum 10. 



Between April 11 and May 30, 34 third-generation wingless in- 

 dividuals developed in an average of 11.8 days, the maximum and 

 minimum periods being, respectively, 18 and 9 days. 



Individual records of generations 4 to 7 were made chiefly from 

 the maximum-generation series and might be grouped best in tabular 

 form as follows: 



Table I. — Development of spring icingless generations of the mealy plum aphis. 

 Walnut Creek, Cah, 1916. 



Genera- 

 tion. 



Dates included. 



Number 

 of indi- 

 viduals. 



Developmental period. 



Ave-a£r P Maxi ' Mini " 

 Ave.age. j mum _ | muiru 



IV.... ' Apr.23-Mav 16 



V May 3-May 18 



VI.... ! May li-Mav 27 

 VH... May 26-June 3 



10 

 27 

 30 

 10 



Days, Days. ! Days. 



9.9 ! 14 9 



11.1 1 14 ! 10 



12.7 ! 16 ! 11 



i 10 11 | 9 



1 June 7, five fourtk-instar wingless individuals died because of an excessive beat wave. 

 Had tbese matured, tbe average developmental period would bave been, sligbtly length- 

 ened. 



Maximum and Minimum Genekations. 



A maximum-generation series of first-born aphids (wingless) is 

 shown in the table following: 



Table II. — Development of m a. ri mum-gene rat ion series of the mealy plum aphis. 

 Walnut Creek, Gal., 1916. 



Genera- 

 tion of 

 individ- 

 ual. 



Date of 

 birtb. 



Date of 

 fourth, 

 molt. 



Develop- 

 mental 

 period. 









Days. 



I 



:iar. 11 



Mar. 28 



17 



II 



Mar. 30 



Apr. 11 



12 



Ill 



Apr. 11 



Apr. 23 



13 



IV 



Apr. 23 



May 3 



10 



V 



May 3 



May 14 



11 



VI 



May 14 



May 26 



12 



VII.... 



May 27 



June 6 



9 



Thus in the space of llf weeks seven complete generations occurred. 

 Had it been possible to have continued the series until winter, cer- 

 tainly 10 more generations would have been produced. The adults 



