6 BULLETIN 774, U. 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 80-April 22. ,The average growing period was 12.6 
days, the maximum 18, and the minimum 10. 
Between Apri! 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 wingless generations of the mealy plum aphis. 
Walnut Creek, Cal., 1916. 
Developmental period. 
Number 
Genera-) Dates included. | of indi- 
z viduals. hone Maxi- Mini- 
se. mum mum. 
Days. Days Days 
TV ....| Apr.23-May 16 10 9.9 | | 
(Mays) ce May 3-May 18 27 41.1 14 10 
VI_...| May 14-May 27 30 12.7 16 11 
VIL...| May 26-June 3 10 110 11 9 
1June 7, five fourth-instar wingless individuals died because of an excessive heat wave. 
Had these matured, the average developmental period would have been slightly length- 
ened. 
MAXIMUM AND MINIMUM GENERATIONS. 
A maximum-generation series of first-born aphids (wingless) is 
shown in the table following: 
Tasrm Il.—Development of maximum-generation series of the mealy plum aphis. 
Walnut Creek, Cal., 1916. 
Genera- Date of | Develop- 
tion of | Date of By 
divide bohicth! fourth | mental 
il molt. period. 
Days 
tale pain 5 Mar. 11 | Mar. 28 1 
Bees Mar. 30) Apr. i1 12 
IW aise Apr. 11 | Apr. 23 13 
TVS: Apr. 23 | May 3 10 
Wisasces May 3] May 14 il 
Wilewsce May 14 | May 26 12 
VII....| May 27| June 6 9 
Thus in the space of 114 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 
