PROGRESS REPORT ON THE EUROPEAN CORN BORER 83 
period may have occurred a few days earlier or later than is indi- 
cated. Since, however, the majority of these records were based 
upon averages of actual field observations, it is believed that the 
seasonal occurrence of the stages as shown in Figure 41 represent 
closely the average occurrence during this five-year period. A de- 
tailed explanation of the manner in which this chart and the three 
following charts were constructed is given in another publication (7). 
During the season of 1922, beginning March 23 and extending to 
September 8, a systematic investigation was conducted to determine 
more accurately the occurrence of P. nubilalis in the field. Field ex- 
aminations were made to ascertain the occurrence and progress of 
pupation and adult emergence in representative localities in the 
Massachusetts area, on practically every working day throughout 
the season when weather conditions permitted. The examinations 
occurred in corn and Xanthium, although some of the more widely 
distributed and susceptible weeds, such as Echinochloa, Bidens, 
Amaranthus, and Polygonum, were included. During the course 
of the season 559 separate examinations were made, involving a 
total of 71,528 individuals. 
With the information concerning the occurrence and progress of 
pupation and adult emergence at hand, similar data relating to eggs 
and larvae were computed from the known duration of the adult 
stages and incubation period, as determined by contemporaneous 
insectary rearings, supplemented by observations in the field. The 
daily percentages of larvae, pupation, and adult emergence which 
were found at the time of examination, averaged for all localities 
and food plants, are shown graphically in Figure 42. The incidence 
of eggs and younger larvae, as well as the larvae entering hiberna- 
tion, is indicated by dotted lines. The prevalence of first-generation 
larvae which hibernated in that stage, consisting of about 1.4 per 
cent of the total larvae present, is shown by a dotted extension. 
The fact should be emphasized that the percentage of each stage 
designated in Figure 42 as being present in the field on a given 
date is an average for all localities and food plants. In general the 
incidence of egg deposition as well as the development of the larvae 
and subsequent pupation in the summer broods was earlier in the 
early corn than in the later corn. For this reason the percentage 
of larvae, pupae, and empty pupal cases (denoting adult emergence) 
often varied considerably on the same date in different corn plantings 
in the same field or farm, during the growing season. Since 1922 
was in most respects a normal season it is believed that the progress 
of seasonal development during that season as represented in Figure 
42 was very nearly typical, although it is realized that this procedure 
must be repeated for several years before definite, reliable informa- 
tion on this point can be secured. 
For the purpose of illustrating more graphically the occurrence 
of each stage of P. mibilalis in the field during 1922, a graph (fig. 
43) has been constructed based upon the field examinations discussed 
above. The black areas represent the actual field counts, while the 
areas inclosed by dotted lines are computed from the occurrence 
and the known duration of the preceding stages, as determined by 
contemporaneous insectary rearings, supplemented by observations 
in the field. 
