22 



BULLETIN 1235, U. S. DEPARTMENT OF AGRICULTURE. 



SEASONAL-HISTORY STUDIES OF 1920. 



The season of 1920 proved to be quite different from that of 1919. 

 According to the Weather Bureau, the spring was the most backward 

 on record. The first three months of the year were about normal, 

 but April was extremely cold, the mean temperature at Yakima being 

 5.1° below normal. In May the mean temperature was 2.6° below 

 normal, and in June the mean temperature was 2.3° below normal. 

 This resulted in greatly retarding the activities of the codling moth. 

 Apple trees bloomed at Yakima May 6 to 10, and the calyx spray 

 was applied about May 20, or 10 days later than in 1919. After July 

 1 the temperatures were above normal. The cold spring resulted in 

 slowing up the various stages of the spring brood of the codling moth. 

 Beginning with the incubation of the first brood of eggs, however, 

 the periods were shorter than the corresponding ones of 1919. In 

 spite of this the seasonal history continued throughout the summer 

 to be later than in 1919. The studies recorded below were carried 

 out in the same way as in 1919, and each table should be considered 

 as a unit. 



WINTERING LARVAE. 



In order to provide material for the 1920 studies, a large number 

 of wintering larva? were secured in the fall of 1919 from reared ma- 

 terial and from banded trees. The extremely cold weather of De- 

 cember, described on page 58, killed all these larva?, and it was neces- 

 sary to collect a new supply in February and March. These were 

 collected wherever they had escaped the freeze, many of them being 

 taken from the soil about the bases of trees and from bands which 

 were covered with snow during the freeze. 



PUP^E OF THE SPRING BROOD. 



Time of pupation. — In Figure 12 is given the time of pupation of 

 160 individuals of the spring brood. The effect of the cold weather 

 in April is very noticeable. Two larva? pupated on April 3, and there 

 were no more until April 12. The maximum occurred April 26 and 



Fig. 12.— Pupation of the spring brood of the codling moth at Yakima, Wash., 1920. 



27, however, which is the same time as the maximum for 1919. 

 Pupation ended on May 17, with a single late straggler OD June 3. 



Length of the pupal stage. — Table 18 shows the length of the pupal 

 stage of 136 pupae, the average being 33.56 days, two days longer 

 than in 1919. 



