30 



BULLETIISr 427, U. S. DEPAETMENT OF AGRICULTURE. 



Tn examiniiig this table it must be borne in mind that the results 

 given are for a particular year, and great variations are possible from 

 similar months in other years. One example will be sufficient to 

 illustrate this point: During the middle of April, 1914, the mean 

 temperature averaged about 66°, and during this time eggs of the 

 tuber moth were hatching in from 8 to 9 days. During the same 

 period of 1915, with the average mean temperature about 59° F., the 

 egg stage lasted from 12 to 14 days. All stages of the insect vary 

 so greatly that it is difficult to foretell how long a generation will 

 take. 



OVERLAPPING OF GENERATIONS. 



From the foregoing examples it is evident that all stages of the 

 tuber moth exist throughout the year in Southern Cahfomia, and 

 consequently the broods can not be distinguished. Many writers have 

 estimated the numbers of broods or generations by the abundance of 

 moths at different times of the year. If conditions are considered, 

 it may be seen that food and temperature govern this condition. In 

 summer, with an abundant food supply, the insect multiplies with 

 great rapidity, adults become abundant, and the impression of the 

 emergence of a brood is given. If plots A (p. 28) and B (p. 29) are 

 compared, the overlapping of generations may be understood. 



By placing one over the other as in plot C, the shaded areas 

 show the time adults are emerging. The broken Unes are from 

 plot B. 



Plot C. 



1st Gen. 



2nd Gen. 



3rd Gen. 



4th Gen. 



5tli Gen. 



1st Gen. 



2nd Gen. 



3rd Gen. 4th Gen. oth G en. 



6th Gen. 



Jan. 



Feb. Mar. Apr. May. June. July. Aug. Sept. Oct. Nov. 



Dec. 



This diagram indicates that although the generations may secure 

 an even start at the beginning of the year, by late summer and fall 

 the first to emerge from the fifth generation are appearing at a time 

 when there are stiU adults from the fourth generation emerging. This 

 explains the presence of all stages of the insect at all times of the year, 

 and indicates that from the economic side a knowledge of the life his- 

 tory of the moth is of httle importance, except as it shows the possi- 

 bihties of reproduction . 



HIBERNATION. 



During the discussion of the effect of temperature on the various 

 stages of the tuber moth the impression was given that there is no 

 hibernation for any length of time in southern California. This is 

 surely the case under normal conditions, though possibly there is no 



