POPtTLATION FACTORS AND SELECTED POPULATION PROBLEMS 



387 



MAY 



JUNE 



JULY 



AUGUST 



SEPT. 



OCT. 



NOV. 



DEC. 



JAN. 



FEB. 



MARCH 



APRIL 



MAY 



JUNE 



u. 



11 



_Spnng flight of motfis ; total possible number 

 may be cut down by 1-9 — 14. 



Eggs deposited by these mcA^s; total 

 possible number may be cut dcwn by 2-3H5. 



Larvae resulting from these egge- total 

 'possible number may be cut down by 4-5—10- 

 13-15-16-17- 1 8-21-22-23. 



XT 



^Pupoe developing from these larvae: total possible 

 \ number moy be cut down by 6—10—17—22. 



\ \Mid-season flight of moths total possible number 

 \ may be cut down by 1—9—14. 



\Eggs deposited by these moths- total possible 

 number may be cut down by 2—13 — 15 — 16. 



Overwintering larvae (single generation)^ 



total number may be cut down by 4-5—8—1 1— 

 12-13 — 17-18 — 19 — 20 — 21. 



y Overwintering larvae {two generations) 



resulting from the eggs^ total number may 

 be cut down by 4-5 — 7— 8 — 10— II —12- 

 13-17—18-19-20-21-24-25. 



Pupae developing from overwintering larvae: 

 total number may be cut down byll— 17— 19. 



Fig. 137. The life cycle of the comborer in New England. The numerals refer to factors tab- 

 ulated in the text that affect the population's abundance. ( After Barber. ) 



already made and as an illustration of the 

 fact that more than one environmental 

 factor typically acts on a particular popu- 

 lation process. 



Barber stresses, as was documented in 

 the last chapter, that many insect popula- 

 tions vary markedly in abundance from 

 year to year and that much is to be gained 

 when the reasons for this variability are 

 understood. He concludes that such infor- 

 mation is of assistance in planning eflFective 

 measures of control for insect pests. His 

 work on the corn borer centered in the 

 New England states, where the popula- 

 tion growth form during the early nine- 



teen-twenties is well known. During 1923 

 a sharp decline in the general population 

 occurred which carried over into 1924, and 

 it is with the factors responsible for this 

 decline that Barber is particularly con- 

 cerned. 



Barber's study will be developed, first, 

 by outlining the life cycle of the corn 

 borer; second, by listing the known envi- 

 ronmental factors that influence the popu- 

 lations; and, third, by showing the method 

 by which, and time at which, these factors 

 operate in producing their efi^ects. 



The life cycle of the com borer is best 

 understood by reference to Figure 137. 



