POPULATION FACTORS AND SELECTED POPULATION PROBLEMS 



389 



flown, a population reduction ensues be- 

 cause the moths are forced to seek less 

 favorable food plants in which to lay their 

 eggs. This measure is most eflFective when 

 combined with the planting of trap crops. 



During years of excessive corn borer 

 abundance, such as that of 1922, a larval 

 overpopulation (Item 24) results, fre- 

 quently attaining a density of one to two 

 million individuals per acre of corn. This 

 is so severe that the corn is reduced to 

 mere shells by larval feeding, and many of 

 the borers die of starvation. Overpopula- 

 tion thus functions as a self-check when 

 abundance outstrips resources. Item 25 re- 

 fers to the practice of rotating weed crops 

 on fields, which crops are progressively 

 less favorable for the borers than the com 

 plant itself and thus gradually reduce the 

 pests' numbers by this means. 



Barber worked out a pattern showing 

 how these twenty-five items afi^ect the var- 

 ious stages of the corn borer hfe cycle and 

 thereby bring about population decline. He 

 was not able, on the other hand, to eval- 

 uate the relative, quantitative weight in 

 terms of number or percentage of individ- 

 uals lost that a certain factor actually ex- 

 erted in a particular year. The latter dem- 

 onstration would be instructive, indeed, 

 but, in its absence, much is to be gained 

 by examining the pattern. This is summa- 

 rized in Figure 137, in which the numbers 

 appearing after the various hfe cycle stages 

 pertain to the specific items. 



A tabulation of the items reported in 

 Figure 137 indicates that there are sixteen 

 instances in which "weather" operates to 

 reduce the size of the corn borer popula- 

 tion; twenty-eight instances in which "ene- 

 mies" are effective; eleven instances of 

 "agricultural practice and control;" one, of 

 "overpopulation," and one, of "change in 

 host plant availability." This tabulation, of 

 course, tells nothing about whether item 8, 

 say, is more, or less, destructive than item 

 11; it only indicates that both are func- 

 tioning. In fact. Barber reached the con- 

 clusion that the decrease of the popula- 

 tion noted in 1923 and 1924 was caused 

 "by an association of factors, some of major 

 importance, others contributorv but in- 

 creased in value throusjh the effect of the 

 major limiting factors." 



Abundant corn borer infestations would 

 be explained on the basis either that there 

 is a generally low level of effect contrib- 



uted by all the hsted items, or else that a 

 particularly severe effect contributed by 

 one or several of the items only would need 

 to be offset by an unusually shght contri- 

 bution on the part of the others. Sparse 

 corn borer infestations would be explained 

 by an especially stringent combination of 

 several items— as, for example, two, eight, 

 seventeen, and eighteen— or a generally 

 effective combination of most of them, or 

 by an aggressive program of control. 



It is interesting, and possibly suggestive, 

 that apart from these man-instituted con- 

 trol measures, Barber was able to identify 

 density-independent and density-dependent 

 items in about equal numbers. It is not 

 possible to suggest whether or not this 

 generalization is true for most natural 

 populations. Additional, comprehensive re- 

 search is needed to clarify the point, re- 

 search concerned especially with the 

 population as a whole interpreted in so far 

 as possible in its complete, spatial, and 

 temporal, effective environment. From our 

 point of view Barber's study has both 

 raised and answered questions, but it has 

 proved most useful as an ilkistration of 

 such a "total ecosystem" approach. 



It is relevant to quote briefly from 

 Stirrett (1938), who also investigated corn 

 borer populations in some detail twelve 

 vears later than did Barber. Stirrett says 

 '(p. 681): 



"The factors causing population fluctuations 

 are complex, and in many cases, interdepend- 

 ent. The important factors, however, causing 

 fluctuations in borer population have been 

 found to be initial moth population, larval 

 survival, egg production, egg mortality, pupal 

 mortality and com refuse clean-up. Tempera- 

 ture, especially temperature of certain definite 

 characteristics, has been shown to have a great 

 influence upon most of these factors, but 

 especially upon the rate of larv^al establishment, 

 pupal mortality, egg mortality, and the number 

 of eggs produced." 



For a more recent study devoted to an 

 analysis something like that of Barber's, 

 but with a greater theoretical content, the 

 reader is referred to Varley (1947). 



POPULATION INTEGRATION 



Before closing this chapter, it is desir- 

 able to draw some of the points together. 

 If there is any validity to the concept that 

 a population is an objective biological 

 entity, and, granting that considerable 



