354 



POPULATIONS 



2. If the culture medium is not re- 

 newed, larval and pupal mortality 

 increase witli increase in density. 

 Also, under these conditions pupae 

 and imagoes reared from the 

 crowded larvae weigh less than do 

 those from sparse cultures. 



3. The duration of the pupal period 

 is not significantly altered by 

 density, sex, or conditioning. 



B. Conditioning brought about by ima- 

 goes. When a series of Tribolium 

 populations are estabUshed consisting 

 of a constant number of larvae (10), 

 but a geometrically increasing number 

 of male imagoes (1, 4, 16, 32, and 

 64), in a constant volume of flour, 

 the following effects are noted: 



1. The duration of the larval period 

 is extended as the density of the 

 imagoes increases. 



2. The larvae living in crowded 

 imago cultures grow more slowly in 

 terms of body weight than those 

 m less crowded imago cultures. 



3. The duration of the pupal period is 

 not significantly affected by the 

 crowding of imagoes with the 

 larvae. 



This outline summarizes the physiolog- 

 ical responses of Tribolium confusum 

 known to be affected by conditioned flour, 

 but makes no attempt to evaluate them as 

 they may be related to population growth 

 form. Such an evaluation is now in order. 

 The original impetus underlying the condi- 

 tioned flour study grew out of the follow- 

 ing facts: (1) Tribulium populations de- 

 cline as they age if their flour is not fre- 

 quently renewed; and (2) the flour in 

 which such populations live becomes pro- 

 gressively more conditioned. The latter fact 

 was recognized as early as 1896 by Chitten- 

 den, who, in discussing the general biology 

 of granary beetles, said, "When the insects 

 [Tribolium'] have time to propagate, they 

 soon convert the flour into a gray, useless 

 mass." It follows from these two points 

 that conditioning is an expression of popu- 

 lation dechne and population size since the 

 number of beetles (themselves) is responsi- 

 ble for the rate and amount of conditioning. 



Because conditioning does go hand-in- 

 hand with population decline, another ques- 

 tion is raised: Is such conditioning a result, 

 a cause, or both? That it is a result is self- 

 evident because, as already mentioned, it 

 reflects population age and density. That 

 it is a cause of decline is not self-evident 



since knowledge of the physiological effects 

 of conditioning upon individual beetles is 

 not available without experimental study. 

 Such experimentation has unearthed the 

 material presented in the outline. Our iiri- 

 mediate task is to sort these facts into 

 those that contribute to population dechne 

 and into those that do not. 



It is clear that conditioning does not 

 influence decline through the agency of 

 egg cannibahsm (I-A). Even though this 

 is definitely affected by conditioning, the 

 effect— i.e., that the adult beetles eat fewer 

 eggs if they are living in conditioned flour 

 than they do if in fresh flour— favors popu- 

 lation increase rather than contraction, since 

 more eggs escape being eaten and thereby 

 the likehhood of their hatching into larvae 

 is increased. 



The relation of conditioning to egg fer- 

 tihty (I-E; II-C) is not important in terms 

 of growth form since it has been impossi- 

 ble to demonstrate that this is either in- 

 creased or decreased by exposure of the 

 ovipositing females to conditioned medium. 

 The studies of oxygen consumption (I-G) 

 also suggest nothing of significance in rela- 

 tion to population dechne. 



It was shown in an earlier chapter that 

 natahty is one of the three primary variables 

 affecting population survival, the other 

 two being mortality and dispersion. Thus, 

 any factor influencing these must be care- 

 fully evaluated. On the basis of the infor- 

 mation presented in the outfine it appears 

 that conditioning brings about population 

 decline largely through its effect on fecun- 

 dity (I-B-C-D; II-A-B) and, to a lesser de- 

 gree, on larval metamorphosis (I-F; III). 

 This is supported not only by the analyses 

 of heavily conditioned flour, but also by 

 those dealing with differential conditioning. 

 The latter have shown, first, that the effect 

 of conditioning on fecundity is cumulative 

 (i.e., it becomes more extreme as the me- 

 dium gets more conditioned), and second, 

 that even a slight amount of conditioning 

 lowers the rate of oviposition. The last 

 point suggests that Tribolium populations 

 are rarely under optimal conditions so far 

 as fecundity is concerned, since the flour 

 typically is conditioned to some extent. It 

 is, of course, possible, even probable, that 

 other factors compensate for this during 

 the earher stages of population growth and 

 so keep conditioning, operating through 



