POPULATION FACTORS AND SELECTED POPULATION PROBLEMS 



353 



of the available food supply and addition of 

 contaminants to the environment, it merits 

 review in some detail. Summarizing state- 

 ments are to be found in the following 

 papers: Park and Woollcott (1937); Park, 

 Miller, and Lutherman (1939); and Park 

 (1941)." 



Obviously, as a population of Tribolium 

 inhabits its flour (which it never leaves), 

 the flour becomes progressively more al- 

 tered, or "conditioned," with time as a re- 

 sult of the beetles' activities. The rate and 

 extent of such conditioning are in propor- 

 tion to the population density. It does not 

 necessarily follow, however, that the con- 

 ditioning will so aflFect the beetles individ- 

 ually that population growth form will be 

 altered. This must be determined experi- 

 mentally, and the particular, causative 

 process or processes identified. 



It has been shown that heavily condi- 

 tioned flour, i.e., flour in which large cul- 

 tures have lived, induces population de- 

 cline primarily through reduction of fecun- 

 dity and increase of the length and 

 hazard of larval development. It has also 

 been shown that less heavily conditioned 

 flour also reduces egg production signifi- 

 cantly. Before discussing the impHcation of 

 these points, an outhne summary of the 

 major eflFects of conditioned flour upon 

 Tribolium confusum are in order. 



I. Flour taken from "run-down" Tribolium 

 populations (i.e., heavily conditioned 

 flour) has the following known relations to 

 the beetles' physiology: 



A. It reduces their egg-cannibalism (p. 

 371) to about haU that of control 

 beetles living in unconditioned flour. 



B. It reduces their fecundity: 



1. Perceptibly, by lowering it three or 

 more times below the fresh-flour 

 control level; 



2. Quickly, since some effect is notice- 

 able within five days after exposure 

 to conditioned flour and since the 

 maximum effect is attained after 

 twenty days; and 



3. Reversihly, since the rate of ovi- 

 position can be returned to control 

 levels by reintroduction of the 

 beetles into unconditioned medium. 



** An "aggregation effect" has been shown for 

 Tribolium confusum populations where, in 

 early stages of population growth, the most 

 rapidly reproducing cultures are neither the 

 smallest nor the largest. This general question 

 is more appropriately discussed in Chapter 23 

 (p. 403). 



C. It may reduce fecundity through its 

 effect on males as weU as through the 

 females. In one group of experiments, 

 females living in fresh flour, when 

 mated with males from conditioned 

 flour, had a significantly lower rate of 

 reproduction than did females from 

 the same source mated to males from 

 fresh flour. 



D. It increases the relative variabiUty ol 

 the females' egg production. 



E. It apparently does not alter in any con- 

 sistent fashion egg fertility. In other 

 words, once laid, eggs from condi- 

 tioned-floiu: beeUes have as good a 

 probability of hatching as do eggs from 

 fresh-flour beetles. 



F. It increases the duration, variability in 

 respect of duration, and mortality of 

 larval development. 



G. It does not significantly alter the rate 

 of oxygen consumption of imago 

 Tribolium, since there is no apparent 

 difference in this between beetles 

 reared in fresh flour as compared with 

 those reared in conditioned flour. How- 

 ever, a definite sexual difference is 

 demonstrable: the females have a 

 higher rate of oxygen consumption 

 than the males. 



II. The "dilution" of heavily conditioned flour 

 with fresh flour to form a graded series of 

 conditioned media (5, 10, 15, 20, 25, 50, 

 and 75 per cent conditioned) has these 

 relations to Tribolium reproduction when 

 compared with fresh flour and conditioned 

 flour controls: 



A. The fecundity of the beetles is roughly 

 inversely proportional to the amount of 

 conditioned flour in the medium. As the 

 flour becomes progressively more condi- 

 tioned, egg production decreases. This 

 statement is subject to this correction: 

 There is no significant difference in 

 fecundity between the beetles in the 

 5, 10, or 15 per cent conditioned flours, 

 yet these three groups are far below 

 the fresh-flour controls and significantly 

 above the beetles in a higher condi- 

 tioned series. 



B. The fecundity of beetles living in 

 conditioned media can be restored to 

 control levels by reintroducing them 

 into fresh flour. 



C. Egg fertifity again is not affected by 

 such graded conditioning. 



III. The relation of conditioning to Tribolium 

 metamorphosis: 



A. Conditioning brought about by larvae: 

 1. If the culture medium is renewed, 

 larval development and pupal mor- 

 tality are not affected by the de- 

 gree of larval crowding. 



