January, 1909.] 



33 



PLANT SANITATION, 



THEORY OP THE PARASITIC 

 CONTROL OP INSECT PESTS. 



All who have recently discussed the 

 question of the possibility of controlling 

 insect pests by the use of parasitic or 

 predaceous insects or by fungous or 

 bacterial diseases, have failed to consider 

 the subject from a very important point 

 of view. 



The conditions determining the life or 

 death of insects are much more com- 

 plicated than is usually appreciated, 

 and the individual factors in the problem 

 are far from independent. The correct 

 estimation of this interdependence of 

 the causes of death in insects is of 

 vital importance in this connection. The 

 efficiency of each factor is so influenced 

 by the efficiency of the others that the 

 elimination of one cause of death or the 

 addition of an entirely new natural 

 enemy will usually have but a slight 

 effect upon the rate of survival or none 

 at all. 



The reproductive powers of most or- 

 ganic beings are very great. Were 

 not all creatures liable to die prema- 

 turely, that is, before they repro- 

 duced themselves, reproduction would of 

 necessity have been limited to two off- 

 spring from each pair. Whenever re- 

 production is at a more rapid rate it is a 

 prima Jacie evidence that the chance cf 

 premature destruction requires it and 

 the greater the reproductive power the 

 higher this normal death rate. Were 

 conditions otherwise, rapid extinction 

 or enormous increase would result. The 

 fact that species maintain themselves 

 for ages with the rate between the 

 birth rate and that of premature death 

 not varying an appreciated fraction of a 

 per cent, is very evident. 



This balance between birth and death- 

 rates is much greater than the numerical 

 stability. Por instance, in the case of a 

 species increasing a hundred fold in a 

 generation, an average disturbance of 

 only a hundredth part of a per cent, in 

 this ratio— i.e,, if onthe aveiage one more 

 individual in ten thousand should come 

 to maturity — this would result in nearly 

 tripling the numbers of individuals 

 within a hundred generations, and one 

 tenth of one per cent, augmentation— 

 e.g., if one more in a thousand should 

 survive — would be an increase in num- 

 bers amounting in the same period to 

 nearly fourteen thousand fold, 



Disturbing, Contributing and 



Effective Factors. 



The various causes of death may be 

 classed into two groups ; first, those 

 that destroy all insects in a certain con- 

 dition or position, irrespective of the 

 numbers present (for instance, frost, 

 which might kill the same proportion 

 whether there was but one to the acre 

 or a hundred thousand) ; and second, 

 those that are more and more efficient 

 as the numbers increase. This is true 

 in general of predaceous and parasitic 

 insects and of diseases. Causes of death 

 of first class will aid in maintaining the 

 balauce in an insect to the extent they 

 are uniform in their action, the regular- 

 ly recurring winter, for instance; but 

 are usually erratic and disturbing 

 rather than balancing. Those of the 

 second category, however, tall tend 

 towards balance and their efficiency is 

 attested by the approximate balance 

 maintained in nature. Probably in all 

 cases numerous parasites and pre- 

 dators and other factors of this same 

 class contribute to form the controlling 

 environment of an injurious species, 

 and each factor has a different potenti- 

 ality. Those of the second category can 

 be further subdivided into two classes, 

 the contributory and the effective. In 

 the former class, the efficiency increases 

 with the increase of the host, but not in 

 a sufficient ratio to ever overtake it. 

 Thus with the host at one hundred per 

 acre it may destroy one third, at two 

 hundred four ninths, at four hundred 

 thirteen twenty-sevenths, etc., never 

 reaching fifty per cent. Any series that 

 does not ultimately pass the percentage 

 of normal death rate is incapable of 

 itself diminishing the numbers of its 

 host. Its only effect is in slowing down 

 the rate of increase until some effective 

 factor becomes operative or until a dis- 

 turbing factor like frost produces a 

 general destruction. 



The effective class of factors is that in 

 which the ratio finally reaches one 

 hundred per cent, Thus with the host 

 at one hundred per acre it may [destroy 

 say one half, at two hundred three 

 quarters, at four hundred, seven 

 eighths, etc.; finally reaching a fraction 

 so large that only those survive that 

 are necessary to maintain the species. 



Every factor of this class has its 

 particular point of balance, One may 

 overtake the host at two hundred per 

 acre and another only at two million 



