PARASITISM IN INSECT CONTROL. 107 



average percentage of parasitism of the fall webworm in eastern 

 Massachusetts, taken over a sufficiently long series of years to make 

 a fair average possible, is the same as the average would be over 

 another similar series of years in the same general region. This 

 could be said of the larvae of any other insect as well as of that of 

 the fall webworm, but the average percentage of parasitism in another 

 would most likely not be the same, but might be very much larger 

 or very much smaller. To put it dogmatically, each species of insect 

 in a country where the conditions are settled is subjected to a certain 

 fixed average percentage of parasitism, which, in the vast majority of 

 instances and in connection with numerous other controlling agencies, 

 results in the maintenance of a perfect balance. The insect neither 

 increases to such abundance as to be affected by disease or checked 

 from further multiplication through lack of food, nor does it become 

 extinct, but throughout maintains a degree of abundance in relation 

 to other species existing in the same vicinity, which, when averaged 

 for a long series of years, is constant. 



In order that this balance may exist it is necessary that among 

 the factors which work together in restricting the multiplication 

 of the species there shall be at least one, if not more, which is 

 what is here termed facultative (for want of a better name), and 

 which, by exerting a restraining influence which is relatively more 

 effective when other conditions favor undue increase, serves to pre- 

 vent it. There are a very large number and a great variety of 

 factors of more or less importance in effecting the control of defoli- 

 ating caterpillars, and to attempt to catalogue them would be futile, 

 but however closely they may be scrutinized very few will be found 

 to fall into the class with parasitism, which in the majority of 

 instances, though not in all, is truly " facultative." 



A very large proportion of the controlling agencies, such as the 

 destruction wrought by storm, low or high temperature, or other 

 climatic conditions, is to be classed as catastrophic, since they are 

 wholly independent in their activities upon whether the insect which 

 incidentally suffers is rare or abundant. The storm which destroys 

 10 caterpillars out of 50 which chance to be upon a tree would doubt- 

 less have destroyed 20 had there been 100 present, or 100 had there 

 been 500 present. The average percentage of destruction remains 

 the same, no matter how abundant or how near to extinction the 

 insect may have become. 



Destruction through certain other agencies, notably by birds and 

 other predators, works in a radically different manner. These 

 predators are not directly affected by the abundance or scarcity of 

 any single item in their varied menu. Like all other creatures they 

 arc forced to maintain a relatively constant abundance among the 



