PARASITISM IX 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 rot raining 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 he 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 too 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 
