108 PARASITES OF GIPSY AND BROWN-TAIL MOTHS. 
other forms of animal and plant life, and since their abundance from 
year to year is not influenced by the abundance or scarcity of any 
particular species of insect among the many upon which they prey 
they can not be ranked as elements in the facultative control of such 
species. On the contrary, it may be considered that they average 
to destroy a certain gross number of individuals each year, and 
since this destruction is either constant, or, if variable, is not corre- 
lated in its variations to the fluctuations in abundance of the insect 
preyed upon, it would most probably represent a heavier percentage 
when that insect was scarce than when it was common. In other 
words, they work in a manner which is the opposite of “facultative” 
as here understood. 
In making the above statement the fact is not for a moment lost 
to sight that birds which feed with equal freedom upon a variety of 
insects will destroy a greater gross number of that species which 
chances to be the most abundant, but with the very few apparent 
exceptions of those birds which kill for the mere sake of killing they 
will only destroy a certain maximum number all told. A little 
reflection will make it plain that the percentage destroyed will 
never become greater, much if any, as the insect becomes more com- 
mon, and, moreover, that after a certain limit in abundance is passed 
this percentage will grow rapidly less. A natural balance can only 
be maintained through the operation of facultative agencies which 
effect the destruction of a greater proportionate number of indi- 
viduals as the insect in question increases in abundance. 
Of these facultative agencies parasitism appears to be the most 
subtle in its action. Disease, whether brought about by some 
specific organism, as with the brown-tail moth, or-through insuffi- 
cient or unsuitable food supply without the intervention of any 
specific organism, as appears at the present time to be the case with 
the gipsy moth, does not as a rule become effective until the insect 
has increased to far beyond its average abundance. There are 
exceptions to this rule, or appear to be, but comparatively only a 
very few have come to our immediate attention. Finally, famine 
and starvation must be considered as the most radical means at 
nature’s disposal, whereby insects, like the defoliating Lepidoptera, 
are finally brought into renewed subjugation. 
With insects like the gipsy moth and the brown-tail moth disease 
does not appear to become a factor until a degree of abundance has 
been reached which makes the insect in question, ipso facto, a pest. 
Whether in the future methods will be devised for artificially ren- 
dering such diseases more quickly effective, remains to be determined 
through actual experimental work continued over a considerable 
number of years. 
