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, if so 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. 



