114 PARASITES OF GIPSY AND BROWN-TAIL MOTHS. 



Without attempting to go into the details of rather elaborate cal- 

 culations, which were made for the purpose of bringing out this point 

 more graphically, attention is merely called to the three divisions 

 into which the elements operative in the natural control of any insect 

 naturally fall as they were outlined in the preceding section. These 

 are, first, the catastrophic (storms, etc.), which result in the destruc- 

 tion of a certain fixed percentage, irrespective of the abundance of 

 the insect; second, that represented by the birds, most other preda- 

 tors, and a part of the parasites which encompass the destruction of 

 a certain gross number, rather than of any given percentage each 

 year or generation; third, the facultative agencies, of which certain 

 parasites are considered to be typical, which increase in efficiency as 

 the insect increases in abundance. 



The elements composing this last group are absent in New England, 

 or, rather, those elements which are present (disease and starvation) 

 and which do not properly belong to it, are inoperative until a state 

 of extreme abundance is attained. 



Such control as is effected by existing agencies would therefore fall 

 into one or the other of the first two groups mentioned, and since 

 both groups together are obviously inefficient, even when the moth 

 is scarce, that due to the operation of the elements falling in the 

 second group would become relatively less efficient as the time went 

 on and the moth increased. This, it is believed, is actually what has 

 happened and what is happening each year in each of the very 

 numerous outlying colonies of the gipsy moth throughout the more 

 recently infested territory, and thus the larger rate of increase is 

 explained. 



As a matter of fact, there is reason to believe that the average rate 

 of increase during the first few years immediately following the intro- 

 duction of the moth in a new locality is actually less than sixfold 

 annually, and that it may even be as low as threefold, or perhaps less. 

 In any case, there is a stage in the progress of the moth in which the 

 average is no greater than that recorded by Forbush and Fernald, 

 and there is no longer room for doubt that the lowest rate of increase 

 is in localities where the moth is relatively a rare or uncommon 

 insect for the time being, while the highest occurs in localities where 

 the moth is rapidly approaching its maximum abundance. 



AMOUNT OF ADDITIONAL CONTROL NECESSARY TO CHECK THE 

 INCREASE OF THE GIPSY MOTH IN AMERICA. 



It was evident in 1907, as it is now, that the problem of the intro- 

 duction of parasites was far from being as simple as it might appear 

 to be upon its surface and as it evidently did appear to be to some 

 who were at that time agitating for a radical change in the methods 

 adopted for its solution. It was plain that the expense incident to 



