EXTENT OF GIPSY-MOTH PARASITISM ABROAD. 117 



greater rate of increase than sixfold would have to be met and offset 

 before the much-to-be-desired consummation could reasonably be 

 expected the prospects looked rather discouraging. Recognition 

 of the correlation which existed between increased abundance and 

 rate of increase served more than anything else to allay the doubts 

 which these reflections created. Field work in 1908, 1909, and 1910 

 showed pretty conclusively that a rate of increase of not in excess of 

 sixfold and possibly considerably less prevailed whenever the moth 

 was in that state of innocuousness incident to the scarcity which 

 it was hoped to bring about and maintain through the introduction of 

 the parasites. An aggregate parasitism of 85 per cent will almost 

 certainly be sufficient, and it may well be that 80 per cent or even 

 75 per cent will answer equally well. Much less than 75 per cent will 

 probably not be effective. 



THE EXTENT TO WHICH THE GIPSY MOTH IS CONTROLLED 

 THROUGH PARASITISM ABROAD. 



While it is true that the work which has been done for the purpose 

 of determining the prevailing rate of increase of the gipsy moth in 

 America leaves considerable to be desired in the way of exactness, in 

 the main the statements made by Forbush and Fernald, as confirmed 

 and modified by later observation, may be accepted as essentially 

 accurate. It is fortunate that the situation is no worse than it appears 

 to be, for if it were necessary to undertake the work of parasite intro- 

 duction with the idea that the maximum rate of increase exhibited by 

 the gipsy moth must be met by the parasites, such an unreasonable 

 percentage of parasitism would be demanded as to make the propo- 

 sition of introducing them a decidedly difficult task. As it is, there 

 seems to be good reason to believe that a parasitism amounting to 

 75 per cent will be sufficient, provided that it can be maintained 

 during the periods when the moth is relatively rare. It may be 

 that less than that will answer equally well, but it would require 

 actual test or else a much more careful study of the actual rate of 

 increase of the moth under favorable conditions to justify such 

 prophecy. In any event, 85 per cent will probably be amply efficient, 

 if it can be established and maintained during all stages in the abun- 

 dance of the moth. Such a rate would undoubtedly prevent the moth 

 from increasing to destructive abundance in new territory or from 

 regaining ground lost through the activities of disease in older infested 

 regions. 



Granted that it is sufficient, the question naturally arises as to 

 whether such a degree of parasitism is to be found abroad in coun- 

 tries where the gipsy moth is present without being considered as a 

 serious pest. 



