EXTENT OF GIPSY-MOTH PARASITISM ABEOAD. 119 



the number of hosts originally involved and that of the number suc- 

 cessfully completing their transformations. If from a lot of 1,000 

 brown-tail caterpillars 250 individuals of Parexorista chelonix and 

 250 moths are reared, it is perfectly safe to assume that parasitism 

 by Parexorista amounts to more than 25 per cent and less than 75 

 per cent. Further than this nothing absolutely definite may be said. 

 Exactly the same is true of the determination of prevailing rates of 

 parasitism of native insects through rearing work. 



On account of the inadequacy of these methods when it comes to 

 the point of securing absolutely authentic information, not nearly so 

 much is known of the parasitism of the gipsy moth or of the brown- 

 tail moth abroad as is needed to carry on the work to its best advan- 

 tage. This much, however, can be said definitely, that in some 

 instances existing parasitism is sufficient to answer the requirements 

 of the situation in America; in others it is obviously insufficient; in 

 most the results of the study of imported material are not sufficiently 

 reliable to support either contention. 



Here, again, was food for serious consideration when it came to 

 the point of making definite recommendations concerning the con- 

 tinuation of the work. Would the foreign parasites certainly meet 

 the demands which would be made upon them in America? 



This has been answered in the affirmative through its considera- 

 tion from quite a variety of different viewpoints. For one thing, 

 the lack of accurate information as to the conditions under which the 

 parasite material was originally collected, has rendered the results 

 of its study in America of difficult analysis. No one, for example, 

 would seriously question the statement that the white-marked tus- 

 sock moth is under well-nigh perfect control in America except in 

 cities. Nevertheless, if it was desired to transport caterpillars or 

 pupae of this insect to Europe in order that its parasites might be 

 reared, the agent intrusted with the collection of the material for 

 exportation would certainly go to the city for it, and the person who 

 received and studied it upon the other side would find so few para- 

 sites present as to justify exactly the same doubts concerning the 

 parasitism of the tussock moth in America as have actually arisen 

 concerning the parasitism of the gipsy moth and the brown-tail moth 

 in Europe. The tussock moth is not often subjected to the full 

 extent of parasitism necessary to effect its control in any locality 

 from which caterpillars can be secured in quantity. It is reasonable 

 to suppose that something of the same sort is true of the gipsy moth 

 or of the brown-tail moth. 



Furthermore, the study of the tussock moth has resulted in demon- 

 strating another fact which is of peculiar interest in this connection, 

 which is, that the parasites which assist in effecting its control in 

 country districts where this control is perfect are sometimes entirely 



