EXTENT OF GIPSY-MOTH PARASITISM ABROAD. 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 chelonie 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 
percent. 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 
pupe 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 
m 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 
