43 
[303 ] 
available means of applying oil in a diluted form. But as the 
proper time to apply this remedy is in the winter or early spring, 
when there is no foliage on the trees, it could undoubtedly be used 
much stronger than it was in Mr. Hobson’s experiments and mine. 
I cannot close this chapter, already, perhaps, too long, without 
briefly referring to a subject of the utmost interest and importance, 
and one directly suggested by the foregoing history, and that is, 
the practicability of transporting beneficial parasites from one 
part of the country to another, or if necessary, importing them 
from abroad. 
The incalculable benefits resulting from the depredations of 
parasitic insects upon those kinds which are injurious to mankind, 
are now generally known, and they can have no more striking 
illustrations than those furnished by the history of the Chalcis-fly 
in a former part of this chapter, and the parasitic Tachina of the 
Tussock-moth, described in the first article of this report. It is 
also a notorious fact that many of our most pernicious insects have 
been imported from abroad, and one reason why they have proved 
so intractable, is, that in introducing the noxious insect, we have 
failed to import with it the natural enemies which held it in check. 
Mr. Walsh was so impressed with the importance of this subject 
that it became almost a hobby with him, and he went so far as to 
advocate the artificial breeding of parasitic insects, if they could 
not be otherwise obtained. However difficult this might be m 
ordinary cases, since we should also be obliged to rear the noxious 
species upon which the parasite subsists, yet that the transporta¬ 
tion of them, at least, is not necessarily impracticable, is very 
clearly shown by the case of the Chalcis of the Bark-louse. One 
brood of this insect passes the winter in the larva or pupa state 
under the scale of the Bark-louse, at whose expense it has sub¬ 
sisted, ready to emerge on the opening of the succeeding summer. 
The twigs of apple trees, where the Chalcis is known to abound, 
could be easily gathered any time in the winter or spring, and 
carried to any "other part of the country, or even to a foreign land, 
and all that would then be necessary would be to tie these twigs, 
here and there, upon the branches of the trees which it is desired 
to protect. That this operation will ever have to be performed 
with this particular species, is not, perhaps, very probable, but 
» 
