OYSTER-SHELL BAEK-LOTTSE. 43 



available means of applying oil in a diluted form, liut 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. Robson'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. 



f 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 diflBcult this might be in 

 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 



