While they habitually accompany injurious insects wherever they are found, it may happen in a restricted region and an isolated plantation that the beneficial forms are absent, and there will be undoubtedly a benefit in introducing them. It often happens that coniferous forests are ravaged by insects without any of their most important enemies, such as Calosoma sycophanta. Will it not be opportune in such a case to transport a lot of these beetles from the region where they exist and acclimatize them in the devastated forests, where they have not appeared naturally?
Then also with sedentary insects, such as the scale insects, which develop often in closely circumscribed localities, it will be possible, when one chances to find a colony particularly invaded by parasites, to cut off certain branches and carry them into other orchards infested by scale insects and less favored from the point of view of the presence of parasites.
In 1871-72, Le Baron, in the United States, made some experiments in the transportation of the small hymenopterous parasite, Aphelinus mytilaspidis, from one locality to another, attaching the branches covered by parasitized scale insects to infested trees which were found in a region where the Chalcidid parasite did not exist. At the end of the year it was stated that the parasite had become domiciled in that locality.
Johnson has noticed that another parasite very close to the preceding, Aphelinus fuscicollis, may be extremely abundant in certain localities invaded by the San Jose scale, and be totally absent, on the contrary, in others, and he succeeded in propagating this insect by suspending upon a tree, at small distances, small baskets containing twigs covered with parasitized scale insects.
In France, Decaux was the promoter of the same method, and in 1872 had the honor of attracting attention to the question, making experiments in the transportation of parasites from one locality to another. However justifiable such practises may be in certain determined cases, one can not deny that they have not the certainty which they should have in order to be perfectly convincing. In fact, with an indigenous species, it is very difficult to say that it is, at a given moment, really absent from a locality. If it is absent to-day there is a great chance that it will appear to-morrow, coming from a neighboring region. The experimenter will find himself also exposed to possible criticism, not without reason, that he has attributed a result to his own work when nature would have perfectly accomplished the same thing without his intervention.
A more profound study of parasites and predaceous insects—of their development, their migrations, their geographical distribution—will show us without doubt and in a more precise way, the real value of the consistent method to be used in transporting indigenous parasites, and