parasitic insects can be utilized to advantage that are not only prolific and will endure the climatic conditions into which they have been artificially introduced, but will maintain very definite relations only with individuals of a single or of a very few host species and destroy them in their earliest possible ontogenetic stage before they can do extensive damage. Such constancy is especially necessary in primary and tertiary parasites, since whenever these show a tendency to become secondaries and quaternaries, as is sometimes the case, they become harmful instead of beneficial.
It is clear that the determination of the constancy or invariability of parasitic reactions as a basis for practical applications requires, if anything, an even greater insistence on the experimental method than does the determination of the range and character of modifiability for purely theoretical purposes. Ever since the days of Redi both theoretical and practical entomologists have resorted to the experimental method and therefore have no reason to regard themselves as behind the times in appreciation of what some zoologists have been heralding as a recent dispensation. In other respects, however, the students of insect life are "old fashioned" and resemble the botanists more closely than the zoologists, in that they are constrained by the extraordinary intricacy of their science to maintain the closest and most sympathetic cooperation with the taxonomists, morphologists, and students of geographical distribution. Without this cooperation their studies of insect parasitism would resolve themselves into a weltering chaos.
- Howard and Fiske (loc. cit., p. 204) express a similar opinion when they say that "it is probably true also that among those parasites which are the most closely restricted in their host relationships are to be found those which are the most effective in bringing about the control of their respective hosts. This is primarily due to the fact that a correlation usually exists between the life and seasonable history of such a parasite and some one or more hosts which it is particularly fitted to attack. The existence of a correlation between parasite and host of such intimate character makes possible the continued existence of the parasite independently of alternate hosts, and it is thus enabled to keep pace with the ODe species upon which it is peculiarly fitted to prey when other circumstances are favorable to its increase. Some of the most interesting examples of correlation of this sort which have yet come to attention are to be found among the tachinid parasites of the gypsy moth or the brown-tail moth, and on this account as well as on a purely empirical basis they are now considered much more likely to become important enemies of these hosts than before their characteristics were so well understood."
- A very instructive case of such instability in hyperparasitism, or rather superparasitism, is seen in Pteromalus egregius, which was introduced into Massachusetts as a primary parasite of the brown-tail caterpillar. This European parasite, as Fiske has recently shown (Howard and Fiske, loc. cit., p. 267 et seq.) has not only spread over a great area in eastern New England, since it was first liberated in 1906 and 1907, but besides acting as a primary parasite, it may also behave as a secondary, tertiary or quaternary superparasite.