INSECT PARASITISM 449 



parasitic insects can be utilized to advantage that are not only prolific 

 and will endure the climatic conditions into which they have been arti- 

 ficially 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. 21 Such constancy is especially necessary in primary and ter- 

 tiary parasites, since whenever these show a tendency to become secon- 

 daries and quaternaries, as is sometimes the case, they become harmful 

 instead of beneficial. 22 



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 Eedi both theo- 

 retical 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 geo- 

 graphical distribution. Without this cooperation their studies of insect 

 parasitism would resolve themselves into a weltering chaos. 



21 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 para- 

 site 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 consid- 

 ered much more likely to become important enemies of these hosts than before 

 their characteristics were so well understood." 



22 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. 



VOL. LXXIX.— 31. 



