PARASITIC AND PREDACEOUS INSECTS. 683 



sequent exceptional injury done by the injurious species. Now, 

 the insecticide method of routing this last, under such circum- 

 stances, too often involves, also, the destruction of the parasitic 

 and predaceous species, and does more harm than good. This is 

 particularly true of those of our Coccidm and Aphidida^, and 

 those of our lepidopterous larvae, which have numerous natural 

 enemies of their own class, and it not only emphasizes the impor- 

 tance of preventive measures which we are all agreed to urge for 

 other cogent reasons, and which do not to the same extent destroy 

 the parasite ; but it affords another explanation of the reason why 

 the fight with insecticides must be kept up year after year, and 

 has little cumulative value. 



But the problem of the wise encouragement and employment 

 of the natural enemies of injurious insects in their own class is 

 yet more complicated. The general laws governing the interac- 

 tion of organisms are such that we can only in very exceptional 

 cases derive benefit by interference with it. The indigenous ene- 

 mies of an indigenous phytophagous species will, ccEteris paribus, 

 be better qualified to keep it in check than some newly introduced 

 competitor from a foreign country, and the peculiar circumstances 

 must decide in each case the advisability of the introduction. 

 The multiplication of the foreigner will too often involve the 

 decrease of some indigene. If a certain phytophage is generally 

 disastrous in one section and innocuous in another, by virtue of 

 some particular enemy, it will be safe to transfer and encourage 

 such enemy, and this is particularly true when the phytophage is 

 a foreigner and has been brought over without the enemy which 

 subdues it in its native home. Icerya had some enemies in Cali- 

 fornia, presumably American ; but they were not equal to the 

 task of subduing it. Vedalia in the icerya's native home, Aus- 

 tralia, was equal to the task, and maintained the same superiority 

 over all others when brought to America. The genus was new to 

 the country, and the species had exceptionally advantageous at- 

 tributes. But there is very little to be hoped from the miscellane- 

 ous introduction of predaceous or parasitic insects for the sup- 

 pression of a phytophage which they do not suppress in their 

 native home or in the country from which they are brought. 

 The results of the introduction by Mr. A. D. Hopkins of Clerus 

 formicarius to contend with the scolytids, which were ruining 

 the West Virginia pines, were doubtful, for the reason that the 

 indigenous species of the genus were already at work in America. 

 Yet the experiment was safe and desirable because the European 

 clerus is more active and more seemingly efi^ective than our indi- 

 genes. The gypsy moth was evidently introduced into Massa- 

 chusetts without its European natural enemies, and as in some 

 parts of Europe it is often locally checked by such natural ene- 



