678 THE POPULAR SCIENCE MONTHLY. 



PARASITIC AND PREDACEOUS INSECTS.* 



By C. V. EILEY, Ph. D. 



THE importance to man, and especially to the horticulturist, 

 of the parasitic and predaceous insect enemies of such species 

 as injure vegetation has been recognized by almost all writers on 

 economic entomology. Indeed, it is a question whether the earlier 

 writers did not attach too much importance to them, because 

 while in the abstract they are all essential to keep the plant-feed- 

 ing species in proper check, and without them these last would 

 unquestionably be far more difficult to manage, yet in the long 

 run our worst insect enemies are not materially afi^ected by them, 

 and the cases where we can artificially encourage the multiplica- 

 tion of the beneficial species are relatively few. While fully ap- 

 preciating the importance of the subject, therefore, it is my pur- 

 pose in this paper to point out the dangers and disadvantages 

 resulting from false and exaggerated notions upon it. 



There are but two methods by which these insect friends of 

 the farmer can be eff^ectually utilized and encouraged, as, for the 

 most part, they perform their work unseen and unheeded by him, 

 and are practically beyond his control. These methods consist in 

 the intelligent protection of those species which already exist in 

 a given locality, and in the introduction of desirable species which 

 do not already exist there. 



In a few cases like this there is no reason why the farmer 

 should not be taught with advantage to discriminate between his 

 friends and his foes, and to encourage the multiplication of the 

 former ; but, for the most part, the nicer discrimination as to the 

 beneficial species, some of the most important of which are micro- 

 scopically small, must be left to the trained entomologist. Few 

 of the men practically engaged in agriculture and horticulture 

 can follow the more or less technical characterization of these 

 beneficial species, and where the discriminating knowledge is pos- 

 sessed it can, as just intimated, only exceptionally be turned to 

 practical account. 



In other cases much good may be done without any special 

 knowledge of the beneficial forms, but as a result of a knowledge 

 of the special facts, which enable the farmer materially to en- 

 courage the multiplication of parasitic species while destroying 

 the plant-feeding host. Very good illustrations of this kind of 

 work are afforded by the rascal leaf-crumpler and the common 

 bag worm, both of which in the larva state live in cases, and 



* Condensed from a paper read before the Association of Economic Entomologists at 

 Madison, Wis., August 15, 1893. 



