326 PRINCIPLES OF ECONOMIC ENTOMOLOGY 



effectively to keep the destructive species in reasonable 

 bounds and often preventing it from causing any serious 

 loss. The Hessian fly is certainly kept in check during a 

 considerable portion of the time by just such agencies, and 

 we can scarcely doubt that if the parasites of this species 

 were eliminated and other conditions unaffected, the losses 

 incurred would be enormously increased. 



With some species it may be possible to assist the para- 

 sitic forms or to preserve them in such manner as to get the 

 advantage of their service. For instance, the cabbage 

 l)utterfly is very commonly parasitized by a small ichneu- 

 monitl {Ftcromahis) which issues from the pupa?, and it is 

 evident that if the pupse were gathered and instead of being 

 crushed were enclosed in wire screen of such mesh as to 

 retain the issuing butterflies but allow the free escape of 

 the minute parasites, there would be a destruction of the 

 healthy butterflies without any reduction in the numbers of 

 the parasites. Again, in the case of the Hessian fly, care 

 as to the time when the stubble is burned or plowed under 

 in order to allow opportunity for escape and survival of 

 the ])arasites, might be of special service in their protection. 

 In California extensive shipments of lady l)ugs from the 

 northern part of the State to the Imperial valley to prey 

 upon the plant lice affecting melons is claimed to have 

 accomplished much in their control. 



On the whole, however, the utilization of parasites in a 

 direct manner can hardly be depended upon as a very great 

 advantage, especially because of the difficulty in so training 

 the average cultivator that he will be able to distinguish 

 between parasites and non-parasitic forms, and adapt his 

 practice to accommodate them. 



A more important phase of the subject perhaps is found 

 in the transportation of parasitic species from one country 

 to another where an injurious species has been introduced 

 without the introduction of its native enemy. It has already 

 been suggested that the original habitat of an injurious 

 insect is a matter of great importance, especially with 

 reference to its natural parasites, and a number of instances 



