84 INSECT ENEMIES OF THE BOLL WEEVIL. 



of other agencies, that the effects of one or two agencies of control 

 with known strength can not be estimated, because of the many 

 other agencies either lmkno^^^l or of unknown strength. 



3. When an injurious insect escapes from its natural surroundings 

 to a region where conditions are favorable for enormous reproduction, 

 it may become a pest, but it is never absolutely free of natural checks. 

 Anthonomus grandis has never been free of its checks although it 

 escaped those of its native home. These agencies of insect control 

 are inherent to all countries. An insect parasite is as Hkely to escape 

 its original surroundings as a phytophagous insect. 



4. When an insect finds in its vicinit}' a variet}^ of food closely 

 related to its native host, and that food is more succulent or more 

 abundant, there is a possibihty that sooner or later the more in^dting 

 food 'will become the normal food. Tliis possibiht}" becomes stronger 

 when the original food supply fails, if the species is to be continued. 

 Not only phj^tophagous but entomophagous insects have frequently 

 been proven to have thus changed their food habits — whether from 

 preference or necessit}^ it is not kno'svn. Leptinotarsa decemlineata 

 (the Colorado potato beetle) is an excellent example of this change of 

 habit among phytophagous insects. All of the boll-weevil parasites 

 are examples of parasites which have adjusted their habits in the 

 presence of their original hosts. 



5. A crisis in the history of a species occurs whenever the food 

 supply fails. The species may either disperse in search of food, as 

 the boll weevil does each autumn, or it may hibernate or sestivate, 

 or it may select a new host, or the species may perish. All these 

 results occur in nature. All of these alternatives may be chosen by 

 different individuals of the same species. It is safe to assume that 

 when a species is found to have many hosts that it has undergone 

 many crises and that the resultant species is a highly developed and 

 adaptable form. A species most limited in food habit is most hable 

 to restriction or extinction and consequently of a lower type than 

 one able to meet any emergency. 



6. When a desirable parasite species is kno\\Ti to be adaptable to 

 various hosts, a crisis may be artificially superinduced by the timely 

 elimination of the favorite hosts, thus forcing the species to attack 

 the most predominant near-by related host in the vicinity, or it may 

 be taken to an entirely remote or foreign locality and placed near a 

 field containing many insects closely related to its original host, 

 which it may learn to thrive upon. 



7. The species most available for utilization are those most adapt- 

 able to changing environments, or those ha^-ing the most hosts in 

 the given locaHty. These other hosts mil serve as nurseries for 

 parasites. 



