April, '08] JOURNAL OF ECONOMIC ENTOMOLOGY 123 



THE CORRELATION BETWEEN HABITS AND STRUC- 

 TURAL CHARACTERS AMONG PARASITIC 

 HYMENOPTERA 



By Charles T. Bbues, PuMic Museum, Milwaukee, Wis. 



The problem of insect parasitism has always been a fascinating one 

 from the standpoint of pure science, but during recent years it has 

 become an increasingly important one for the economic entomologist. 

 Indeed it has been discussed so fully and in so many aspects that I. 

 feel much hesitancy in adding the present remarks to the writings of 

 many able entomologists better acquainted with the more or less 

 heterogeneous mass of facts so far accumulated on the subject. My 

 only desire is to present the matter in a somewhat different light. 



The rapidity of increase among injurious insects which become in- 

 troduced into new regions where they are not kept in check by their 

 parasites was early noted and commented upon by entomologists, and 

 certain experiences in our own country within the past few decades 

 have brought out very clearly the fact that of all the forces which 

 control the comparative abundance of related insects, the presence of 

 their parasites is the most vital. 



The balance maintained by the struggle for existence between species 

 is immediately and violently disturbed if the parasites of any par- 

 ticular species be removed. Such a form suddenly begins to increase 

 in numbers, reproducing itself at a phenomenal rate approaching the 

 geometrical progression, which would theoretically obtain if every 

 individual were permitted to reach maturity and reproduce itself. 

 When the food supply is sufficient it will quickly become dominant 

 over related species. Such conditions of rapid increase occur almost 

 exclusively as the result of the introduction of an insect into a locality 

 where its natural parasites do not occur, and are on this account most 

 often brought to our notice by the rapid spread of injurious species. 



Following the acceptance of this principle, was the attempt on the 

 part of economic entomologists to combat accidentally introduced in- 

 sects by purposely introducing the parasites which prey on them in 

 their native region. The experiment has been tried a number of times 

 under varied conditions and has proven almost universally successful 

 in measure to warrant its trial whenever feasible. 



There are vast numbers of parasitic insects, particularly Hymenop- 

 tera. These are widely distributed, and a very close relationship ex- 

 ists between allied species and genera inhabiting widely separated 

 regions. It would be natural to suppose, therefore, that the transfer 

 of an insect from one region into another would lay it open to attack 



