418 Bulletin No. 116. [_Augicst, 



tude and climate, between northern and southern Illinois; hence 

 each species must be studied in different parts of the state. The 

 most important species — that is the most abundant ones — are not 

 the same in- all parts of Illinois, and the relative importance of the 

 various species must be made out for each region separately. The 

 unexplained sudden disappearance, several times noticed by us, of 

 nearly the whole grub population of badly infested fields before 

 their transformation to the pupa, suggests the occurrence among 

 these insects of contagious diseases — a supposition borne out by sev- 

 eral field and insectary observations ; and the whole subject of their 

 bacterial and other fungus parasites consequently invites attention. 

 The enormous effect of the rapid multiplication, under favorable 

 conditions, of certain insect parasites — a hymenopterous enemy, 

 Tiphia inornata, especially — requires a critical and complete study 

 of the life history of these parasitic insects also, especially as there 

 is some probability that we shall be able to increase their efficiency 

 by artificial measures. 



No one has heretofore undertaken to work out to a finish this 

 difficult but highly important economic problem, and our present 

 knowledge of the white-grubs is a patchwork of fragments, contrib- 

 uted by a considerable number of observers working on various 

 species and in different parts of the country. The nearest approach 

 thus far made to a continuous investigation of the subject was car- 

 ried on in this office in the years of 1886-1890, and its results were 

 published in 1891 in Volume III. of "Insect Life."* 



They were also incorporated, with some later studies, in the 

 Eighteenth Report of the Illinois State Entomologist, printed in 

 1894. In the year 1906 I finally began what I now hope may prove 

 to be a steady and comprehensive study of this problem for the 

 state of Illinois ; and I have now to report some of the first results 

 of this work, which I have incgrporated, however, with other data 

 and conclusions in a way to give us a fairly full synopsis of the 

 present state of our knowledge, and a clear view, consequently, of 

 its deficiencies. 



The Economic Species. 

 The kinds of white-grubs common enough in Illinois to attract 

 attention because of their injuries, belong, so far as we now know, to 

 nine species, although it is likely that this list would be longer if the 

 injurious grubs of southern Illinois were as well known to us as 

 those of the c entral part of the state. Besides these nine injurious 



*"0a ihe Life History of the White-grnbs." By S. A. Forbes. Insect Life, Vol. III.. No. 

 5, pp. 239-245. 



