30 



The corn acreage does not vary in Southern Illinois, as com- 

 pared with our grades of total damage, in a way to make it worth 

 discussing. 



Diagram III. The diagram of total injury for Central Illinois 

 (III. ) conveys some extremely interesting and useful information, es- 

 pecially with respect to the relation of oats culture to the chinch bug. 

 The average wheat acreage makes, on the w^hole, a rapid rise as 

 the total loss by chinch bugs increases (line A); while the corre- 

 sponding average for oats (line B) decreases at first, — that is where 

 chinch bugs are less numerous, — but increases for the higher grades 

 of loss, — where the bugs are more abundant. Otherwise stated, in 

 those townships of Central Illinois where the chinch bug is in- 

 jurious but has not yet become destructive, its numbers vary di- 

 rectly with the acreage of wheat and inversely with that of oats; 

 but in those townships where it has become very abundant, it has 

 already begun to breed in oats, and thenceforth its multiplication 

 is stimulated by an increased oats acreage not less than by 

 an increased area in wheat. We have here fully developed the 

 fact barely noticeable in Diagrams I. and II., that where this in- 

 sect injury is not yet great, it will, as a rule, be heaviest w^here 

 there is most wheat and least oats, but that where it becomes 

 severe, oats and wheat combine to increase its severity. 



The corn and grass figures of the tables from which Diagram 

 III. was drawn are too variable to have any significance, and hence 

 have not been diagramed. 



Diagram IV. The same may be said for that for Northern 

 Illinois, only the column for wheat (Diagram IV.) having any 

 particular meaning; and even here the ascending slope of the line 

 for wheat is but slight. Still, it is to be noticed that the 

 wheat area in the northern districts most injured by the chinch 

 bug, was twenty-five per cent, greater, on an average, than in 

 those not injured at all. 



Diagrams V. and VI. Finally, for a concluding summary of 

 all the diagrams and tables for 1887, we turn to diagrams V. and 

 YL, showing the acreage in each crop for the whole State, corre- 

 sponding to the various degrees of total chinch-bug damage. The 

 larger number of observations here included obliterates many of the 

 more prominent irregularities of the other diagrams, and brings out 

 without complication the main features of a broad general con- 

 clusion. We see repeated here the marked contrast of conditions 

 between Central and Southern Illinois (due, as already often ex- 

 plained, to the widely different stage of insect increase): the rapid 

 rise of the wheat line (V., B. ) and the slower falling away of the line 

 for oats (V., A.) for the lower grades of injury; the rapid rise of the 

 latter line and the slower falling of that for wheat for the higher 

 grades, — tlie upward turn in the oats line being made a little in 

 advance of the downward turn of that for wheat; the similar but 

 less marked contrast between the lines for corn and grass (Dia- 



