240 



THE AMERICAN ENTOMOLOGIST. 



TEMPERATURE AND RAINFALL AS AFFECTING 



THE CHINCH BUG-PERIODICITY IN 



ITS INCREASE. 



BY PROF. CYRUS THOMAS. 



While engaged in preparing my work of 

 1879 — which related in part to the chinch 

 bugs — for publication in the third annual 

 report of our Commission, I concluded to 

 make a more careful study of the meteoro- 

 logical conditions bearing upon their de- 

 velopment than had been heretofore made. 



This attempt has brought to light certain 

 facts which I think will be of interest to 

 the readers of the Entomologist. For 

 the purpose of this investigation I selected 

 Illinois and the immediately adjoining 

 portions of Iowa and Missouri, first, be- 

 cause the history of the chinch bug has 

 been more thoroughly written up for this 

 section than any other, and second, because 

 the meteorological records, though not 

 complete, are fuller for this section than 

 any other portion of the chinch bug area. 

 Confining my investigations to this area, 

 excluding the Cairo record as not belong- 

 ing to the same limited climatic type, and 

 rejecting the early Sandwich record as 

 doubtful, I have combined the rainfall rec- 

 ords of the other stations, and taken the 

 average for each year. 



Commencing with the Athens record in 

 1840, I have calculated the annual average 

 up to 1877, the latest records I have at 

 hand. Having ascertained the general 

 average, which I find to be 38.30 inches 

 per year, I next found the variation of 

 each year from this average. 



These variations I have represented by 

 curves, as here shown. The temperature 

 will also be considered, and is shown on 

 the diagram. 



The points of this graphic representa- 

 tion, to which I desire to call attention at 

 present, are as follows: 



First. — The fact that the series appears 

 to be divided into cycles of seven years 

 each. For example: if we commence with 

 1876, which is considerably above the 

 average, and count back by sevens, we 

 shall find that each seventh year was above 

 the average, to wit: 1869, 1862, 1855, 1848, 



1 841. If we commence with one below 

 the average we find the same rule holding 

 true in rhost cases; thus, commencing with 

 1874, and counting back, we find 1867, 

 i860 and 1853 below the average, 1846 

 forming an exception. Commencing with 



1842, which was below the average, we 

 find 1849 on the average line, and 1856, 

 1863 and 1870 all below it. 



Secondly. — A careful examination reveals 

 the fact that this septenary period is 

 divided into two sub-periods of four and 

 three years. 



This will be apparent if we commence 

 with 1848 and count forward; the ternary 

 period coming first, and the quarternary 

 second; thus, 1851, then 1855, 1858, then 

 1862, 1865, then 1869, 1872, then 1876. 



Thirdly. — The relation of the rainfall to 

 the appearance of the chinch bugs. As I 

 have argued in Bulletin No. 5, U. S. Ento- 

 mological Commission, and elsewhere, and 

 as I believe is now generally admitted, two 

 successive dry years are necessary to the 

 development of these insects in large and 

 injurious numbers. According to this 

 theory, as applied to our graphic delinea- 

 tion, the only years of the series repre- 

 sented which could have been serious 

 chinch-bug years are the following : 1854, 

 1857, i860 or 1861, 1864, 1871 and 1874 

 or 1875. 



The chronological history of the species 

 shows that, although appearing in inter- 

 mediate years in limited localities, the 

 great chinch-bug years in the region desig- 

 nated were 1850, 1854, 1871 and 1874, the 

 northern region suffering considerably from 

 its depredations in 1861 and 1864. In this 

 series 1850 forms an apparent exception 

 to the rule; but if we take the average rain- 

 fall for the first twelve years — 1840-- 

 185 1 — we find it to be 40.97 inches, instead 

 of 38.30 inches. The rainfall of 1849 was 

 38.49 inches, and 1850 but 40.69 inches, 

 showing that the year 1850 really formed 

 no exception to the rule. In 1862 these 

 insects appeared in considerable numbers 

 in some sections, but were destroyed by 

 the wet weather ; a considerable number 

 appeared also in 1865 in northwestern Illi- 



