46 



north.* Harris's Bark-louse, on the contrary, flourishes vigorously, 

 to my certain knowledge, so as to be a great pest in the latitude of 

 Philadc]p].'ia, which is somewhat south of Champaign; and in Mis- 

 souri it probably extends to a point at least 180 miles further south, 

 where it does a great amount of damage. It occurs also in consid- 

 erable numbers throughout the wliole S.tate of Illinois, but is no- 

 where anything like so destructive as the Imported Species. Indeed 

 almost all our worst Noxious Insects have been imported from the 

 Old World, and are far more destructive than the corresponding spe- 

 cies indigenous to North America — a curious fact which I have 

 explained and illustrated at some length in the Practical Entomolo- 

 gist, (Vol. I., No. 12.) 



"But," the reader will ask, '^low am I to distinguish these two 

 Bark-lice, the one from the other?" The answer is short and simple. 

 The scale of the Oyster-shell Bark-louse is the shape of a very 

 elongate pear, considerably hunched, and of the exact color of the bark. 

 That of Harris's Bark-louse is usually the shape of an egg, almost 

 entirely fiat, and of a pure milk-white color. Lift up the former 

 with the point of a penknife any time between the middle of Sep- 

 tember and the middle of May, and you will see underneath it a score 

 or two of very minute milk-white eggs, many of which will often drop 

 out and on any dark surface look like so many grains of corn-meal. 

 Lift up the latter in the same manner and at the same period, and 

 you will find that the eggs, though of the same size and shape as those 

 of the other species, are not milk-white, but blood-red. If these dis- 

 tinctive characters are not sufficient — and they certainly ought to be 

 sufficient in all conscience — the reader can refer, in addition, to the 

 figures of the two kinds of scales given in the Practical Entomologist, 



"Since the above was writteiij I have received the Oyster-shell Bark-louse 

 from Mr. J. Hiiggins, of Macoupin county, Central Illinois, with a statement 

 that it swarms at Ship)uan, in that county, on two trees that were imported 

 eight years ago from New York, though "the other trees in the orchard are 

 not yet seriously affected." On examining the infested twigs sent by Mr. 

 Huggins, I found tliat about 19-20ths of the eggs under the scales had been 

 destroyed by the same Cannibal Mite that, as will be afterwards shown, 

 operates upon them in Northern Illinois. Now, in North Illinois, the lar- 

 gest proportion of eggs, that I ever found to be destroyed by this Mite, was 

 only two-thirds. Hence, I infer that the Mite is a far more efficient check 

 upon the multiplication of this Bark-louse in Southern than in Northern lat- 

 itudes. Certainly if this Bark-loiise had been introduced into any county 

 in North Illinois eight years ago, it would have been all over the county 

 long before now ; whereas, in . Macoupin county it seems to have scarcely 

 spread beyond the two trees on which it was originally imported eight years 

 ago. 



