ILLINOIS HORTICULTURAL SOCIETY. 259 



€xcite particular attention, but come to multiply them the next year by 

 five hundred, or even two hundred and fifty, supposing but half of them 

 to be females, and we become painfully conscious of their presence. 



The insects which I have just named, however, are not those which 

 are to-day of the greatest interest to the horticulturists of northern lUi- 

 Bois. The two insects which I believe have the highest claim to this 

 ■distinction, are the oyster-shell bark -louse and the canker worm. These 

 are both long known insects, but to each of them a peculiar interest at- 

 taches at this time, and especially in this northern section of the State. 



The oyster-shell bark-louse, which was formerly the worst scourge 

 •which the apple orchardist had to contend against throughout all the 

 northern states, has been, for a considerable number of years past, ap- 

 parently in a process of gradual extinction. This is emphatically a 

 northern insect, never having been known to do much damage south of 

 the thirty-ninth parallel of latitude; and for a considerable range north 

 of that line, this insect does not appear to flourish. Mr. Walsh, in his 

 report of r868, speaks of having received apple twigs infested with bark- 

 lice scales, from as far south as Macoupin county in this State, but he 

 found that under most of them the eggs had been destroyed, as he had 

 reason to suppose, by acari, or mites. 



Farther north, in the latitude of Chicago, another minute parasitic 

 enemy of the bark-louse is found to prevail, which is more destructive 

 to it than even the acari. This is a beautiful little chalcis fly, less than 

 the twentieth of an inch in length, of a lemon yellow color, which your 

 speaker had the pleasure of discovering in the fall of 1870. This fly is 

 so minute that it would in all probability have escaped our notice, were 

 it not for the fact that after its larvae has devoured the eggs of the bark- 

 louse under the scale, the perfect fly makes its escape by cutting a per- 

 fectly round hole through the scale, and these holes constitute indelli- 

 ble marks which last for several years, that is, as long as the scales ad- 

 here to the tree, and which thus enable us to estimate with mathemati- 

 cal certainty what proportion of the bark-lice are by this means 

 destroyed. It appeared from observations made in the counties of Kane 

 and DuPage, that more than half of the bark-lice had been destroyed 

 by the larvce of this minute fly. 



The great difficulty in contending with bark-lice has arisen from what 

 might be called their centrifugal instinct, that is, their disposition to 

 disseminate themselves outwardly upon the terminal twigs, where our 

 destructive applications can but very partially reach them, liut where 

 man, with his clumsy appliances is baflled, these tiny parasites are in 

 their element. They follow their victims, as weasels follow rats, to their 

 ultimate resorts, and having found them, they adhere to them with a more 

 than brotherly tenacity. They enter the domicil pf their victim with- 

 out ceremony ; sit down at his table without being bidden ; devour his 

 substance without compunction ; and finish their rei)ast by killing and 

 eating the host himself. I will not undertake to defend their course 

 upon strictly moral considerations. But I do not advise any one to in- 



