46 IMPORTED PARASITES. 



PROGRESS OF THE PARASITES IN MASSACHUSETTS. 



Before it is possible accurately to predict the progress of an 

 insect in a new country, it is necessary to know, first, the average 

 rate of increase under the new conditions; and, second, the 

 average rate of dispersion. Neither may be determined other- 

 wise than by actual observation in the field. In the countries 

 to which the insect in question is native, the rate of increase 

 is balanced more or less perfectly, and, although it always fiuc- 

 tuates somewhat in relative abundance from one year to another, 

 there is no permanent gain or loss. The dispersion of an insect 

 in a country where it is native and generally distributed is im- 

 possible of determination; the progeny of any given parent or 

 of the parents within a given area are at once confused with 

 the progeny of parents in any part of the surrounding country 

 into which they may chance to spread. 



The introduction of an insect into a new country is usually 

 followed by a steady increase, which is sustained until it has 

 established a balance with the native insects; or by a steady 

 decrease, which results in its final extinction. 



There are a few instances on record in which the progress of 

 an introduced insect has followed neither path. The Chinese 

 lady-bird (Chilchorus similis) was introduced into Georgia in 

 1902 as an enemy of the San Jose scale, and for one year in- 

 creased at a very rapid rate, and spread over a considerable 

 territory from the point where it was liberated. It passed the 

 first winter successfully, and for a time bade fair to become 

 so numerous as to be of valuable assistance in the fight against 

 this scale; but in 1904 its numbers showed a decided decrease, 

 and at the present time it appears not to be at all common. The 

 causes for this are very obscure, and no satisfactory explanation 

 has ever been advanced. 



The history of the Oriental moth in Boston, where a few 

 years ago it appeared to have become firmly established, is an- 

 other case in point. At the present time it is far from common, 

 and it is very possible that eventually it will become extinct. 



It will never be known how many insects have been introduced 

 into America from abroad, but the number is undoubtedly far 



