1908.] PUBLIC DOCUMENT- -No. 73. 185 



of parasites, which check it. This shifting relationship is an 

 illustration of the " balance of nature," so called, the tilting 

 adjustment of an insect to its enemies. 



The canker worm and tent caterpillar are equally good illus- 

 trations, except that the period of their ravages often extends 

 over two or three years before it is checked. Practically the 

 same thing holds good in Europe, with the two notable imported 

 caterpillar pests with which we in ISTew England, particularly 

 in Massachusetts, are contending, - - the gypsy and brown-tail 

 moths. The mid-temperate zone of Europe is their natural 

 home. Wherever the apple, pear and oak flourish, there they 

 may be found. They do not occur in the frozen north; they 

 are not found in the tropics ; but in the life zone mentioned 

 their periodical ravages are notable, and equally notable is the 

 fact that, no matter how severe may be their attacks on orchard 

 and forest trees, in a given year they soon subside and for many 

 years the insects are harmless, if not practically unnoticed. 

 This state of affairs is brought about by the same controlling 

 influences that govern the army worm, the canker worm and 

 the tent caterpillar in America; viz., the silent but mighty 

 work of parasitic enemies, which soon develop in multitudes 

 wherever the natural host is abundant. This increase of both 

 these important enemies of fruit and forest trees is eventually 

 checked and their ravages stopped by their parasites. 



While the work of exterminating the gypsy moth in Massa- 

 chusetts was in progress from 1890 to 1900, no effort was made 

 to secure the importation of parasites, since from the very na- 

 ture of the case such an effort would be an anomaly. The former 

 State work was for absolute extermination. Parasites require 

 a host to feed upon, and the State was bending all its energies 

 to absolutely destroy the host. When the work was stopped in 

 1900 the case became an altogether different one. It was known 

 by those best qualified to judge that the moth was bound to in- 

 crease and spread, and that after a few years control measures 

 would be the only ones available to protect our trees ; hence the 

 desirability of employing every such measure possible was ap- 

 parent. Among these none seemed to offer greater promise of 

 aid than the importation of the parasitic and predaceous enemies 

 of the moths in their native home. The moth pests in Europe 



