68 INSECTS AND HUMAN WELFARE 



During the past decade certain parts of eastern Canada 

 and a considerable portion of the State of Maine have suf- 

 fered from an invasion of the spruce bud-worm (Tortrix fumi- 

 ferana), an insect which defoliates the balsam fir and spruce. 

 This insect has moved like the incoming tide and extended 

 its destructive range from year to year. The spruce woods of 

 northern Maine that furnish much of the pulpwood from 

 which some of our better class papers are produced have 

 suffered severely for several years. During this period certain 

 areas have in succession lost a goodly part of their tree popu- 

 lation of spruce and balsam fir, after which the insects have 

 suddenly disappeared. If we can judge by the past history 

 of this species, it will not again reappear in destructive abun- 

 dance for many years. As a result of its ravages, however, 

 there is grave danger that these forests may be replaced by 

 more rapidly growing and less valuable hardwood trees which 

 would not under natural conditions again give way to spruce 

 and fir for perhaps two or three hundred years. In the his- 

 tory of the world this is a mere incident, but it is a very se- 

 vere blow to one of our important present-day industries. 



In the northwestern states and western Canada there is a 

 small, pale yellowish-white butterfly, quite similar to the 

 common European cabbage butterfly now so abundant in the 

 United States. It feeds during its larval stages upon the fo- 

 liage of several kinds of western conifers, and over the short 

 period during which it has been observed, has several times 

 suddenly appeared in incredible numbers in rather definitely 

 circumscribed areas. At such times, extensive portions of the 

 forests are defoliated, and the trees succumb as a result of the 

 injury. 



This insect and others that produce the general defoliation 

 of coniferous trees may not always cause the immediate death 

 of the trees, but in cases where the weakened trees might 

 otherwise recover, they open the way for depredations on 

 the part of another group of destructive insects known as the 

 bark-beetles. The latter are small, brownish or blackish, 

 hard-bodied insects, represented by numerous species that 



