FOREST INSECTS 



67 



as parasitic insects, in combination with a scarcity of food, 

 are undoubtedly responsible for the curbing of many in- 

 vasions of this kind. 



In both Europe and North America the larch tree is af- 

 fected by the leaf-feeding, caterpillar-like larva of the larch 

 sawfly, an insect common to both continents. Like many 



FIG. 30. The nun moth (Psilura monachal, an important enemy of 

 coniferous trees in Europe. This insect was once found in Brooklyn, 

 N. Y., but was stamped out before it had opportunity to spread. 



other forest insects, this species has in a number of instances, 

 appeared suddenly in great abundance in localities where it 

 had previously been hardly noticed for many years. Since 

 the middle of the last century, there have been about half a 

 dozen outbreaks of this kind, culminating in a very wide- 

 spread one in 1903-1907 which involved a considerable part 

 of the area in the eastern part of America where the larch is 

 native, and caused the death of a large proportion of the ma- 

 ture larches in this region. 



