70 INSECTS AND HUMAN WELFARE 



by the spruce bark-beetle (Dendroctonus piceaperda). With 

 this, as with other bark-beetles, the parent beetles excavate 

 galleries in the inner bark or cambium, along which they 

 deposit their eggs. When these eggs hatch, the larvae burrow 

 off to the sides where they feed upon the cambium layer until 

 they attain full growth, after which they bore their way 

 through the outer bark, and leave the tree. Extensive injury 

 to the cambium interferes with the metabolism of the tree 

 and kills it after the manner of the girdler's axe (Fig. 32) . 



More spectacular than the work of the spruce bark-beetle, 

 has been that of the Black Hills beetle (Dendroctonus ponder- 

 osce. Following its sudden appearance in large numbers in 

 the Black Hills National Forest of South Dakota, during the 

 season of 1897, its ravages assumed the proportions of an 

 epidemic there, and Hopkins has estimated that trees con- 

 taining nearly a billion feet of timber were killed before the 

 epidemic subsided several years later. 



As mentioned on a previous page, mixed forests present a 

 somewhat different problem to the entomologist from that 

 just discussed in connection with the unmixed, or pure 

 growths of coniferous forests. Our mixed forests are com- 

 posed mainly of deciduous, or broad-leaved trees, and these 

 ordinarily contain a well-mixed assortment of tree-species 

 belonging to at least several of the natural families of plants, 

 combined with much shrubby and herbaceous growth. They 

 at once exhibit a great diversity of plants, and support a 

 correspondingly large and varied insect fauna. A consider- 

 able proportion of these insects are species dependent upon 

 the plants other than trees; some live upon the rotting wood 

 or bark of fallen trees, others upon fungi, many are predatory 

 or parasitic upon the foregoing, till only a small number re- 

 maining, are actually destructive forest pests. These in turn, 

 are each almost entirely restricted to one or several related 

 trees as food plants, so that the inter-relationship of trees 

 and insects becomes a very complicated one. It would seem 

 that such an environment should tend to equalize the damage 

 done by insects to specific kinds of trees, from year to year, 



