322 



YEARBOOK OF THE DEPARTMENT OF AGRICULTURE. 



conditions for the multiplication and destructive ravages of the two- 

 lined chestnut borer; therefore all such trees, together with those dying 

 from insect attack, should be felled during the summer or the winter 

 following, and the bark removed and burned. Very often such trees 

 can be utilized for fuel, so that nothing is lost in the operation. 



The birches, aspens, cottonwoods, and balm-of-gileads are killed or 

 seriously injured by other species of Agrilus, which have habits similar 

 to those of the oak-destroying bark-borer, hence require the same 

 treatment. 



The work of these insects has been observed in the birches, aspens, 



a, ~b 



FIG. 34. Mines of a destructive bark-borer: a, healed-over mine in cotton wood; b, mine grooved in 



surface of wood. (Original.) 



and cottonwoods from Maine to West Virginia, and westward to 

 northern Idaho and northern New Mexico. 



The curious embossed effect on the surface of the wood (fig. 34, #), on 

 trees which have been infested or killed by one of these bark-borers, 

 is the result of healed-over grooves made by the Iarva3 in the outer 

 layers of wood beneath the bark (fig. 34, b) when the trees were living 

 and growing. Therefore, these healed wounds furnish conclusive 

 evidence that the trees are attacked while living. The long winding 

 burrows beneath the bark, as shown in PI. XXXIX, fig. 2, show how 

 the trees are easily girdled and killed when thickly infested. 



