318 



YEARBOOK OF THE DEPARTMENT OF AGRICULTURE. 



' BARK-BEETLE INJURIES TO OAK TREES. 



Different kinds of oaks, of sizes ranging from a few inches in 

 diameter to large trees, are frequently found in the woods dying or 

 recently dead, with no evidence of external injury. If, upon remov- 

 ing the bark from a place on the trunk some 4 or 5 feet above the 

 base of one of these trees, the inner surface is found to be grooved 

 with great numbers of minute transverse burrows, similar to fig. 30, &, 



which are also faintly marked on 

 the surface of the wood, it is the 

 work of the oak-destroying bark- 

 beetle (Pityophthorus pruinosus 

 Eichh.). 



This is an exceedingly small 

 dark-brown or nearly black beetle, 

 less than 2 millimeters (0.08 of an 

 inch) in length, which, notwith- 

 standing its small size, sometimes 

 occurs in such vast numbers that 

 large oak trees are attacked and 

 .killed by it in a few weeks. It 

 passes the winter in the adult and 

 larval stage in the inner bark of 

 trees and limbs where it bred the 

 previous summer. It commences 

 to fly early in the spring, and pre 

 fers to enter the bark of trees re- 

 cently felled or injured by storm 

 or other causes. It also enters the 

 bark of large and small branches 

 recently broken or cut from living 

 trees. In the inner bark of these 

 the parent adults excavate short 

 double transverse primary galle- 

 ries, from which the broods of 



FIG. 30.-Brood galleries of the oak bark-beetle, } T OUng larV86 eXCRVate long lateral 

 showing character of primary gallery at b; m i nes (fig. 30), Up and down, 



larval or brood mines at a. . , . . 



through the inner bark, and folio #- 



ing the bark fibers in such a way that they are difficult to recognize. 

 Enormous numbers of these brood galleries are often found within a 

 small area of bark. When a large number of trees and branches are 

 infested the swarms of beetles of the first generation will emerge later in 

 the summer, and if they fail to find sufficient felled and injured trees in 

 the right condition to attract them, they will concentrate their attack on a 

 few living trees, which soon perish. The leaves of such trees will first 

 fade, then turn brown, and remain on the branches until the next spring. 



