17 



HOW THE TREES ARE ATTACKED AND KILLED. 



Many hundreds of trees were examined during the investigation, 

 including those that were living and perfectly healthy, living and 

 freshly attacked, infested and dying, recently dead, and old dead ones 

 which bore evidence of having been killed by the pine-destroying 

 beetle. All stages of the insect, including the adult, the egg^ different 

 stages of the larva, the pupa, and recently transformed beetles, were 

 observed and studied, as were also all stages of the primary entrance, 

 the gallery and brood mines in the living, dying, and dead bark, and 

 also the primary gallery grooves on the surface of the wood of old 

 dead trees and logs from which the bark had fallen and decayed. 



The evidences gathered from these studies, and from information 

 conveyed in Mr. Dewey's letter, quoted on another page, indicate that 

 the principal attack is made in August, when it would seem the beetles 

 migrate in swarms from the dying trees and settle on the living ones, 

 which they attack and infest in large numbers from near the base to 

 the upper part of the main trunk or stem. 



The trees that are attacked by a sufficient number of the beetles to 

 overcome the resistance exerted by the vital forces of the plant com- 

 mence to decline, and by winter or the following spring they die and 

 the leaves turn yellow and red. Those not attacked by sufficient num- 

 bers of the beetles to overcome this vital resistance recover and are 

 usually exempt from future attacks; the wounds heal and are covered 

 over by subsequent layers of wood, thus causing pitch spots or gum- 

 streak defects in the wood. 



The details of the work of the attacking force of beetles on a living 

 tree may be briefly described as follows: 



Both sexes settle on their victim, usually in large numbers, and the 

 males (?) a commence to excavate the entrance burrows, which are usu- 

 ally hidden in a crevice or beneath a flake of the outer bark. The 

 reddish, sawdust-like borings thus produced and thrown out fall to the 

 ground around the base and lodge in the loose outer bark on the trunk. 

 When they enter the inner living bark, or bast, the tree commences 

 to exert its resistance by throwing out pitch to fill and heal the fresh 

 wounds in the living tissue. Then the struggle between the resisting 

 force of the plant and the beetles begins in earnest. Each female 

 joins her mate, and together they continue the excavation. The bor- 

 ings and pitch are disposed of by being pushed out and formed into 

 a pitch tube at the mouth of the entrance burrow (PI. VII, figs. 1, 3, 

 and 4). The inner bark is entered obliquely and subtransversely to 

 the cambium and surface of the wood, where a broadened cavity is 

 excavated for the accommodation and temporary occupation of the 



a While it was not positively determined that the male of this species excavates 

 the first entrance, it is the habit of many other bark beetles, and is probably followed 

 by this. 



16274— No. 32—02 2 



