16 



BULLETIN 255, U. S. DEPARTMENT OF AGRICULTURE. 



After the } 7 oung larva has penetrated to the cambium, which 

 usually occurs about August, or at a time when the growing period 

 of the tree for the season is ended, it excavates a tortuous tunnel 

 from 1 to 2 inches in length, parallel with and transverse to the 

 grain of the wood. The length of the tunnel at completion depends 

 largely upon the growth of the infested tree. In slow-growing trees 



Fig. 10. — Pitch tube covering larvse of the Douglas fir pitch moth 

 the second year after infestation. Natural size. (Original.) 



it reaches occasionally a length of from 5 to 6 inches at the time of 

 moth emergence, while in very vigorous growers the larva maintains 

 its well-being within a circular-shaped pit, not more than 2 inches 

 in width, deeply embedded in the woody tissues of the cambium. 



The surface of the wound is invariably covered by a pitch tube of 

 the color of the bark. When the outer crust of the tube is removed it 

 is found to contain pitch and the excreta of the larva. If the same 

 wound is reinfested, soft pitch mixed with the old crusty pitch 



