8 BULLETIN 255, IT. S. DEPARTMENT OF AGRICULTURE. 



50 years old, when the bark has roughened and thickened to an 

 extent to render it practically immune. However, on many trees 

 the bark over previous sesiid wounds is frequently in a condition 

 which invites reinfestation, and thus larvae are often found in trees 

 which are ready for logging. Especially is the latter a frequent 

 occurrence, if, as a result of the circular pitch seam inside, caused by 

 the moth many years ago, the tree is split by wind strain from that 

 seam straight to the surface of the bark. Toward the end of the 

 first-year work of the larvae, usually about August, the effect of 



Fig. 2. — Work of the Douglas fir pitch moth : Evolution of pitch blister and pitch seam. 



(Original.) 



their presence is shown in the growth of the new layer of wood, 

 which is restricted immediately above and below and to some ex- 

 tent on the sides of the wound. Pitch formation in this new layer 

 of wood and in the underlying layers, in an attempt by the tree to pro- 

 tect itself, forms the nucleus of the pitch blister. The inflexibility 

 of this blister, even if the timber is but normally swayed by the winds 

 as the tree grows taller, is bound to cause a further parting of the 

 tissues and ultimately results in the pitch seam, which so greatly 

 depreciates the value of the logs as lumber material. The afflicted 

 tree endeavors, during the three years required by the larva to de- 

 velop to adult, to wall up the wound, and the larva tries to main- 

 tain its established position; this struggle results in the wound 



