8 BULLETIN 260, U. 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 larve 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-vear work of the larve, usually about August, the effect of 
' 
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Fic. 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 
