THE ZIMMERMAN PINE MOTH. 9 



show a tendency to develop into tops, are reached and the trees 

 killed, and on mature ones to a point where the thickness of the bark 

 fails to suit the insect. (PL VIII.) 



On wounds infested by a single larva a pitch tube, resembling that 

 produced by sesiid pitch moths, is usually formed, presumably because 

 one larva alone is not capable of cutting as near the surface as when 

 several work together in one space. In the latter case the tunnels 

 cross and recross continuously, and when a larva strikes the tunnel of 

 another, which must happen frequently, it usually cuts to the surface 

 in order to avoid the solidified pitch. To the presence of the larva 

 so very near and even at the surface of the bark must be attributed 

 its rather heavy parasitization in localities where its parasite exists, 

 because larvge living singly are very seldom parasitized. 



EFFECT OF INFESTATION ON TREE GROWTH AND FOREST. 



It is obvious that the killing of many trees in stands preferred by 

 the moth results in too great a thinning out of the stand. This 

 wastage of ground is further augmented by the permanent stunt- 

 ing of a still greater number of trees by the insect's work, because 

 the space taken up by such scrubs would just as readily accommodate 

 thrifty, well-formed trees. 



Moreover, the wood from trees that have been infested by the moth 

 is invariably* so permeated with pitch that the lumber cut from 

 such logs is either materially reduced in value or is rendered wholly 

 unfit for commercial use. (PL IX.) From one part of southeastern 

 Montana, where this moth is especially abundant and a large per- 

 centage of the trees are pitch soaked, the lumber is, for this reason, 

 only used for sheds, etc., where shrinkage can be discounted; the 

 users find it cheaper to have the better material shipped in than to 

 pick it out of the local stuff and throw half of it away unless it is 

 needed for the less particular purposes indicated. To the writer this 

 practice at first seemed rather to indicate prejudice against the home 

 product, because there is a large amount of first-grade lumber pro- 

 duced along with the bad. However, the pine moth is responsible for 

 this condition, as was abundantly proved by examination of its 

 injury in the district. The manner in which the moth's work "pitch- 

 ifies " the wood is best seen in the well-known tops which have been 

 infested by it. From these tops the bark has dropped off, but the 

 surface of the wood has a roughened appearance and the tissues are 

 literally saturated with pitch, while at the lower end, where the 

 infestation ended and the wood was not pitchified in the process, the 

 spike is rotted off from the tree. This insect's work alone accounts 

 for the fact that the extreme top of a tree may be excessively pitchy, 

 while the rest of the same tree is not. 



