THE GENUS DENDROCTONUS. 161 



pine and spruce, especially the former. While it sometimes infests 

 the bark 8 or 10 feet above the base and in sufficient numbers to kill a 

 few trees, it is an abnormal habit and result. It sometimes com- 

 pletely separates the bark around the base, but the wounds are so 

 completely covered by resin that a tree so affected rarely dies from this 

 injury alone, but often from secondary injury by fire or other insects. 

 The broad larval chambers separating the bark over areas of greater 

 or less extent might in many cases heal without serious harm, but the 

 dead bark with the pitch masses on the outside and the dried resin 

 and borings beneath offer the most favorable conditions for subse- 

 quent injuries by forest fires, and thus are the primary cause of a very 

 large percentage of the so-called "fire scars" or fire wounds (fig. 100) 

 which are so v prevalent at the base and lower portion of the trunks of 

 living trees in the dry pine areas of the western mountains. One or 

 more years after the injury by the beetle, a forest fire may burn the 

 bark and resin and expose the wood, which becomes dry and is then 

 bored by round-headed and flat-headed wood-borers, or it becomes 

 pitchy or decayed, so that the next fire burns deep into the wood 

 and kills a larger area of the bark. Thus each subsequent fire con- 

 tributes to an extension of the wound until in many cases the tree 

 is so weakened that it is broken down by wind or attacked and killed 

 by other barkbeetles. In the aggregate, this primary injury by the 

 beetle results in very extensive losses of some of the best timber. 



EVIDENCES OP ATTACK. 



The first evidence of the work of the beetle is found in the fresh 

 masses of pitch or large pitch tubes, mixed with reddish borings, at 

 or near the base of living trees and stumps of recently felled ones. 

 Subsequent evidence, until destroyed by fire, is found in the old 

 pitch masses on the surface or traces of primary galleries under the 

 bark or on the wood, and round holes in the loose, dead bark over the 

 wound. 



EFFECTS ON COMMERCIAL VALUE OF THE WOOD. 



The commercial value of the trees injured by this beetle is not 

 materially affected until after the injury has been extended into the 

 trunk by fire, wood-boring insects, decay, etc., to a point where the 

 vitality is greatly reduced or the tree becomes worthless. 



FAVORABLE AND UNFAVORABLE CONDITIONS FOR THE BEETLE. 



Favorable conditions for the multiplication of this beetle and for 

 its injury to living timber are found in sections where for several years 

 a large amount of pine timber has been killed or injured by insects, or 

 felled and broken by storms, lumbering operations, etc., followed by a 

 year in which no timber dies or is injured or killed. Under these con- 

 89535— Bull. 83, pt. 1—09 12 



