THE GENUS DENDROCTONUS. 



119 



numbers as it does the living, standing trees. It infests the red 

 spruce, black spruce, and white spruce, but, so far as known, does 

 not attack any of the pines or the larch. 



The beetles enter the bark of healthy trees at a point from 6 to 10 

 feet from the base, and that of trees weakened by disease or other 

 causes from near the base to the larger branches. 



In the living trees the entrance burrow is gradually extended 

 obliquely upward, or subtransversely, thence in a longitudinal direc- 

 tion upward through the inner bark and often grooving the surface 

 of the wood. Along the sides of 

 this gallery, which is usually about 

 three times as broad as the beetle, 

 the eggs are placed singly in small 

 cavities or in groups in an elon- 

 gated cavity. The eggs are then 

 protected by a mass of borings, 

 closely packed and cemented with 

 gum, which, with the exception of 

 a small inner burrow or subgallery, 

 fill up the broad egg gallery. The 

 original entrance is first packed; 

 then an opening to the outside is 

 made in the roof of the gallery a 

 few inches from the entrance, an- 

 other section is excavated and 

 packed, another hole is made 

 through the roof, and so on until 

 the gallery is completed. After 

 all is finished the adults make one 

 or two short, irregular, lateral 

 branches at the farther end, ap- 

 parently for an abiding place until 

 they die. 



The gum flowing into the wounds 

 made by the beetles when they are 

 excavating the entrance is pushed out and the holes kept open 

 through it, thus forming the pitch tubes, which are so conspicuous on 

 the bark of freshly attacked trees. After the vitality of a tree is 

 weakened by numerous wounds and by an excessive flow of resin 

 the subsequent entrances are not marked by pitch tubes. Or, if a tree 

 is decidedly weakened from other causes before it is attacked, or 

 when a large number of beetles is boring into the outer bark, and 

 the boring-dust falls down and lodges in the flakes of bark and in the 

 moss on the tree, pitch tubes are not formed. 



Fig. 74.— The eastern spruce beetle. Old gal- 

 leries marked on surface of tree: Grooves on 

 surface of wood of tree that has been dead 

 about twelve years. (Author's illustration.) 



