THE GENUS DENDROCTONTJS. 
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. ) 
