THE GENUS DENDROCTONUS. 17 
Species 21 is recorded as attacking spruce and pine principally, and 
as sometimes attacking larch, and fir (Abies), and is recognized as a 
destructive enemy. 
Species 22 attacks the different species of pine within its range and 
has also been found in spruce. It is sometimes destructive to living 
trees, but more often is simply injurious to the base of the trunks in 
causing basal scars. Species 23 attacks all of the pines and is some- 
times found in spruce and larch. It rarely causes the death of trees 
but causes serious damage to the base of living trees, resulting in the 
common defect known as basal scars and fire wounds. 
EVIDENCES OF ATTACK. 
In all of the species the first evidence of attack on living trees is 
the presence of pitch tubes on the trunks, mixed with reddish bor- 
ings, or the presence of reddish boring dust in the loose bark and 
around the base of the trees. Later the fading, yellowish, or reddish 
condition of the foliage is conspicuous evidence of the barkbeetles' 
destructive work. 
Successful attacks by species 1 to 8 are followed by a rapid death 
of the trees. The leaves fade in a month or two and turn yellow and 
reddish before winter. Successful attacks by species 9 to 11 are fol- 
lowed by a slow death of the trees. While tlfe trees attacked during 
the summer will have the bark on the trunks killed, the leaves will 
not turn yellow until the following May. 
Attacks by species 12 and 13 are not as a rule indicated by pitch 
tubes, but the reddish boring dust in the crevices of the bark, in the 
loose bark, and around the base of the trunk of Douglas fir or larch 
is quite conclusive evidence of their presence. Douglas fir attacked 
in the summer will have the leaves fading and turning pinkish in the 
fall and winter, and reddish in the spring. The leaves on the larch 
probably fall before they fade, although some of them may remain 
on the trees after the normal time for them to fall. 
In species 14 and 15, pitch tubes and red boring dust, mixed with 
resin, on the trunk and around the base, are evidences of attack. 
The trees attacked in the early summer will shed their green needles 
before fall. Those attacked later in the summer will have the bark 
on the trunks killed before winter, but the leaves may remain green 
until growth starts in the spring, when they will fall. Thus in May 
and June one often finds the ground beneath the infested trees 
covered with the green needles. After the leaves have fallen the 
bare twigs will cause the tops of infested trees to present a reddish 
appearance. 
Species 18 to 20 appear to confine their attack to or toward the 
base of the trees, where large pitch or gum tubes are formed, indi- 
eating their presence. In Europe, species 21 sometimes attacks the 
89535— Bull. 83, pt. 1—09 3 
