18 THE SCOLYTID BEETLES. 
trunk at points some distance above the base, where the large pitch 
or gum tubes indicate their presence. 
Species 22 and 23 nearly always attack the base of the trees, where 
the very large pitch tubes and masses of pitch indicate their work. 
TVhen the main trunk is infested by these species to a sufficient extent 
to kill the trees, the evidence of infestation is found in the large 
pitch tubes and yellow foliage of the dying trees. 
SEASONAL HISTORY. 
The important features in the seasonal history of these beetles are 
the hibernation or overwintering of the broods, the beginning of 
activity in the spring, the emergence and flight of the adults, the 
beginning and ending of the period of principal attack, the period of 
larval development, the principal period of transformation from the 
larvae to the pupoe and adults, the beginning and ending of the period 
of emergence, and the number of generations annually. 
Certain features in the seasonal history of all of the species are 
similar, but as a rule each species or series of closely related ones 
has peculiarities which are more or less distinctive. A knowledge of 
these facts, therefore, is of prime importance as a basis for advice 
relating to the exact species involved in a given trouble and the 
successful methods of control. 
The broods of all of the species pass the winter as adults and 
larvae in the bark of the trees, logs, or stumps attacked during the 
preceding spring, summer, or fall. All excavate galleries through the 
inner living bark in which to deposit eggs, and the larvae of all feed 
on the inner bark; all become more or less active as soon as the 
weather conditions are favorable in the spring, especially the larvae 
and overwintered parent adults, the former extending their larval 
mines and the latter their egg galleries. The principal differences 
in seasonal history are brought out in the following references to 
the general features of the different species and in the detailed dis- 
cussion under each species farther on. 
In species 1 and 2, under average conditions, there is one complete 
generation and a partial second during the season of activity. 
In species 1 the first attack is made during the last week in June 
and first week in July, and the more advanced broods develop and 
begin to emerge about the last of August, but are not all out before 
cold weather. The first eggs of the second generation are deposited 
about the first of September, but the broods do not develop beyond 
the larval stage before hibernation begins in October. In species 2 
the seasonal history is practically the same as in species 1, except 
that the attack and subsequent stages begin a month earlier. The 
attack begins during the last of May, and the advanced broods begin 
to emerge during the latter part of July. 
