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. 

 When 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 pupce 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, 



